Showing papers in "AAPG Bulletin in 1969"

Deep-Water Evaporite Deposition: A Genetic Model

Abstract: Reasoning by analogy with modern salinas, many geologists believe that ancient marine evaporite deposits formed in shallow, slowly subsiding basins in regions of arid or semiarid climate. If this "salina" model is considered critically in the light of knowledge of several well-studied ancient deposits, however, inconsistencies arise which suggest the need for an alternative hypothesis. Sedimentation rates inferred from ancient salt deposits appear to be compatible with temperate rather than arid climates. More significantly, the thickness of evaporite salts in many localities requires either extremely rapid subsidence or salt deposition over a period substantially longer than that permitted by present stratigraphic control. In addition, the petrography, stratigraphy, and bromine content of the halite in such basins can be reconciled with the salina model only with great difficulty. The observed inconsistencies may be overcome by postulating that the salts were deposited in basins several hundreds to several thousands of meters deep. A "deep-basin" model of evaporite deposition is shown to be both geologically reasonable and oceanographically tenable, and to be consistent with the depositional histories of the Zechstein, Salina, Castile, and at least part of the Elk Point evaporites. Direct stratigraphic evidence of deep-water deposition is available in the Castile Sea. The Elk Point (Prairie or Muskeg) Evaporites of Alberta are examined in the light of the deep-basin model, and implications of the model for the exploration geologist in the region are developed.

Holocene Shallow-Water Carbonate and Evaporite Sediments of Khor al Bazam, Abu Dhabi, Southwest Persian Gulf

Abstract: Holocene shallow-water carbonate and evaporite sediments are forming in the Khor al Bazam, a saline lagoon on the coast of Abu Dhabi, southwest Persian Gulf. The sediments are composed of (1) skeletal grains, including whole and fragmented mollusks, corals, calcareous algae, bryozoans, foraminifers, and ostracods; (2) nonskeletal grains, including oolites, pellets, and pellet aggregates; (3) carbonate mud; (4) noncarbonate minerals including gypsum, anhydrite, and terrigenous quartz; and (5) organic material incorporated within mud and carbonate grains. Lithofacies are (1) coral and coralline-algae facies, (2) oolitic sand facies, (3) pellet-aggregate and pellet facies, (4) mud and pellet facies, (5) molluscan sand facies, (6) algal mat facies, and (7) evaporite facies. Nonskeletal calcium carbonate sediment types are related to wave energy; oolites form in the most turbulent environments, pellet aggregates in moderately sheltered environments, and pellets and muds in areas of low wave energy. Skeletal material is altered by blue-green algae. The grains contain algal cells and threads invested in mucilage. Respiration and photosynthesis by algae dissolve the original shell material and reprecipitate microcrystalline aragonite. Chemical changes within the investing organic material cause blackening of some of the carbonate grains.

Permian Cyclic Strata, Northern Midland and Delaware Basins, West Texas and Southeastern New Mexico

Abstract: Permian cyclic rocks of Wolfcampian-Guadalupian age in the northern Permian Basin, West Texas and southeast New Mexico, are grouped into five regionally extensive lithofacies: (1) shelf evaporite-carbonate, (2) shelf detritus, (3) shelf-margin carbonate, (4) basin carbonate, and (5) basin detritus Recognition of these lithofacies within an unconformity-bounded sequence suggests the following sedimentary model During normal sea-level conditions, Wolfcampian-Guadalupian shelf-margin reefs and banks formed near sea level The resultant backreef lagoon was shallow but very broad; therefore little terrigenous sand reached the distant basin Deposition of shelf-margin carbonate was at a maximum and the sediments accumulating in the basin were chiefly pelagic mud and micrite Relative lowering of sea level, possibly eustatic-epeirogenic, initiated regression, causing continental and nearshore sand and mud to prograde across the lagoon Continued progradation enabled shelf detritus to enter the basin through numerous reentrants and submarine canyons dissecting the shelf margin; additi nal regression subaerially exposed the shelf clastic beds, providing an unconformity-delimited datum surface Flooding of the shelf by transgression restricted the supply of detritus and reactivated normal carbonate deposition Correlation of a shelf-detritus top with a coeval basin-detritus top provides the framework for Wolfcampian-Guadalupian shelf-to-basin correlations

