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Showing papers in "Geological Society of America Bulletin in 1997"


Journal ArticleDOI
TL;DR: In this article, a classification of channel-reach morphology in mountain drainage basins synthesizes stream morphologies into seven distinct reach types: colluvial, bedrock, and five alluvial channel types (cascade, step pool, plane bed, pool rime and dune ripple).
Abstract: A classification of channel-reach morphology in mountain drainage basins synthesizes stream morphologies into seven distinct reach types: colluvial, bedrock, and five alluvial channel types (cascade, step pool , plane bed, pool rime, and dune ripple). Coupling reach-level channel processes with the spatial arrangement of reach morphologies, their links to hillslope processes, and external forcing by confinement, ripar­ ian vegetation, and woody debris defines a process-based framework within which to assess channel condition and response potential in mountain drainage basins. Field investigations demonstrate character­ istic slope, grain size, shear stress, and roughness ranges for different reach types, observations consistent with our hypothesis that alluvial channel morphologies reflect specific roughness configurations ad­ justed to the relative magnitudes of sediment supply and transport ca­ pacity. Steep alluvial channels (cascade and step pool) have high ratios of transport capacity to sediment supply and are resilient to changes in discharge and sediment supply, whereas low-gradient alluvial channels (pool rime and dune ripple) have lower transport capacity to supply ra­ tios and thus exhibit significant and prolonged response to changes in sediment supply and discharge. General differences in the ratio of transport capacity to supply between channel types allow aggregation of reaches into source, transport, and response segments, the spatial distribution of which provides a watershed-level conceptual model linking reach morphology and channel processes. These two scales of channel network classification define a framework within which to in­ vestigate spatial and temporal patterns of channel response in moun­ tain drainage basins.

1,889 citations


Journal ArticleDOI
TL;DR: In this paper, the authors examine possible causative relations between tectonics and environmental and biologic changes during the Neoproterozoic and Paleozoic eras by reconstructing Rodinia and Pannotia, supercontinents that may have existed before and after the opening of the Pacific Ocean basin.
Abstract: The ever-changing distribution of continents and ocean basins on Earth is fundamental to the environment of the planet. Recent ideas regarding pre-Pangea geography and tectonics offer fresh opportunities to examine possible causative relations between tectonics and environmental and biologic changes during the Neoproterozoic and Paleozoic eras. The starting point is an appreciation that Laurentia, the rift-bounded Precambrian core of North America, could have been juxtaposed with the cratonic cores of some present-day southern continents. This has led to reconstructions of Rodinia and Pannotia, supercontinents that may have existed in early and latest Neoproterozoic time, respectively, before and after the opening of the Pacific Ocean basin. Recognition that the Precordillera of northwest Argentina constitutes a terrane derived from Laurentia may provide critical longitudinal control on the relations of that craton to Gondwana during the Precambrian-Cambrian boundary transition, and in the early Paleozoic. The Precordillera was most likely derived from the general area of the Ouachita embayment, and may have been part of a hypothetical promontory of Laurentia, the “Texas plateau,” which was detached from the Cape of Good Hope embayment within Gondwana between the present-day Falkland-Malvinas Plateau and Transantarctic Mountains margins. Thus the American continents may represent geometric “twins” detached from the Pannotian and Pangean supercontinents in Early Cambrian and Early Cretaceous time, respectively—the new mid-ocean ridge crests of those times initiating the two environmental supercycles of Phanerozoic history 400 m.y. apart. In this scenario, the extremity of the Texas plateau was detached from Laurentia during the Caradocian Epoch, in a rift event ca. 455 Ma that followed Middle Ordovician collision with the proto-Andean margin of Gondwana as part of the complex Indonesian-style Taconic-Famatinian orogeny, which involved several island arc-continent collisions between the two major continental entities. Laurentia then continued its clockwise relative motion around the proto-Andean margin, colliding with other arc terranes, Avalonia, and Baltica en route to the Ouachita-Alleghanian-Hercynian-Uralian collision that completed the amalgamation of Pangea. The important change in single-celled organisms at the Mesoproterozoic-Neoproterozoic boundary (1000 Ma) accompanied assembly of Rodinia along Grenvillian sutures. Possible divergence of metazoan phyla, the appearance and disappearance of the Ediacaran fauna (ca. 650–545 Ma), and the Cambrian “explosion” of skeletalized metazoans (ca. 545–500 Ma) also appear to have taken place within the framework of tectonic change of truly global proportions. These are the opening of the Pacific Ocean basin; uplift and erosion of orogens within the newly assembled Gondwana portion of Pannotia, including a collisional mountain range extending ≈7500 km from Arabia to the Pacific margin of Antarctica; the development of a Pannotia-splitting oceanic spreading ridge system nearly 10 000 km long as Laurentia broke away from Gondwana, Baltica, and Siberia; and initiation of subduction zones along thousands of kilometres of the South American and Antarctic-Australian continental margins. The Middle Ordovician sea-level changes and biologic radiation broadly coincided with initiation of the Appalachian-Andean mountain system along >7000 km of the Taconic and Famatinian belts. These correlations, based on testable paleogeographic reconstructions, invite further speculation about possible causative relations between the internally driven long-term tectonic evolution of the planet, its surface environment, and life.

1,053 citations


Journal ArticleDOI
TL;DR: The 3.8 km 3 Osceola Mudflow as discussed by the authors was a water-saturated avalanche during phreatomagmatic eruptions at the summit of Mount Rainier about 5600 years ago.
Abstract: The 3.8 km 3 Osceola Mudflow began as a water-saturated avalanche during phreatomagmatic eruptions at the summit of Mount Rainier about 5600 years ago. It filled valleys of the White River system north and northeast of Mount Rainier to depths of more than 100 m, flowed northward and westward more than 120 km, covered more than 200 km 2 of the Puget Sound lowland, and extended into Puget Sound. The lahar had a velocity of ≈19 m/s and peak discharge of ≈2.5×10 6 m 3 /s, 40 to 50 km downstream, and was hydraulically dammed behind a constriction. It was coeval with the Paradise lahar, which flowed down the south side of Mount Rainier, and was probably related to it genetically. Osceola Mudflow deposits comprise three facies. The axial facies forms normally graded deposits 1.5 to 25 m thick in lowlands and valley bottoms and thinner ungraded deposits in lowlands; the valley-side facies forms ungraded deposits 0.3 to 2 m thick that drape valley slopes; and the hummocky facies, interpreted before as a separate (Greenwater) lahar, forms 2–10-m-thick deposits dotted with numerous hummocks up to 20 m high and 60 m in plan. Deposits show progressive downstream improvement in sorting, increase in sand and gravel, and decrease in clay. These downstream progressions are caused by incorporation (bulking) of better sorted gravel and sand. Normally graded axial deposits show similar trends from top to bottom because of bulking. The coarse-grained basal deposits in valley bottoms are similar to deposits near inundation limits. Normal grading in deposits is best explained by incremental aggradation of a flow wave, coarser grained at its front than at its tail. The Osceola Mudflow transformed completely from debris avalanche to clay-rich (cohesive) lahar within 2 km of its source because of the presence within the preavalanche mass of large volumes of pore water and abundant weak hydrothermally altered rock. A survey of cohesive lahars suggests that the amount of hydrothermally altered rock in the preavalanche mass determines whether a debris avalanche will transform into a cohesive debris flow or remain a largely unsaturated debris avalanche. The distinction among cohesive lahar, noncohesive lahar, and debris avalanche is important in hazard assessment because cohesive lahars spread much more widely than noncohesive lahars that travel similar distances, and travel farther and spread more widely than debris avalanches of similar volume. The Osceola Mudflow is documented here as an example of a cohesive debris flow of huge size that can be used as a model for hazard analysis of similar flows.