Salt Tectonics and Pleistocene Stratigraphy on Continental Slope of Northern Gulf of Mexico

Abstract: During a sparker and core-drill program conducted by Shell, salt was cored on 10 prominent structures on the continental slope. Broad salt swells and pillows are typical structures in this region. The Sigsbee scarp appears to be the surface expression of a salt front. A zone of active down-to-the-ocean faults follows the Texas shelf edge. They appear to be related to the flow of salt at depth away from the advancing clastic wedge. Upper Cretaceous through Holocene deep-water sedimentary beds were cored on the continental slope. East of Brownsville the salt is overlain by redbeds of unknown age. Core holes at the shelf edge penetrated deltaic and shoreline deposits of the Pleistocene low-sea-level stages. Submarine slides and turbidity currents carried sediments down the slope and filled deep synclinal basins between the salt uplifts.

Physiography and Sedimentary Processes of La Jolla Submarine Fan and Fan-Valley, California

Abstract: The depositional environments of La Jolla canyon, fan-valley, and fan are well known from closely spaced sounding lines, deep-diving vehicle observations, numerous undisturbed box cores, and continuous reflection profiles. The narrow rock-walled canyon changes seaward at 300 fm (549 m) to a wider valley cut into the compacted clayey sediments of a fan, and bordered by discontinuous leveelike embankments. The fan-valley merges gradually into the relatively flat floor of San Diego trough. Numerous dives into the fan-valley have shown precipitous walls along the outside of the bends of the winding channel. Slumping is taking place actively from these walls and large slump blocks of clay are common on the floor. Small scour depressions around isolated erratics suggest the ero ive effect of relatively weak currents in some places but, for the most part, the muddy floor seems to have been little disturbed in recent years. Diagonal tension cracks cut the floor locally. Box cores show that most of the sediment deposited on the valley floor in the past few thousand years consists of poorly sorted clayey silt, underlain by discontinuous layers of well-sorted fine-grained sand with a few coarse sand grains, gravel, and mud balls. Sand layers occur in 94 percent of the valley axis cores, of which 26 percent are graded; 59 percent have parallel laminations; and 41 percent have current-ripple cross-laminations. Sand layers are less common in the cores from levees and from the small discontinuous terraces along the sides of the fan-valley. Cores from the open fan have less and finer grained sand. In all these environments the sand shows no consistent or systematic grain-size variation with increasing water depths. Some of the coarsest sediments, including g avel and mud balls, are found in sand farthest from shore and at the greatest depths. The character of the sand and the finding of shallow-water Foraminifera indicate the probability that sand has been carried from the coastal area along the valley axes and spilled over the levees onto the open fan. However, there is little evidence of recent high-velocity, high-density turbidity currents, because, in general, the covering mud layer is distinctly separated from the underlying sand deposits, and therefore does not suggest deposition at the terminus of a turbidity current. Also, the discontinuous character of the sands and series of laminae with heavy mineral concentrations indicate introduction by a traction type of pulsating current, such as has been seen during vehicle dives, and also has been measured in the few available current-meter records. The locally precipitou fan-valley walls and outcrops of gravel, and the sand layers on the levees and open fan, may be the product of stronger currents that moved down the valley during earlier more pluvial periods, when greater quantities of sediment entered the canyon heads. Possible confirmation of this idea comes from the available C-14 dates in plant layers, which suggest that deposition in the past few thousand years may have been considerably slower than that indicated for the Pleistocene. The finer sediments may be largely the result of slow downslope movement of slightly higher density muddy waters coming from the coastal areas. Continuous reflection profiles have shown that the inner La Jolla fan has only a thin cover of unconsolidated sediments overlying the folded and faulted Miocene-Pliocene rocks. The outer fan and adjacent San Diego trough contain a thick section (more than 1,000 m) of Quaternary sediments with probable buried older channels and possible thick lenses of sand sediments.

Escarpments, Reef Trends, and Diapiric Structures, Eastern Gulf of Mexico

Abstract: Dredging, coring, and arcer profiling of the Florida escarpment southward from 28°1.5^primeN, 86°24^primeW, to the Florida Strait, of Jordan Knoll (23°20^primeN, 83°45^primeW) in the Florida Strait, and of the Campeche escarpment northeast of Yucatan (23°39^prime-23°45^primeN, 85°22^prime-85°26^primeW) have revealed the presence in all three areas of late Aptian-Albian reef and forereef facies which lithologically and paleontologically are nearly identical to the Glen Rose-Stuart City reefs of the U.S. Gulf Coast and the El Abra-Golden Lane reefs of eastern Mexico. The late Aptian-Albian reefs--or banks--apparently were not continuous from the Florida escarpment to the Campeche escarpment, but were separated by a deep-sea channel wh ch crossed Pinar del Rio Province, western Cuba. Jordan Knoll may have been an Early Cretaceous atoll, isolated from the reefs of the Florida escarpment and similar to the Golden Lane atoll of eastern Mexico. A core from Jordan Knoll penetrated a late Pliocene carbonate mud containing abundant angular limestone clasts up to 2.1 cm in diameter. The clasts range in age from late Aptian-Albian through middle Miocene to early Pliocene. The source of the clasts is unknown but regional geologic data eliminate a southern source; the clasts most probably were derived from Jordan Knoll itself. The lithology and paleontology of the clasts show that the Jordan Knoll region was a shallow-water bank until latest Albian or early Cenomanian time; that the present Florida Strait area deepened steadily from Cenomanian through Santonian times; and that, from Santonian time until the present, bathyal conditions generally prevailed. The knoll may have been uplifted during middle Eocene time as a result of the "Laramide" deformation of Cuba. The eastern Gulf of Mexico has had a complex geologic history since Early Cretaceous time or before; the geologic data from Cuba, Yucatan, Florida, and our samples reveal some of this complexity, and suggest that the site of the Florida Strait has shifted through time. Much of the complexity is related to middle Cretaceous and "Laramide" tectonic events in Cuba. An understanding of the tectonic and stratigraphic history of South Florida, the Florida Strait, and the Campeche (Yucatan) platform ultimately may lead to the discovery of substantial petroleum accumulations in this region.

Pennsylvanian Evaporite-Carbonate Cycles and Their Relation to Petroleum Occurrence, Southern Rocky Mountains

Abstract: The Paradox and Eagle basins of southeastern Utah and western Colorado contain a thick series of cyclic evaporite deposits of Pennsylvanian (Desmoinesian) age. In response to marked salinity gradients, halite and potash were deposited in the deeper parts of these basins, whereas on the basin margins, or shelves, carbonates were the predominant facies. Thus each evaporite cycle of the inner basin has a shelf counterpart in the carbonate facies. Each cycle has its own particular facies pattern, which is a reflection of biologic and chemical response to changes in sea level. Within the shelf carbonate facies some cycles contain lens-shaped buildups of biogenic carbonates. These carbonate mounds form the productive reservoir rocks on the southwest shelf of the Paradox basin. imilar, but as yet unproductive and relatively untested, carbonate mounds are present in the Eagle basin. Most of the petroleum production in the Paradox basin is from porosity development associated with moundlike buildups of algal and leached oolite limestones. Aneth, the major field on the southwest shelf of the basin, contains an estimated 300 million bbl of oil. Some production comes from the inner-basin evaporite facies from thin beds of intensely fractured black shale and dolomite. The Paradox basin is only one of the many areas of the world where petroleum resources are associated with evaporites. A close appraisal suggests that the evaporite environment plays a direct role in generation and accumulation of petroleum. Barred basins, which probably have contributed the greatest volume of marine evaporites, not only concentrate oceanic salts but also form an efficient trap for organic matter. In addition, much of this organic matter is in solution, which may facilitate its conversion to petroleum. Evaporation losses in a large basin create a strong basinward flow of nutrient-rich water. If this water moves across a shallow shelf, it may stimulate the activities of the shelf's biologic community and accelerate production of organic matter. Anoxic conditions within the evaporite basin, caused by low solubility of oxygen in high-salinity brines, combined with high levels of H2S produced by biogenic sulfate reduction, retard the decay process and allow preservation of most of the organic matter swept in from the shelf.