356 citations


Journal ArticleDOI
TL;DR: In this paper, the authors provide a unique data set comprising rates of thrust advance and basin fill migration for the Tertiary foreland basin of the European Alps, and suggest that the Alpine basin of central Switzerland migrated with an approximately steady state geometry for at least 210 km northwestward over the European craton.
Abstract: Advances in the development of quantitative models of foreland basin stratigraphy have outpaced the observational data used to constrain the input parameters in such models. Underfilled peripheral foreland basins comprise a broad threefold subdivision of depositional realms that translates into three stratigraphic units which are commonly superimposed during basin migration; these units are here termed the “underfilled trinity.” The three units of the trinity reflect (1) carbonate deposition on the cratonic margin of the basin (the lower unit), (2) hemipelagic mud sedimentation offshore from the cratonic margin of the basin (the middle unit), and (3) deep water turbiditic siliciclastic sedimentation toward the orogenic margin of the basin (the upper unit). Theoretical predictions of how such a complex basin fill initiates and evolves through time are not currently available; hence this study reviews the stratigraphy of underfilled peripheral foreland basins and provides a unique data set comprising rates of thrust advance and basin fill migration for the Tertiary foreland basin of the European Alps. The Paleocene to Oligocene Alpine foreland basin of France and Switzerland comprises a well-developed underfilled trinity that is preserved within the outer deformed margins of the Alpine orogen. Structural restorations of the basin indicate a decrease in the amount of basin shortening from eastern Switzerland (68%) to eastern France (48%), to southeastern France (35%). Structurally restored chronostratigraphic diagrams allow rates of basin migration to be calculated from around the Alpine arc. Paleogeographic restorations of the Nummulitic Limestone (lower unit) illustrate a radial pattern of coastal onlap on to the European craton. Time-averaged rates for northwestward coastal onlap of the underfilled Alpine basin across Switzerland were between 8.5 and 12.9 mm/yr. Time-equivalent westward to southwestward coastal onlap rates in France were between 4.9 and 8.0 mm/yr. The direction of migration of the cratonic coastline of the basin was parallel to the time-equivalent thrust motions, and oblique to the Africa-Europe plate motion vector. By comparing rates of thrust propagation into the orogenic margin of the basin to rates of coastal onlap of the cratonic margin of the basin, it is possible to suggest that the Alpine foreland basin of central Switzerland migrated with an approximately steady state geometry for at least 210 km northwestward over the European craton. The westward and southward decrease in the basin migration rate around the Alpine arc was associated with an increase in the degree of syndepositional normal faulting on the European plate; this is thought to relate to the opening of the Rhine-BresseRhone graben system.

348 citations


Journal ArticleDOI
TL;DR: In this paper, an inspection of the available stratigraphic and geochronological data on sedimentary, volcanic, and plutonic units of the southern Central Andes of northern Chile and northwestern Argentina reveals a lull in magmatic and metamorphic activity lasting for ~100 m.y., from Early Silurian to early Late Carboniferous time.
Abstract: In Ordovician time, Gondwana in the area of northwestern Argentina and northern Chile had a west-facing active margin. The evolution of this margin culminated in the Ocloyic orogeny at the end of Ordovician time. This orogeny was caused by the collision of the allochthonous Arequipa-Antofalla terrane with this margin. The early Paleozoic evolution of northwestern Argentina and northern Chile contrasts markedly with the accretionary history of central Argentina and central Chile, where the Precordillera and Chilenia terranes docked in the Late Ordovician and Late Devonian periods, respectively. An inspection of the available stratigraphic and geochronological data on sedimentary, volcanic, and plutonic units of the southern Central Andes of northern Chile and northwestern Argentina reveals a lull in magmatic and metamorphic activity lasting for ~100 m.y., from Early Silurian to early Late Carboniferous time. This is interpreted as corresponding to a tectonic scenario in which the present Andean margin was a passive margin of Gondwana. This passive margin developed in response to the rifting off of a part of the Arequipa-Antofalla terrane; the present location of this block is unknown. Late Carboniferous time marks the renewed onset of subduction, initiating the Andean plate tectonic setting still prevalent today. Recently proposed models explain the Late Ordovician orogeny by the collision of Laurentia with western South America during Laurentia’s clockwise motion around South America and away from its position in the Neoproterozoic supercontinent. In its present form, this hypothesis is difficult to reconcile with the Paleozoic tectonostratigraphic evolution of the southern Central Andean region.

230 citations


Journal ArticleDOI
TL;DR: In the case of the Cayara Formation, the most recent erosional unconformity at the base of Cayara is 58.2 Ma as mentioned in this paper, which separates the Upper Puca and Corocoro supersequences in Bolivia and is thus coeval with the Zuni-Tejas sequence boundary of North America.
Abstract: Integration of sequence stratigraphy, magnetostratigraphy, Ar/Ar dating, and paleontology considerably advances knowledge of the Late Cretaceous–early Paleogene chronostratigraphy and tectonic evolution of Bolivia and adjacent areas. The partly restricted marine El Molino Formation spans the Maastrichtian and Danian (°73–60.0 Ma). Deposition of the alluvial to lacustrine Santa Lucia Formation occurred between 60.0 and 58.2 Ma. The widespread erosional unconformity at the base of the Cayara Formation is 58.2 Ma. This unconformity separates the Upper Puca and Corocoro supersequences in Bolivia, and is thus coeval with the Zuni-Tejas sequence boundary of North America. The thick overlying Potoco and Camargo formations represent a late Paleocene–Oligocene foreland fill. The onset of shortening along the Pacific margin at °89 Ma initially produced rifting in the distal foreland. Santonian–Campanian eastward-onlapping deposits indicate subsequent waning of tectonic activity along the margin. Significant tectonism and magmatism resumed along the margin at °73 Ma and produced an abrupt increase in subsidence rate and other related phenomena in the basin. Subsidence was maximum between °71 and °66 Ma. Due to the early Maastrichtian global sea-level high, marine waters ingressed from the northwest into this underfilled basin. Subsidence decreased during the Late Maastrichtian and was low during the Danian. It increased again in the latest Danian, for which a slight transgression is recorded, and peaked in the early Selandian. Tectonism between 59.5 and 58.2 Ma produced a variety of deformational and sedimentary effects in the basin and correlates with the end of emplacement of the Coastal batholith. The subsequent 58.2 Ma major unconformity marks the onset of continental foreland basin development, which extended into Andean Bolivia during the late Paleocene–Oligocene interval. This basin underwent internal deformation as early as Eocene time in the Altiplano and Cordillera Oriental. These early structures, previously assigned to the late Oligocene–early Miocene orogeny, probably accommodated observed tectonic rotations in the Eocene–Oligocene.

213 citations


Journal ArticleDOI
TL;DR: In this paper, a late Pliocene and early Pleistocene (approximately 2.6-1.7 Ma) succession of cyclothems of shelf origin exposed in the Rangitikei River valley in the eastern part of Wanganui basin is represented by an individual depositional sequence comprising transgressive, highstand, and regressive systems tracts.
Abstract: This study is based on a late Pliocene and early Pleistocene (approximately 2.6‐1.7 Ma) succession about 1 km thick of 20 sixth-order (41 k.y. duration) cyclothems of shelf origin exposed in the Rangitikei River valley in the eastern part of Wanganui basin. The cyclothems correlate with δ 18 O isotope stages 100‐58, and each 41 k.y. glacialinterglacial stage couplet is represented by an individual depositional sequence comprising transgressive, highstand, and regressive systems tracts. Unlike most examples inferred from the stratigraphic record, these systems tracts were deposited during phases of known sea-level cycles indicated by the contemporary oxygen isotope ice-volume curve. Because of the high rate of subsidence of Wanganui basin, glacioeustatic sea-level falls during most cycles were not of sufficient magnitude to expose the outer shelf. Thus, the Rangitikei section provides an exceptional example of regressive strata deposited landward of the contemporary shelf break. Simple one-dimensional modeling shows that moderate to high rates of basin subsidence (1‐2 mm/yr) and low rates of sedimentation (<0.2 mm/yr) during transgressions combined to produce an accommodation surplus at the relative highstand. This surplus accommodation was infilled during the late highstand and ensuing fall partly by aggradational, highstand systems tract shelf siltstone, and chiefly by strongly progradational shoreface sediments of the regressive systems tract. Rangitikei regressive systems tracts are distinguished from forced regressive systems tracts (sensu Hunt and Tucker, 1992) by their different stratal geometry. By definition, forced regressive systems tracts display an erosional contact with the underlying highstand systems tracts and typically occur as a series of downstepped disjunct shoreline wedges stranded on the shelf and/or slope. In contrast, regressive systems tracts exhibit a gradational lower contact, above which parasequences are stacked in a strongly progradational pattern terminated by the superjacent sequence boundary. Cyclothems display two types of motif termed Rangitikei dt (depositional transgression), and Rangitikei nt (nondepositional transgression), which include the following architectural elements in ascending stratigraphic order: (1) a basal sequence boundary that is coincident with either the transgressive surface of erosion, which displays small-scale (up to 50 cm) erosional relief and may be penetrated by the ichnofossil Ophiomorpha, or its deeper water correlative conformity; (2) either a thick (5‐30 m) transgressive systems tract comprising a deepening upward nearshore to inner shelf, mixed carbonate-siliciclastic lithofacies succession (depositional transgression), or a thin (<2 m) transgressive systems tract comprising condensed fossiliferous facies deposited on the sediment-starved offshore shelf (nondepositional transgression); (3) a sharp downlap surface separating condensed fossiliferous facies of the transgressive systems tract from terrigenous siltstone of the superjacent highstand systems tract; (4) a highstand systems tract comprising a 10‐20-m-thick interval of aggradational, shelf siltstone; and (5) a thick (up to 45 m) progradational inner shelf to shoreface lithofacies assemblage ascribed to the regressive systems tract. Condensed shell beds are associated with intrasequence and sequence-bounding discontinuities, and, together with the sedimentological and stratal characteristics of the sequences, are important indicators of stratigraphic architecture. Four types of shell bed are associated with surfaces formed by four different types of stratal termination; onlap, backlap, downlap, and flooding surface shell beds (cf. Kidwell, 1991) are associated, respectively, with the transgressive surface of erosion, “apparent truncation” at the top of the transgressive systems tract, the downlap surface, and local marine flooding surfaces. A fifth shell-bed type, termed a compound shell bed, forms in offshore environments where the downlap surface converges with the sequence boundary, and elements of both the downlap and the backlap shell beds become mixed or superposed. The shell beds mark zones of stratal attenuation and can be used as surrogates for seismic discontinuities when applying sequence stratigraphic concepts at outcrop scale.