Depositional Systems in Wilcox Group (Eocene) of Texas and Their Relation to Occurrence of Oil and Gas

Abstract: Regional investigation of the lower part of the Wilcox Group in Texas in outcrop and subsurface indicates seven principal depositional systems: (1) Mt. Pleasant fluvial system developed updip and in outcrop north of the Colorado River; (2) Rockdale delta system, present primarily in the subsurface, chiefly between the Guadalupe and Sabine Rivers; (3) Pendleton lagoon-bay system in outcrop and subsurface largely on the southern flank of the Sabine uplift; (4) San Marcos strandplain-bay system in outcrop and subsurface mainly on the San Marcos arch; (5) Cotulla barrier-bar system in the subsurface of South Texas; (6) Indio bay-lagoon system developed updip and in outcrop in South Texas; and (7) South Texas shelf system, an extensive system entirely within the subsurface. Th Rockdale delta system consisting of large lobate wedges of mudstone, sandstone, and carbonaceous deposits, is the thickest and most extensive of the lower Wilcox depositional systems. It grades updip to the thinner terrigenous facies of the Mt. Pleasant fluvial system. Deposits of the Rockdale delta system were the source of sediments redistributed by marine processes and deposited in laterally adjacent marine systems. Delineation of depositional systems and, more specifically, delineation of component facies of the several systems permit establishment of several regional oil and gas trends which show the relation of producing fields and distribution of potentially productive trends.

Late Cretaceous Eugeosynclinal Sedimentation, Gravity Tectonics, and Ophiolite Emplacement in Oman Mountains, Southeast Arabia

Abstract: The Oman Mountains border the Gulf of Oman from the Arabian Sea to the mouth of the Persian Gulf. The Jebel Akhdar anticline forms the central core of the mountains and exposes a section of massive limestones which range in age from Permian (transgressive on a Precambrian nucleus) to late Tertiary; the latter fringe the uplift. Intercalated in this sequence, between well-dated latest Cretaceous sediments, is a thick contorted assemblage of rocks composed, from base to top, of turbidite limestone and radiolarian chert (Hawasina Group), massive exotic blocks of Permo-Triassic limestone, and a thick sheet of serpentinic igneous rock (Semail igneous series). This association of rocks, which is comparable to that known from many complex Laramide orogenic belts, can be studied with advantage in Oman where the sequence has been little disturbed since its emplacement during the latest Cre aceous (Campanian-Maestrichtian). The absence of recognizable latest Cretaceous fossils in the Hawasina, the common occurrence of well-preserved Permian to middle Cretaceous species, and the contorted features of the strata have led some geologists to postulate that these sediments were deposited outside of their present location during a prolonged pre-Late Cretaceous interval and then tectonically emplaced during the latest Cretaceous. However, it is concluded from field data presented herein that the Hawasina was deposited in its present location during latest Cretaceous time. Regional stratigraphic correlations show that northeast Oman was situated far out on the Arabian platform where quiet carbonate sedimentation persisted from Permian to middle Cretaceous time during prolonged regional tectonic quiescence. Sedimentary and tectonic quiescence ended during latest Cretaceous time when the thick Aruma pelagic shale, originating from a northerly source, was deposited across northeastern Oman concomitant with major normal faulting. Hawasina facies, north of the Aruma province, give evidence in the form of basal boulder beds, turbidites, and massive terminal gravity slides of sharp contemporaneous tectonism. Lower Hawasina deep-water turbidite sediments have yielded an inverted faunal sequence of shallow-water fossils, whereas the overlying cherty facies contains only pelagic Radiolaria. Thus the whole sequence cannot be older than the reworked youngest middle Cretaceous fossils present in the basal beds. Distribution, grading, and constitution of the carbonate clastic material in the Hawasina, and the alignment of exotic limestone blocks indicate that the sediment source area was a northwest-southeast-trending uplift of Permian to middle Cretaceous carbonate rocks north of the Oman Mountains. The absence of terrigenous clastic material and terminal submarine volcanism suggest that the source area was a submerged seamount. Erosion from this high is believed to have been by means of turbidity currents activated by repetitive and at times strong block-fault movement. Hawasina sediments compare with present deep-water sediments in the Puerto Rico Trench where faunally barren siliceous oozes of abyssal facies are interbedded with calcareous turbidites rich in reworked older and contemporaneous shallow-water fauna. The absence of contemporaneous (latest Cretaceous) shelf fauna in the Hawasina is attributed to the seamount source area being deeper than neritic, whereas the absence of contemporaneous pelagic calcareous fauna was the result of dissolution during descent to abyssal depths. The latest Cretaceous Aruma Shale is compared with Holocene pelagic globigerinal sediment occurring above 4,500-m depth. The deep trough which received Hawasina sediments appears to have been bounded by a steep block-faulted northeastern limb whereas the southwestern limb became shallower gradually through the neritic Aruma belt to the Arabian carbonate platform. At the close of the time of Hawasina deposition, volcanism and catastrophic tension-relief faulting dislodged the remnants of Permo-Triassic limestone sedimentary rocks from the seamount, and these descended into the trough as huge gravity slides (e.g., Jebel Kawr, 250 sq mi), leaving the seamount as a denuded basement uplift. Regional tension relief was accomplished finally by crustal separation and flood eruption of Semail ultrabasic magma which blanketed the abyssal landscape and cooled slowly, under great hydrostatic pressure, along the axi of the trough. According to this interpretation, the sedimentation of Hawasina beds is considered to have been contemporaneous with the deposition of the latest Cretaceous Aruma Shale, and major block-fault tectonism in the orogenic belt is correlated with known latest Cretaceous tension faulting on the southwest basin slope. Compressional features in the Hawasina are attributed to contemporaneous gravity slumping of the beds on the steep northeast limb of the trough, combined with the subsequent End_Page 626------------------------------ "bulldozing" effect which massive slides of exotic blocks and the thick Semail eruptives had on the incompetent sediments. Oil prospects in the Hawasina belt are difficult to explore and assess for several reasons, not the least of which is the masking effect which contorted chert has on seismic and gravity exploration for more simple structures in the competent sub-Hawasina limestone succession, which is hydrocarbon-bearing below Aruma Shale at Fahud, Natih, and Yibal oil fields.