201 citations


Journal ArticleDOI
TL;DR: A detailed investigation of seawater Sr-isotope stratigraphy based on foraminifers and, where available, on inoceramid bivalves from 12 mid-Cretaceous Deep Sea Drilling Project and Ocean Drilling Program sections is reported in this paper.
Abstract: Large variations exist between published mid-Cretaceous (late Barremian to early Turonian stages) seawater Sr-isotope stratigraphies; this has resulted in disparate interpretations of crustal production rates. We report on a detailed investigation of seawater Sr-isotope stratigraphy based on foraminifers and, where available, on inoceramid bivalves from 12 mid-Cretaceous Deep Sea Drilling Project and Ocean Drilling Program sections. The effects of diagenesis are assessed using scanning electron microscope observations and traceelemental analyses, but are best distinguished by comparing the 87 Sr/ 86 Sr values of similarage samples from different sites. Strontiumisotope analyses compiled from 9 of 12 sites that have detailed age control define one band of common values. This band is used as a composite curve, which presumably represents seawater 87 Sr/ 86 Sr values. The composite curve shows a “trough” of markedly lower 87 Sr/ 86 Sr values in the Aptian and early Albian stages, higher but constant values for the middle Albian-Cenomanian stages, followed by a decrease in 87 Sr/ 86 Sr values in the early Turonian.

200 citations


Journal ArticleDOI
TL;DR: In this paper, an analysis of the Upper Jurassic-Lower Cretaceous Morrison and Cedar Mountain Formations of Utah and Colorado has resulted in a general sequence-stratigraphic model for nonmarine rocks.
Abstract: An analysis of the Upper Jurassic–Lower Cretaceous Morrison and Cedar Mountain Formations of Utah and Colorado has resulted in a general sequence-stratigraphic model for nonmarine rocks In this model, nonmarine deposition is governed by changes in basin accommodation development and corresponding shifts in depositional base level These fluctuations result in deposition of systematically varying facies and architectural elements that allow nonmarine depositional sequences to be recognized Internally, nonmarine depositional sequences comprise three systems tracts—degradational, transitional, and aggradational—which are analogous to the lowstand, transgressive, and highstand systems tracts of marine depositional sequences Degradational systems tracts overlie sequence-bounding unconformities and consist of relatively coarse-grained, low-sinuosity fluvial deposits that are either contained within incised valleys or deposited as widespread, thin sheets above shallow erosion surfaces Transitional systems tracts represent an increase in basin accommodation development following degradational systems-tract deposition They are marked by the transition from laterally continuous, low-sinuosity fluvial channel sandstones and conglomerates to lenticular and ribbon-like, meandering and anastomosing channel sandstones Aggradational systems tracts are characterized by meandering-anastomosing channel sandstones and abundant fine-grained overbank and lacustrine deposits The Upper Jurassic–Lower Cretaceous nonmarine rocks of the study area contain three depositional sequences The first of these, the UJ-1 sequence, consists primarily of an aggradational systems tract overlain by a sequence-bounding unconformity However, the lower parts of this sequence are transitional with older marine rocks and can be considered the late stages of a marine highstand systems tract The upper Morrison sequence (UJ-2) consists of degradational, transitional, and aggradational systems tracts Above the UJ-2 sequence are a sequence-bounding unconformity and degradational and transitional systems tracts of the LK-1 sequence represented by the Buckhorn Conglomerate The Buckhorn is overlain by a sequence-bounding unconformity and transitional-aggradational systems tracts of the LK-2 sequence that is composed of the upper part of the Cedar Mountain Formation The Upper Jurassic–Lower Cretaceous sequences in Utah and Colorado can be traced regionally and correlated with nonmarine depositional sequences in central Utah and sequences that contain nonmarine, transitional, and marine rocks in central Wyoming These sequences were deposited in the back-bulge, forebulge, and distal foredeep depozones of the Late Jurassic–Early Cretaceous foreland-basin system

185 citations


Journal ArticleDOI
TL;DR: In this paper, the Triassic succession of the Nakhlak area in central Iran consists of (1) the LithologicallAlam Formation, which is a sequence of shallowing and coarsening-upward marine turbidites deposited equon the forearc side of an accretionary prism; (2) the Baqoroq Formation, a resentatsequence of coarse to fine, polymictic, fluvial conglomerates; and (3) the Ashin For-mation, which comprises alternating, distal marine shales and sand
Abstract: The Triassic succession of the Nakhlak area in central Iran consists of (1) the LithologicallAlam Formation, which is a sequence of shallowing- and coarsening-upward marine turbidites deposited equon the forearc side of an accretionary prism; (2) the Baqoroq Formation, a resentatsequence of coarse to fine, polymictic, fluvial conglomerates; and (3) the Ashin For- mation, which comprises alternating, distal marine shales and sand- stones that have turbiditic characteristics. These rocks are wnot litholog- ically similar to time-equivalent lithostratigraphic units of Late Permian – Triassic age of the Aghdarband area of northeastern Iran (which are interpreted to be forearc deposits), but they may have formed in close association with them in a single tectonic and sedimen- tary framework. Accepting the 135° counterclockwise rotation of the central-east Iranian microcontinent authorwith respect to the Turan plate since Triassic time, and assuming that the Triassic rocks of the Nakhlak and the Late Permian to Triassic rocks of the Aghdarband formed in a single tectonosedimentary framework on the no rthern side of the paleo-Tethyan oceanic realm, we present here a sequential develop- ment. In this scheme, rocks of the bNakhlak and Aghdarband areas are considered deposits of a forearc, basin-ridge-slope centralenvironment. The separation of the Nakhlak succession from the rest of the Turan plate and its transportation to central Iran might have occurred as (1) a lithospheric segmen t of the Turan plate, first detached from Turan and then attached to the Iranian plate, and finally rotated with it in a coun- terclockwise direction to successionits present site; or (2) as a thin thrust slice first obducted over th e Iranian continental shelf and then displaced to cen- tral Iran by its counterclockwise rotation. INTRODUCTION

181 citations


Journal ArticleDOI
TL;DR: In this paper, the distribution and petrologic character of all three assemblages reflect slab-window magmatism triggered by decompression melting of upwelling mantle in the Neogene San Andreas transform system.
Abstract: Cenozoic volcanic rocks of coastal California were erupted west of the magmatic arc trend related to subduction along the continental margin. Two assemblages, representing discrete pulses of mid-Tertiary (Oligocene–Miocene) and mid-Miocene volcanism, occupied relatively compact tracts, as restored palinspastically prior to disruption and dispersal by tectonic displacements within the Neogene San Andreas transform system. A third assemblage records migratory post–mid-Miocene volcanism at centers located progressively farther north. The distribution and petrologic character of all three assemblages reflect slab-window magmatism triggered by decompression melting of upwelling mantle. Paleogeographic reconstructions, based on analysis of magnetic anomaly patterns offshore and restoration of on-land features prior to San Andreas transform slip and associated transrotational deformation, indicate the paleotectonic positions of the Cenozoic volcanic fields before structural disruption. The pulses of mid-Tertiary and mid-Miocene volcanism were related to transient episodes of mantle upwelling generated successively by rise-trench encounters associated with subduction of the Vancouver-Farallon and Monterey-Arguello plates, respectively, along the continental margin off southern California. Migratory post–mid-Miocene volcanism in central California accompanied the incremental expansion of a growing slab window as the Mendocino triple junction migrated northward along the continental margin. Mid-Miocene Columbia River basalt volcanism in the Pacific Northwest was coeval with the mid-Miocene pulse of coastal volcanism and may have reflected tectonism induced by the final demise of offshore microplates to allow initial full integration of the San Andreas transform as a coherent plate boundary. Columbia River volcanism may have stemmed from mantle perturbation caused by torsional deformation of the continental block in response to shear imposed by the Pacific plate.

Journal ArticleDOI
TL;DR: In this paper, the authors measured cosmogenic 36 Cl in 56 samples from boulders on moraines and fluvial terraces in the vicinity of the Wind River Range, Wyoming.
Abstract: We measured cosmogenic 36 Cl in 56 samples from boulders on moraines and fluvial terraces in the vicinity of the Wind River Range, Wyoming. We also measured 10 Be in 10 of the same samples. Most of the 10 Be ages were in good agreement with the 36 Cl ages, indicating that rock-surface erosion rates were very low. The oldest moraine investigated, the type Sacagewea Ridge site, yielded only a limiting minimum age of >232 ka. The oldest moraines in the type Bull Lake complex also could be constrained only to >130 ka. The main sequence of type Bull Lake moraines yielded age distributions indicating deposition within the intervals 130 to 100 ka and 120 to 100 ka; the best estimates are closer to the upper limits of these ranges, and associated uncertainties are in the range of 10% to 15%. These uncertainties could permit deposition in either marine isotope stage 6 or stage 5d. We found no evidence of glacial deposits dating to marine isotope stage 4. Both Bull Lake–age moraines from Fremont Lake, on the opposite side of the Wind River Range, and boulders on a fluvial terrace above the Wind River, gave age distributions very similar to that of the second oldest Bull Lake advance (ca. 130 to 100 ka). The distribution of boulder ages for Pinedale moraines at Bull Lake indicated deposition between 23 and 16 ka, nearly identical to the distribution of 10 Be ages previously reported for the type Pinedale moraines at Fremont Lake.

Journal ArticleDOI
TL;DR: In this article, the authors show that stromatolites and calcite cements are precipitating in Lake Tanganyika, Africa and provide a wealth of paleolimnologic and paleoclimatic information for the late Holocene.
Abstract: Fossil and living stromatolites are abundant around the margins of Lake Tanganyika, Africa, and provide a wealth of paleolimnologic and paleoclimatic information for the late Holocene. Six lines of evidence show that stromatolites and cements are precipitating in the lake today: (1) carbonate saturation state calculations, (2) documentation of living stromatolites and their depth distribution, (3) new stable isotope data showing the lake’s present mixing state and ancient evaporation and inflow balance, (4) new radiocarbon data and a reevaluation of apparent 14 C ages derived from Lake Tanganyika carbonates, (5) the presence of modern Mg-calcite cements derived from lake waters, and (6) the presence of modern, biologically mediated Mg-calcite precipitates in the lake. Lake Tanganyika’s lake levels have been remarkably stable over the past 2800 yr, fluctuating around the marginally open to marginally closed level through most of this time period. Lake lowstands and high δ 18 O values from the ninth century B.C. to the early fifth century A.D. indicate that the lake basin was comparatively dry during this time. However, the period prior to the most recent opening of Lake Kivu into the Lake Tanganyika basin (ca. A.D. 550) was not marked by major lake lowstands, nor was this opening accompanied by a dramatic lakelevel rise. The Kivu opening was roughly coincident with a significant shift toward isotopically lighter (δ 18 O and δ 13 C) lake water, which persists today. The lake remained close to its outlet level between the sixth and thirteenth centuries A.D. Lake levels rose between the fourteenth and sixteenth centuries. At some time between the late sixteenth and early nineteenth centuries, lake level fell to perhaps its lowest level in the past 2800 yr. By the early nineteenth century, lake level had begun to rise to the overflow level, apparently the result of a regional increase in precipitation/evaporation ratios. Weak δ 18 O/δ 13 C covariance for late Holocene carbonates suggests that the surface elevation of the lake has remained near the outlet level, with only occasional periods of closure. However, there is no simple relationship between solute input from Lake Kivu, isotope input from Lake Kivu, and lake levels in Lake Tanganyika. Lake Kivu waters are the primary source of major ions in Lake Tanganyika, but are much less important in controlling the δ 18 O and the lake level of Lake Tanganyika. Because the Ruzizi River’s discharge into Lake Tanganyika is largely derived from sources other than Lake Kivu, the overflow events in the two lakes have been uncoupled during the late Holocene.

Journal ArticleDOI
TL;DR: In this article, a three-dimensional finite element model is used to calculate mantle flow beneath North America during Phanerozoic time in response to episodes of subduction at cratonic margins and two cycles of supercontinent formation and breakup.
Abstract: Models integrating geodynamic and stratigraphic processes show that some gross features of Phanerozoic North American cratonic strata can be explained with dynamic topographies generated by subduction and cycles of supercontinent aggregation and dispersal. A three-dimensional finite-element model is used to calculate mantle flow beneath North America during Phanerozoic time in response to episodes of subduction at cratonic margins and two cycles of supercontinent formation and breakup. Dynamic topographies calculated by the flow models are used as input to a stratigraphic model that also includes background subsidence, eustasy, denudation, clastic and carbonate deposition, compaction, and isostasy. These models successfully reproduce aspects of the Sloss sequences; the best matches were obtained by combining two wavelengths of dynamic topography with second-order eustasy. Long-wavelength dynamic topography generates first-order stratal cyclicity. Periods of erosion were shorter when North America was over a dynamic topography low than when it was over a high. Long-wavelength dynamic topography also explains the absence of Mesozoic strata on the eastern portion of the craton. Characteristic stratal patterns are shown to result from subduction-related dynamic topography, although sensitive to sediment supply and other subsidence mechanisms. Aspects of Upper Cretaceous stratal patterns may be explained by the effects of Farallon plate subduction. Generally, strata deposited in a dynamic topography depression have low preservation potential because the topography is reversible. Thus, ancient subduction-related dynamic topography is most likely to be represented by unconformities.

Journal ArticleDOI
TL;DR: In this paper, the SHRIMP (II) ion microprobe was used to analyze six samples of granitic rock from the Idaho-Bitterroot batholith.
Abstract: Granitic plutonism and extension are broadly contemporaneous in many metamorphic core complexes. However, the relationship between magmatism and extension is rarely unambiguous. The northern Idaho batholith (Idaho-Bitterroot batholith), Montana and Idaho, composes the footwall for most of the Bitterroot metamorphic core complex and thus is an ideal area for assessing the relationships between magmatism and extension. We analyzed zircon from six samples of granitic rock from the Idaho-Bitterroot batholith using the SHRIMP (II) ion microprobe. Three samples of mylonitic granite from the Bear Creek pluton, Lost Horse Canyon, give a weighted mean 206 Pb/ 238 U age of 54.3 ± 0.7 Ma. A protomylonitic granite from the central part of the Bitterroot core complex (also Bear Creek pluton) gives a similar 206 Pb/ 238 U age of 54.6 ± 0.8 Ma. Mylonitic megacrystic granite from Sweathouse Canyon yields an age of 63.6 ± 0.6 Ma. A granite sample from the Lochsa Canyon, in the central Idaho-Bitterroot batholith, gives an age of 56.7 ± 1.0 Ma. Inherited zircon from the granitoids ranges in age from 800 to 1820 Ma, but the majority of grains have formation ages of 1750–1800 Ma. This suggests that Paleoproterozoic crust dominates the source region of the Idaho-Bitterroot batholith. Hornblende 40 Ar- 39 Ar age spectra for mafic dikes intruded during late-stage crystallization of main-phase granite in the central Idaho-Bitterroot batholith suggest crystallization of the main-phase plutons in this area at ca. 57 Ma. New and previously published 40 Ar- 39 Ar and K-Ar apparent ages of biotite and muscovite from the Lochsa River area and the western and central Bitterroot core complex are 50 to 47 Ma. Younger mica ages (46–43 Ma) are restricted to the vicinity of the Bitterroot mylonite zone. These results indicate that the cessation of main-phase magmatism within the Bitterroot metamorphic core complex migrated east with time, and that most of the plutons in the core complex were intruded during the Paleocene and early Eocene. When the regional tectonic setting changed from compression to extension at ca. 50 Ma, the late stages of mid-crustal, peraluminous plutonism appear to have been localized within the Bitterroot core complex. The presence of the youngest mid-crustal plutons in this area may have focused extensional deformation leading to the thick mylonite zone, as a consequence of rheological contrasts with cooler areas to the east and west. A progression of K-Ar and 40 Ar- 39 Ar cooling ages from west to east within the core complex part of the batholith is consistent with top-to-the-east shear indicators in the mylonite zone. Thermochronology indicates that the western part of the Bitterroot metamorphic core complex was below ≈350°C at the same time as the last stage of granite emplacement and metamorphism in the east. Therefore, the transition from mylonitization to brittle deformation to inactivity of the shear zone was progressive from west to east across the core complex from ca. 50 to 44 Ma. These features offer an explanation for the previously enigmatic occurrence of amphibolite facies ductile deformation in the eastern part of the core complex coincident with emplacement of epizonal, alkali-feldspar granite plutons along the western side of the complex.