Chenier Versus Barrier, Genetic and Stratigraphic Distinction

Abstract: Barrier islands and cheniers are elongate narrow sand bodies which may appear similar where preserved in the sedimentary record. However, their modes of origin and sequence of development are distinctive. Differentiation of these features is important in the interpretation of the depositional environments, paleogeography, and geologic history of coastal areas. Chenier development begins with progradation by deposition of clay, silt, and sand sediments. Rapid sedimentation precludes removal of fines. Progradation is followed by a period of reworking, shore retreat, and formation of a ridge along the landward side of the beach. Fines are transported seaward and along the shore. Sand is concentrated on the upper beach and on top of the adjacent marsh and is transported along the shore; it may accumulate in areas not being actively eroded. The contact of the chenier with underlying marsh and mudflat deposits is disconformable in areas of reworking and shore retreat, but the chenier may intertongue with finer sediments where the sand has been transported laterally along the shore. A return of conditions favoring rapid sedimentation reinitiates m dflat progradation and the sand ridge is left as a chenier. Holocene cheniers are commonly less than 15 ft thick. Barriers originate from a topographic ridge along the landward side of a beach which subsequently is partly submerged. Lagoonal-marsh sediments are deposited behind the barrier; however, continued submergence accompanied by transgression may result in complex intertonguing of barrier and lagoonal-marsh sediments. Barriers also form from spits, but this is not believed to be the general mechanism of barrier formation. Barriers, like cheniers, may be eroded, reworked, and moved landward over the adjacent marsh. Generally, barriers predate the lagoonal-marsh sediments, although, with continued submergence, synchronous deposition may occur. The sand ridge of the chenier develops on, and seaward of, existing marsh and mudflat deposits.