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TL;DR: In this article, a detailed history of hydrocarbon seepage using radiocarbon and U-series isotopes was investigated using radiometric and ionium-based dates. But the results showed that the ionium dates were highly discordant, due to dilution of the 14 C pool with fossil-hydrocarbon-derived carbon.
Abstract: Massive abiotic carbonates and calcareous shells of the chemosynthetic mytilid Bathymodiolus sp. containing a detailed history of hydrocarbon seepage were investigated using radiocarbon and U-series isotopes. Stable carbon isotopes and 87 Sr/ 86 Sr ratios were also determined in order to provide insights on the carbon source and the nature of the hydrocarbon-rich fluids. Samples from five seepage sites on the northern Gulf of Mexico sea floor overlying subsurface salt diapirs and encompassing depths from 125 to >2000 m were selected as representative of the spectrum of active and extinct seeps examined from submersible dives. In general, paired 14 C and 230 Th dates of carbonate buildups consisting of aragonite, high-Mg calcite, and dolomite mineralogies are highly discordant. The cause of the discordance, established on the basis of paired Δ 14 C and δ 13 C values (Δ 14 C = −898‰ to −992‰ δ 13 C = −9.5‰ to −53.3‰, for n = 27), lies with the impairment of the radiocarbon dates resulting from dilution of the 14 C pool with fossil-hydrocarbon–derived carbon. The validity of the ionium dates based on U-rich (2.4–7.6 ppm) samples is demonstrated by the concordance between 234 U/ 238 U and 230 Th/ 234 U evolution in time, and by the ( 234 U/ 238 U) o activity ratios that are generally within the range of sea-water value of 1.14 ± 0.04 (2 sigma). Some dolomite-rich samples are exceptional because their ( 234 U/ 238 U) o ratios are significantly higher (1.22–1.38) than sea water, suggesting deposition from anoxic pore waters where the soluble U reached anomalous 234 U/ 238 U ratios. The formation of the carbonates from sea-water–derived fluids, rather than from formation fluids advecting from deep aquifers, is supported by the 87 Sr/ 86 Sr composition of the samples (mean 0.709145 ± 19 × 10 −6 , n = 14) that compares well with modern nonseep marine carbonates and the ambient Gulf of Mexico sea water (0.709171 ± 8 × 10 −6 ). Calcareous shells, the δ 13 C values of which indicate a carbon source in sea water (δ 13 C = −4.3‰ to −1.1‰), yield valid radiocarbon ages and show fair concordancy between their radiocarbon and ionium dates. Isotope migration attested by the observed U uptake in the fossil shells is likely to affect the accuracy of their ionium dates. Radiometric ages from extinct and senescent seep sites at upper bathyal depths indicate that hydrocarbon seepage occurred there during late Pleistocene time (195–13 ka). Ages derived from nascent seep sites at mid-bathyal and abyssal depths (12.3–0.0 ka) indicate that currently vigorous seepage was initiated at the end of the last deglaciation. These radiometric ages most likely reflect the time of sedimentary loading and associated salt diapirism that activated the fault conduits to the sea floor.

Journal ArticleDOI
TL;DR: The authors used an integrated modeling philosophy, first modeling the seismic-refraction data to obtain a final velocity model, and then modeling the long-wavelength features of the gravity data to get a final density model that is constrained in the upper crust by the velocity model.
Abstract: The nature of the Great Valley basement, whether oceanic or continental, has long been a source of controversy. A velocity model (derived from a 200-km-long east-west reflection-refraction profile collected south of the Mendocino triple junction, northern California, in 1993), further constrained by density and magnetic models, reveals an ophiolite underlying the Great Valley (Great Valley ophiolite), which in turn is underlain by a westward extension of lower-density continental crust (Sierran affinity material). We used an integrated modeling philosophy, first modeling the seismic-refraction data to obtain a final velocity model, and then modeling the long-wavelength features of the gravity data to obtain a final density model that is constrained in the upper crust by our velocity model. The crustal section of Great Valley ophiolite is 7–8 km thick, and the Great Valley ophiolite relict oceanic Moho is at 11–16 km depth. The Great Valley ophiolite does not extend west beneath the Coast Ranges, but only as far as the western margin of the Great Valley, where the 5–7-km-thick Great Valley ophiolite mantle section dips west into the present-day mantle. There are 16–18 km of lower-density Sierran affinity material beneath the Great Valley ophiolite mantle section, such that a second, deeper, “present-day” continental Moho is at about 34 km depth. At mid-crustal depths, the boundary between the eastern extent of the Great Valley ophiolite and the western extent of Sierran affinity material is a near-vertical velocity and density discontinuity about 80 km east of the western margin of the Great Valley. Our model has important implications for crustal growth at the North American continental margin. We suggest that a thick ophiolite sequence was obducted onto continental material, probably during the Jurassic Nevadan orogeny, so that the Great Valley basement is oceanic crust above oceanic mantle vertically stacked above continental crust and continental mantle.

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TL;DR: In this article, the long-term lateral-slip rate for the Fish Lake Valley fault zone since about 10 Ma is 5 mm/yr (3-12mm/yr).
Abstract: Several well-dated stratigraphic markers permit detailed assessment of the temporal and spatial variation in slip rates along the interconnected faults of the Fish Lake Valley, Emigrant Peak, and Deep Springs fault zones in west-central Nevada and east-central California. Right-lateral motion on the Fish Lake Valley fault zone apparently began ca. 10 Ma (11.9–8.2 Ma). Associated extensional faulting probably began ca. 5 Ma (6.9–4 Ma) and resulted in the opening of Fish Lake Valley and Deep Springs Valley. The long-term lateral-slip rate for the Fish Lake Valley fault zone since about 10 Ma is 5 mm/yr (3–12 mm/yr). Our preferred lateral-slip rate for the central, most active part of the Fish Lake Valley fault zone decreased from about 6 to 3 mm/yr from the late Miocene to the early Pleistocene, increased to about 11 mm/yr during the middle Pleistocene, and decreased to about 4 mm/yr during the late Pleistocene. Extension may account for some of the change in lateral-slip rate during the Pliocene. The large increase in lateral-slip rate during the middle Pleistocene is circumstantially linked to an increase in vertical-slip rates on the Fish Lake Valley and Deep Springs fault zones at about the time of the eruption of the Bishop ash (0.76 Ma). Vertical-slip rates along the three fault zones are also related to fault strike; vertical rates are highest on north-striking faults and approach zero on northwest-striking faults. The long-lived slip history of the Fish Lake Valley fault zone fits a tectonic model in which the Death Valley–Furnace Creek–Fish Lake Valley fault system is integrated with right-lateral shear on faults of the central Walker Lane and the Eastern California shear zone to accommodate part of the Pacific–North American relative plate motion. Our research demonstrates that the Fish Lake Valley fault zone accounts for about half the rate of 10–12 mm/yr of Pacific-North American plate-boundary shear accommodated within the Basin and Range province between about lat 37° and 38°N.

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TL;DR: Pollen and plant macrofossils from eight sedimentary basins on the west slope of the Colorado Rocky Mountains document fluctuations in upper and lower timberline since the latest Pleistocene as mentioned in this paper.
Abstract: Pollen and plant macrofossils from eight sedimentary basins on the west slope of the Colorado Rocky Mountains document fluctuations in upper and lower timberline since the latest Pleistocene. By tracking climatically sensitive forest boundaries, the moisture-controlled lower timberline and the temperature-controlled upper timberline, paleoclimatic estimates can be derived from modern temperature and precipitation lapse rates. Pollen data suggest that prior to 11 000 yr B.P., a subalpine forest dominated by Picea (spruce) and Pinus (pine) grew 300–700 m below its modern limit. The inferred climate was 2–5 °C cooler and had 7–16 cm greater precipitation than today. Abies (fir) increased in abundance in the subalpine forest around 11 000 yr B.P., probably in response to cooler conditions with increased winter snow. Pollen and plant macrofossil data demonstrate that from 9000 to 4000 yr B.P. the subalpine forest occupied a greater elevational range than it does today. Upper timberline was 270 m above its modern limit, suggesting that mean annual and mean July temperatures were 1–2 °C warmer than today. Intensification of the summer monsoon, coupled with increased summer radiation between 9000 and 6000 yr B.P., raised mean annual precipitation by 8–11 cm and allowed the lower limit of the subalpine and montane forests to descend to lower elevations. The lower forest border began to retreat upslope between 6000 and 4000 yr B.P. in response to drier conditions, and the upper timberline descended after 4000 yr B.P., when temperatures cooled to about 1 °C warmer than today. The modern climatic regime was established about 2000 yr B.P., when the summer precipitation maxima of the early and middle Holocene were balanced by increased winter precipitation.

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TL;DR: In this article, the internal architecture and timing of development of five strand plains of late Holocene beach ridges along the Lake Michigan coastline were studied, and relative lake-level curves for each site were constructed by determining the elevation of foreshore (swash zone) sediments in the cliff faces and by dating basal wetland sediments between ridges.
Abstract: Lake level is a primary control on shoreline behavior in Lake Michigan. The historical record from lake-level gauges is the most accurate source of information on past lake levels, but the short duration of the record does not permit the recognition of long-term patterns of lake-level change (longer than a decade or two). To extend the record of lake-level change, the internal architecture and timing of development of five strand plains of late Holocene beach ridges along the Lake Michigan coastline were studied. Relative lake-level curves for each site were constructed by determining the elevation of foreshore (swash zone) sediments in the beach ridges and by dating basal wetland sediments in the swales between ridges. These curves detect long-term (30+ yr) lake-level variations and differential isostatic adjustments over the past 4700 yr at a greater resolution than achieved by other studies. The average timing of beach-ridge development for all sites is between 29 and 38 yr/ridge. This correspondence occurs in spite of the embayments containing the strand plains being different in size, orientation, hydrographic regime, and available sediment type and caliber. If not coincidental, all sites responded to a lake-level fluctuation of a little more than three decades in duration and a range of 0.5 to 0.6 m. Most pronounced in the relative lake-level curves is a fluctuation of 120–180 yr in duration. This ≈150 yr variation is defined by groups of four to six ridges that show a rise and fall in foreshore elevations of 0.5 to 1.5 m within the group. The 150 yr variation can be correlated between sites in the Lake Michigan basin. The ≈30 and 150 yr fluctuations are superimposed on a long-term loss of water to the Lake Michigan basin and differential rates of isostatic adjustment.

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TL;DR: In this article, a well-cut section of Miocene-Oligocene mudrocks from Kenedy County, Texas, spanning a depth range of 2130 to 5490 m (7000 to 18 000 ft), have been studied petrographically and geochemically.
Abstract: Cuttings from a well through a thick section of Miocene–Oligocene mudrocks from Kenedy County, Texas, spanning a depth range of 2130 to 5490 m (7000 to 18 000 ft), have been studied petrographically and geochemically. On the basis of whole-rock chemical analyses, the deepest samples have lost ≈18 wt% (and approximately vol%), mostly as CaCO3, mineral-bound H2O, and SiO2, but including additional Ca, as well as Sr, light rare earth elements (REE) (La, Ce, Nd, Sm), Fe, and Li. K2O and Rb have been added to the deeper rocks. The large chemical changes are accompanied mineralogically by loss of detrital calcite, kaolinite, K-feldspar, Ca-plagioclase, and muscovite, gain of chlorite and albite, and continued reaction of smectitic illite/smectite (I/S) to more illitic (and K-rich) compositions throughout the entire depth interval of the well. The large chemical changes in this thick mud-rich interval almost certainly require advection of water (free convection?) to accomplish the mass transfer. Initial variation in sediment composition is ruled out as a cause of the observed compositional changes with increasing depth because (1) a variety of “immobile” elements (Al2O3, TiO2, Zr, Hf, heavy REE [Er, Yb], Th, and Sc) remain constant relative to each other despite their uneven distribution across various particle size fractions in the sediments; (2) deep Frio shales are unlike Quaternary Gulf of Mexico sediments or average shales; and (3) unreasonable primary mineralogic compositions would be necessary to explain the chemical composition of the deep samples. These results indicate that burial diagenesis of argillaceous sediment can be a considerably more open chemical process than is conventionally assumed, that it can account for the two major chemical cements (calcite and quartz) in associated sandstones, and that it mirrors secular changes in shales throughout geologic time.

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TL;DR: In this paper, structural analysis and geochronologic data indicate a nearly orthogonal, late Eocene-Oligocene flow pattern in migmatitic infrastructure immediately beneath the kilometer-thick, extensional, mylonitic shear zone of the Ruby Mountains metamorphic core complex, Nevada.
Abstract: Structural analysis and geochronologic data indicate a nearly orthogonal, late Eocene–Oligocene flow pattern in migmatitic infrastructure immediately beneath the kilometer-thick, extensional, mylonitic shear zone of the Ruby Mountains metamorphic core complex, Nevada. New U-Pb radiometric dating indicates that the development of a northward-trending lineation in the infrastructure is partly coeval with the development of a pervasive, west-northwest–trending lineation in the mylonitic shear zone. U-Pb monazite data from the leucogranite orthogneiss of Thorpe Creek indicate a crystallization age of ca. 36–39 Ma. Zircon fractions from a biotite monzogranite dike yield an age of ca. 29 Ma. The three dated samples from these units exhibit a penetrative, approximately north-south–trending elongation lineation. This lineation is commonly defined by oriented bundles of sillimanite and/or elongated aggregates of quartz and feldspar, indicating a synmetamorphic and syndeformational origin. The elongation lineation can be interpreted as a slip line in the flow plane of the migmatitic, nonmylonitic infrastructural core of the northern Ruby Mountains. A portion of this midcrustal flow is coeval with the well-documented, west-northwest sense of slip in the structurally overlying kilometer-thick, mid-Tertiary mylonitic shear zone. Lineations in the mylonitic zone are orthogonal to those in the deeper infrastructure, suggesting fundamental plastic decoupling between structural levels in this core complex. Furthermore, the infrastructure is characterized by overlapping, oppositely verging fold nappes, which are rooted to the east and west. One of the nappes may be synkinematic with the intrusion of the late Eocene orthogneiss of Thorpe Creek. In addition, the penetrative, elongation lineation in the infrastructure is subparallel to hinge lines of parasitic folds developed synchronous with the fold nappes, suggesting a kinematically related evolution. The area is evaluated in terms of a whole-crust extension model. Magmatic underplating in the lower crust stimulated the production of late Eocene–early Oligocene granitic magmas, which invaded metasedimentary and Mesozoic granitic rocks of the middle crust. The midcrustal rocks, weakened by the magmatic heat influx, acted as a low-viscosity compensating material, decoupled from an extending upper crust. The fold nappes and lineation trends suggest large-scale flow of the weakened crust into the study area. The inflow pattern in the migmatitic infrastructure can be interpreted as a manifestation of midcrustal migration into an area beneath a domain of highly extended upper crustal rocks. At present the inferred Eocene–early Oligocene phase of upper-crust extension remains unknown, but available data on relative and geochronologic timing are not inconsistent with our model of return flow into an area already undergoing large-scale upper-crustal extension.