Paleobathymetry of Cretaceous Greenhorn Sea in Eastern Colorado

Abstract: The Cenomanian and Turonian (Cretaceous) Greenhorn sea reached maximum extent and presumably maximum depth at about the time of upper Greenhorn (Bridge Creek Member) and lower Carlile (Fairport Member) deposition. This depth is estimated in two ways--from the percentage of planktonic specimens in the very diverse normal marine foraminiferal fauna of the lower Bridge Creek, and from inference of the paleoslope that the sea transgressed. Chiefly because calcareous benthonic specimens are missing from most of the Graneros-Greenhorn-Carlile sequence, the foraminiferal profile does not correspond to one expected from simple transgression and regression on a modern continental shelf. Therefore planktonic percentages that prevail on continental shelves and slopes today cannot be used to infer depth for most of the foraminiferal profile. However, planktonic percentages might indicate approximate depth for the time of lower Bridge Creek deposition, when conditions temporarily become favorable for diverse calcareous benthonic species. Planktonic percentages for the Bridge Creek, everywhere 80 percent or more, suggest that the water depth was at least 1,640 feet (500 m). If the slope of the western margin of the marine basin remained about the same between deposition of the sandstone of the Lytle Formation (lower Dakota) and deposition of the Greenhorn, the stream slopes necessary to transport Lytle pebbles 400 mi from a western Utah source to the Colorado Front Range foothills area were the same as the slope which the sea transgressed later. When it was deepest, the Greenhorn sea reached western Utah, advancing as far as the source area for the Lytle. If the upper 50 mi of the Lytle stream courses is eliminated from consideration (here slopes were steepest and tectonics least stable), total fall of the streams in the remaining 350 mi was probably of the order of 2,000 ft or more. This figure, modified by sedimentation and isostatic depression of the ea floor, provides an estimate for the maximum depth of the sea in eastern Colorado.

Trace Fossils as Criteria for Recognizing Shorelines in Stratigraphic Record: ABSTRACT

Abstract: Biogenic sedimentary structures offer a new and exciting approach to the interpretation of ancient sedimentary environments. Although trace fossils have been studied extensively by European geologists since early in this century, it is only in recent years that they have received much more than passing mention in North America. The increased interest in tracks, trails, burrows, and borings is due primarily to the environmental or facies approach to the study of sedimentary rocks. Whereas, in the past, attention has been directed toward descriptive studies of rock units, the present paleoecologic approach demands a genetic interpretation of the sedimentary record. Appreciation of biogenic sedimentary structures as facies indicators has been influenced significantly by the emphasis on studies of physical sedimentary structures which in the past two decades have introduced many new keys to paleoenvironment interpretation. Additional impetus to the utilization of trace fossils has come from detailed studies of modern sediments which illustrate clearly the important relations that exist between the animals and sediments in a particular environment. In the study of ancient and present-day nearshore sedimentary environments, the facies significance of biogenic sedimentary structures can be demonstrated readily. Striking similarities exist between nearshore clastic facies of Holocene and Pleistocene sediments of the Georgia coast and Upper Cretaceous shorelines of the western interior. Such similarities dramatically point out the value of trace fossils in environmental interpretation. These comparisons exist not only on the regional stratigraphic level but also between and within specific facies. Field studies which utilize trace fossils in conjunction with physical sedimentary structures, lateral and vertical changes in the sedimentary sequence, and geometry of the rock body offer new opportunities in the search for stratigraphic ccumulations of oil and gas. End_of_Article - Last_Page 723------------

Phylloid Algal-Mound Complexes in Outcropping Upper Pennsylvanian Rocks of Mid-Continent