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TL;DR: The Tatara-San Pedro complex has been used to define the chronostratigraphy of the Andean southern volcanic zone as mentioned in this paper, where the authors used geologic mapping, together with 73 new K-Ar and 40Ar/39Ar age determinations of 45 samples from 17 different volcanic units, plus paleomagnetic orientations, geochemical compositions, and terrestrial photogrammetry.
Abstract: Geologic mapping, together with 73 new K-Ar and 40Ar/39Ar age determinations of 45 samples from 17 different volcanic units, plus paleomagnetic orientations, geochemical compositions, and terrestrial photogrammetry are used to define the chronostratigraphy of the Tatara–San Pedro complex, an eruptive center at 36°S on the volcanic front of the Andean southern volcanic zone. The Tatara–San Pedro complex preserves ≈55 km3 of lavas that erupted from at least three central vent regions. Remnant, unconformity-bound sequences of lavas are separated by lacunae that include significant periods of erosion. Quaternary volcanism commenced ca. 930 ka with eruption of voluminous dacitic magma, followed 100 k.y. later by the only major rhyolitic eruption. From 780 ka onward, more than 80% of the preserved volume is basaltic andesite (52%–57% SiO2), but petrographically and geochemically diverse dacitic magmas (63%–69% SiO2) erupted sporadically throughout this younger, dominantly mafic phase of activity. A few basaltic lavas (49%–52% SiO2) are present, mainly in portions of the complex older than 230 ka. The number of vents, the petrologic and geochemical diversity, and the temporal distribution of mafic and silicic lavas are consistent with emplacement of many separate batches of mafic magma into the shallow crust beneath the Tatara–San Pedro complex over the past million years. Nearly two-thirds of the preserved volume of the Tatara–San Pedro complex comprises the two youngest volcanoes, which were active between ca. 188–83 ka and 90–19 ka. Repeated advances of mountain glaciers punctuated growth of the complex with major erosional episodes that removed much of the pre-200 ka volcanic record, particularly on the south flank of the complex. Dating the inception of a glaciation on the basis of preserved material is difficult, but the age of the oldest lava above a lacuna may be used to estimate the timing of deglaciation. On this basis, the argon ages of basal lavas of multiple sequences indicate minimum upper limits of lacunae at ca. 830, 790, 610, 400, 330, 230, 110, and 17 ka. These are broadly consistent with global ice-volume peaks predicted by the oxygen isotope-based astronomical time scale and with age brackets on North American glacial advances. Estimated growth rates for the two young volcanoes are 0.2 to 0.3 km3/k.y.; these are three to five times greater than a growth rate estimated from all preserved lavas in the complex (0.06 km3/k.y.). Removal of up to 50%–95% of the material erupted between 930 and 200 ka by repeated glacial advances largely explains this discrepancy, and it raises the possibility that episodic erosion of mid-latitude frontal arc complexes may be extensive and common.

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TL;DR: In this article, the upper Miocene to Pliocene Bouse Formation in the lower Colorado River trough, which consists largely of siltstone with basal tufa and marl, has been interpreted as estuarine on the basis of paleontology.
Abstract: The upper Miocene to Pliocene Bouse Formation in the lower Colorado River trough, which consists largely of siltstone with basal tufa and marl, has been interpreted as estuarine on the basis of paleontology. This interpretation requires abrupt marine inundation that has been linked to early rifting in the Gulf of California and Salton trough. New strontium isotope measurements reported here from carbonates and invertebrate shells in the Bouse Formation reveal no evidence of marine water, but are consistent with deposition in a lake or chain of lakes fed by the Colorado River. Furthermore, the absence of a southward decrease in 87 Sr/ 86 Sr within the Bouse Formation does not support the estuarine model in which low 87 Sr/ 86 Sr marine Sr would have dominated the mouth of the hypothetical Bouse estuary. Elevation of originally marine 87 Sr/ 86 Sr in the Bouse Formation to its present level, due to postdepositional interaction with ground water, is unlikely because Sr from secondary calcite above, below, and within the Bouse Formation is consistently less radiogenic, not more, than Bouse marl and shells. In contrast to Bouse Sr, strontium from mollusks in tidal-flat and delta-front paleoenvironments in the contemporaneous Imperial Formation in the Salton trough and from the subsurface south of Yuma was derived from sea water and confirms the dominance of marine strontium near or at the mouth of the late Miocene to early Pliocene Colorado River. Inferred post–early Pliocene uplift of the Bouse Formation from below sea level to modern elevations of up to 550 m has been used to support a late Cenozoic uplift age for the nearby Colorado Plateau. This constraint on uplift timing is eliminated if the Bouse Formation is lacustrine.

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TL;DR: In this article, structural, metamorphic, and U-Pb thermochronologic data from the Wood Hills and Pequop Mountains, coupled with a regional tectonic reconstruction, reveals substantial Cretaceous metamorphism, contraction, and extension in the Sevier hinterland in northeast Nevada.
Abstract: The Pequop Mountains–Wood Hills–East Humboldt Range region, northeast Nevada, exposes a nearly continuous cross section of Precambrian to Mesozoic strata representing middle to upper crustal levels of the Mesozoic hinterland of the Sevier orogen. These rocks preserve the transition from unmetamorphosed Mesozoic upper crust to partially melted middle crust. Integration of new structural, metamorphic, and U-Pb thermochronologic data from the Wood Hills and Pequop Mountains, coupled with a regional tectonic reconstruction, reveals substantial Cretaceous metamorphism, contraction, and extension in the Sevier hinterland in northeast Nevada. We report two phases of contraction not previously recognized that are accommodated by top-to-the-southeast thrust faults, the Windermere and Independence thrusts. Contraction was succeeded by two phases of extension along west-rooted normal faults, the Late Cretaceous Pequop fault and Tertiary Mary9s River fault system. The earliest phase of thrust faulting resulted in as much as 30 km of crustal thickening and an estimated minimum of 69 km of shortening along an inferred fault called the Windermere thrust. The timing of this thrusting event is bracketed between Late Jurassic (ca. 153 Ma) and Late Cretaceous (84 Ma). Relaxation of crustal isotherms following and perhaps during thrusting resulted in Barrovian-style metamorphism of footwall rocks, and partial melting of metapelite at deep levels. Peak metamorphism was attained ca. 84 Ma, and by this time hinterland crustal thickening had reached a maximum. During 84–75 Ma another minor pulse of shortening and thickening along the Independence thrust was followed by partial exhumation of the metamorphic rocks and as much as 10 km of crustal thinning along the Pequop fault. Thus the interval from 84 to 75 Ma in northeast Nevada marks a fundamental, and apparently permanent, change from horizontal contraction to extension in the upper to middle crust in the hinterland. Final exhumation of the metamorphic rocks was accomplished by the Tertiary Mary9s River fault system. Our data indicate that much of the metamorphism and some of the contraction in the Sevier hinterland in northeast Nevada, which was previously thought to be largely Late Jurassic, is actually Cretaceous in age. Furthermore, the data indicate that widespread metamorphism of the middle crust is a byproduct of tectonic burial, and that hinterland and foreland thrust faulting were coeval, suggesting that thrust faults in the Sevier orogen do not form a simple foreland younging sequence.

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TL;DR: In this article, the authors used morphostratigraphic mapping, river-terrace longitudinal profile analysis, and tephrochronology to place the age of Sacagawea Ridge moraines in the Dinwoody Lakes type area at about 660 ka.
Abstract: The distribution and ages of moraines in the Wyoming-type localities for pre-Pinedale Rocky Mountain glacial deposits are not well known. Through morphostratigraphic mapping, river-terrace longitudinal-profile analysis, and tephrochronology, we place the age of Sacagawea Ridge moraines in the Dinwoody Lakes type area at about 660 ka. Outwash and river terrace deposits in the Bull Lake type area, previously considered to be Sacagawea Ridge, are actually older by one or two glacial cycles. In the Bull Lake type locality, 15 individual moraines are divided into 4 groups on the basis of relative age criteria. Using 36Cl/10Be cosmogenic dating of boulders, the two older groups must be >130 ka; the younger groups are given age ranges of 130 to 100 ka and 120 to 95 ka. A terrace along Wind River can be morphostratigraphically traced to the youngest of these moraine groups; 36Cl dating of boulders on the terrace provides an age range of 125 to 100 ka. The combined relative age and cosmogenic nuclide chronology is consistent with previously published evidence from Yellowstone National Park demonstrating that mountain glaciers advanced not only prior to and during the later portion of marine isotope stage 6 but also during marine isotope stage 5d. In contrast with some other areas of the Rocky Mountains, we found no evidence for advances during marine isotope stage 4. At Bull Lake, Pinedale moraines that have a 36Cl/10Be age range of 23 to 16 ka suggest that glacial advances were limited to the later part of marine isotope stage 2. A river terrace that can be traced to these moraines has an age range of 23 to 16 ka. A possible glacial advance during early marine isotope stage 2 is indicated by a river terrace whose age range is estimated by incision-rate modeling at 46 to 28 ka, but no moraines were identified. Along Wind River, there are eight terrace levels capped with outwash that are older than Sacagawea Ridge; although the terraces are the record of early to middle Pleistocene climatic cycles, there are no correlative moraines preserved at the land surface.

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TL;DR: The Ordovician seismites as mentioned in this paper provide information about epicenter location and the recurrence interval of large earthquakes in the foreland basin of the U.S. Appalachians.
Abstract: Synsedimentary ball-and-pillow beds, breccias, and faults in late Middle to Late Ordovician foreland basin rocks of Kentucky, southwest Ohio, and Virginia indicate broad zones of seismicity near the Cincinnati arch and Taconic orogenic front during deposition. Earthquake-induced liquefaction formed seismites, that include ball-and-pillow beds and rare sedimentary breccias that are correlative over large areas (hundreds to thousands of square kilometers). Comparison of these features with liquefaction structures in Holocene sediments indicates that the Ordovician ball-and-pillow beds were probably generated by large earthquakes (magnitudes >6). The Ordovician seismites also provide information about epicenter location and the recurrence interval of large earthquakes in the Ordovician foreland basin. Some were produced by faulting in the foreland and accretionary prism. However, horizons of resedimented lithoclastic breccias in the Jeptha knob cryptoexplosive structure appear to correlate with several ball-and-pillow beds on Jessamine dome, along the Cincinnati arch, suggesting that some of the seismites may be genetically related to this enigmatic structure.

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TL;DR: The Ferrar Dolerite constitutes the hypabyssal phase of the tholeiitic Ferrar Group of Antarctica and has been analyzed by the 40 Ar/39 Ar method on feldspar and biotite separates as discussed by the authors.
Abstract: The Ferrar Dolerite constitutes the hypabyssal phase of the tholeiitic Ferrar Group of Antarctica. Sills with compositions representing most of the range of geochemical variation of the Ferrar Dolerite, and separated by distances of as much as 1400 km, have been analyzed by the 40 Ar/ 39 Ar method on feldspar and biotite separates. The 40 Ar/ 39 Ar ages for five individual sills range from 176.2 to 177.2 Ma and show no significant difference. These ages reflect crystallization at 176.7 ± 1.8 Ma (where the uncertainty includes provision for systematic uncertainty in the age of the neutron-fluence monitor calibrated relative to MMhb-1 at 513.5 Ma). Combining data from these sills with previous determinations on coeval lavas and underlying pyroclastic units indicates an age of 176.6 ± 1.8 Ma for the Ferrar tholeiitic rocks as a whole. The duration of magmatic activity was less than approximately 1 m.y. By extension, other rocks in the Ferrar magmatic province, which occur from southeastern Australia, along the Transantarctic Mountains to the Theron Mountains, are inferred to have this age. The short duration of magmatic activity as well as the consistent pattern of geochemical variation and distinctiveness of the Ferrar rocks suggest that magmas were transported laterally by an extensive dike swarm which is inferred to have originated in the Weddell Sea sector of the province.

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TL;DR: In this paper, the authors focus on the textural nature of phenocrysts of feldspar and quartz in tuff and provide a basis for characterization of the shapes and for interpretation of the origin of felsic phenocryst in ash-flow tuffs.
Abstract: Surprisingly little attention has been devoted to the textural nature of phenocrysts of feldspar and quartz in tuff. Although many geologists have briefly alluded to “broken” phenocrysts, none have addressed their origin in any detail. Petrographic study of 117 cooling units in the middle Tertiary ash-flow province of the Great Basin, United States, provides a basis for characterization of the shapes and for interpretation of the origin of felsic phenocrysts in ash-flow tuffs. Although not proven to be wholly ineffective, breakage of phenocrysts by mutual impact in the erupting magma and pyroclastic flow is doubtful for at least four reasons. First, the statistical probability of mutual collision between phenocrysts diminishes exponentially as their proportion to vitroclasts diminishes (e.g., only 1% probability for 10% phenocrysts); collision is less likely if pyroclasts move by laminar rather than turbulent flow. Second, the coating of glass and/or melt on the phenocrysts provides a cushion that absorbs the impact force. Third, plagioclases broken by impact in the laboratory have unusual shapes unlike those seen in Great Basin tuffs. Fourth, euhedral phenocrysts of feldspar are commonplace in many Great Basin tuffs, and in some they constitute a significant proportion of the phenocrysts, indicating that mutual impact does not modify all intratelluric crystals during explosive eruption. The two most populated categories of phenocryst shape in Great Basin tuffs probably correspond to what has been previously called “broken” phenocrysts. Somewhat less than half of the plagioclase and many sanidine phenocrysts are subhedral to anhedral. These are similar in shape, size, and composition to grains in polycrystalline aggregates within the same thin section. Kindred aggregates and discrete phenocrysts could have been derived from holocrystalline to partly crystalline material in the magma chamber that was disaggregated to varying extents during explosive eruption. More than half of the plagioclase and all of the quartz phenocrysts in Great Basin tuffs consist of irregularly shaped fragments with cuspate, embayed outlines, resembling pieces of a jigsaw puzzle, which we call phenoclasts. Inclusions of glass are common and are especially evident in larger, more or less whole crystals. Textural features of some phenocrysts in cognate pumice clasts in the tuffs reveal that they broke apart while still in the vesiculating but unfragmented magma. As the erupting magma decompressed, vesiculation of the melt that was entrapped at higher pressures as inclusions within the phenocrysts blew them apart, forming the phenoclasts. Shapes of felsic phenocrysts in volcanic rocks provide insight into their mode of emplacement. Euhedral phenocrysts are common in ash-flow tuffs as well as lava flows. Phenoclasts, however, are diagnostic of ash-flow tuffs, because they do not occur in Plinian ash-fall deposits and are rare in lava flows. These textural contrasts are useful for interpretation of generally older, but in any case altered and recrystallized, volcanic rocks. In such rocks, critical groundmass features and field relations that could provide clues to their origin have been obscured, but the shapes of relict phenocrysts are commonly well preserved.

Journal ArticleDOI
TL;DR: In this paper, the authors analyzed the Greenland Ice Sheet Project 2 (GISP2) ice core from 10 500 to 14 000 yr ago and found that the increase in mean number and mass diameter is a proxy for increased strength in zonal winds (westerlies).
Abstract: Oscillations in the time series of insoluble microparticle characteristics between 0.7 and 11.0 μm in the Greenland Ice Sheet Project 2 (GISP2) ice core reflect changes in environmental conditions in the Northern Hemisphere from 10 500 to 14 000 yr ago. Elevated values in microparticle number and mass, especially during the Younger Dryas, are related to Northern Hemisphere aridity and the subsequent increase in dust available for long-range transport to Greenland. This scenario occurs with the colder climatic conditions that result from a more expanded (spatially and temporally) polar vortex. Peaks in mean grain size based on number (mean number diameter) are a proxy for increased strength in zonal winds (westerlies). Highs in mean number diameter in the earlier part of the record often coincide with number and mass peaks reflecting the increased temperature and pressure gradients with an expanded polar vortex. Highs in mean grain size based on mass (mean mass diameter) reflect greater deposition of the coarser size fraction, and thus are a proxy for increased storminess associated with better developed synoptic-scale pressure systems in the northernmost Atlantic region. Peaks in mean mass diameter often lead these other parameters by 100–200 yr, suggesting an increase in storminess with the initial southward migration of the mean position of the polar front prior to full development of a more expanded polar vortex. The general decline in number and mass trends (decreased aridity with a contracting polar vortex) together with increasing mean number diameter trends (strengthening zonal winds) following the maxima in the early Younger Dryas suggest an expansion of mid-latitude circulation systems (subtropical highs), thereby maintaining latitudinal temperature and pressure gradients. Increased variability in mean mass diameter during the warm Preboreal, compared to the colder Younger Dryas, may be a function of the greater seasonality during warmer climatic periods, and thus more frequent storms associated with higher frequency oscillations in the position of the polar front with changing seasons and increased interannual variability in climate.