Abstract: A phylloid algal-mound complex is a local to subregional thickening of limestone attributed chiefly to the presence of a distinctive suite of rock types containing leaflike or phylloid algae. Twenty-three such mound complexes are present at or near the southern ends of most limestone units in Missourian and lower Virgilian (Upper Pennsylvanian) rocks exposed in eastern Kansas, northeastern Oklahoma, and northwestern Missouri. Mound complexes are composed of two facies, (1) the mound, consisting primarily of massive algal calcilutite to algal sparite, and (2) mound-associated facies, consisting primarily of thin- and cross-bedded skeletal and oolitic calcarenite capping and flanking the mound. Overlying shale beds thin across the tops of mound complexes. Most mound complex s grade northward into thinner, more diversely fossiliferous, open marine limestone beds, and grade abruptly southward into thin limestone beds and lenses and thick terrigenous clastic strata. Pennsylvanian phylloid algae are comparable with Holocene calcareous codiacean green algae and coralline red algae, and flourished in shallow sunlit water where they were sediment suppliers and stabilizers. Mounds probably began on topographic highs favorably situated between a region of great clastic influx and the open sea, and grew as the algae proliferated and produced sufficient sediment to compensate for subsidence. Mound growth allowed the algae to continue to flourish in their optimum sunlit environment. Stacking of mound complexes may reflect positive topographic influence of underlying mounds on the sea bottom, and shifts in the stackings probably resulted from shifts in northward extent of the clastic influx.

Carbonate Geochemistry of Superior Deep Test Well, Andros Island, Bahamas

Abstract: The 14,585-ft Superior Oil Company well at Stafford Creek, Andros Island, Bahamas, penetrated only limestone and dolomite. Cuttings and cores from the well were analyzed for carbonate mineralogy by X-ray diffraction and for strontium by X-ray fluorescence. Samples from contemporary Bahama marine sediments and from partly indurated surface outcrops were collected and similarly analyzed for comparison. In Holocene sediments, as well as in the surface rocks, aragonite is more abundant than calcite. In contrast, aragonite is not present in the samples from the well, and some evidence indicates that the depth at which complete conversion takes place is approximately 40 ft. High-magnesium calcite (12 percent MgCO3) is present in all the Holocene marine sediments which contain appreciable percentages of calcite. No dolomite was found. The subsurface rocks are completely dolomitized at 400 ft. Four sequences of dolomite, each capped with a unit of limestone or partly dolomitized limestone, are present in the well section and can be correlated regionally as seismic reflecting and refracting horizons. They record four major cycles of deposition, each of which was terminated by a period of erosion and dolomitization. Dolomitization occurs by the solution-cannibalization of the more unstable carbonate phases during uplift. The periods of uplift are related to Cuban orogenic episodes in the Late Cretaceous, middle Eocene, end of Eocene, and early Miocene.

Tertiary Geology of Part of South Makran, Baluchistan, West Pakistan

Abstract: The South Makran basin in the southern part of the Baluchistan basin was the site of considerable clastic sedimentation from the Oligocene onward. The sediments formed a wedge thickening seaward from the present Gokh Prush Band at a rate of 160 m/km to a total thickness of at least 10,000 m along the coast of the Arabian Sea. The sequence comprises the Panjgur Formation of Oligocene to early Miocene age and the Miocene to Pliocene Makran Group. The rock types are shale, mudstone, conglomerate, and minor coquina and limestone. The predominance of argillaceous and sandy detritus indicates marked subaerial erosion of uplifts on the north. The dominant structural trend is east-west. The structure on the north is complex, with tightly folded asymmetric anticlines and synclines, many overturned, associated with transverse faults and high-angle thrust faults. There are indications that salt intrusions were important in producing local structure along the coast and offshore. The folds are believed to be disharmonic, having been produced by southward gliding over plastic argillaceous sediments above the Eocene to Cretaceous carbonates that formed the rigid floor of the post-Eocene basin. Only four wells have been drilled for oil in the study area, and they tested only three structures. Many untested possibilities are believed to remain. Water resources can be augmented by construction of a series of dams to trap the seasonal rainfall in the intermittent stream courses.

Edwards Formation (Lower Cretaceous), Texas: Dolomitization in a Carbonate Platform System

Abstract: The Edwards Formation is characterized by rudist bioherms, carbonate grainstone and mudstone, and evaporites which were deposited on an extensive, shallow-water, marine platform bounded by deeper water basins in which chiefly carbonate muds were deposited. Rudist bioherms were constructed principally along platform edges peripheral to an evaporitic lagoon. Main dolomite deposits are in a concentric belt marginal to the lagoonal facies. Two main types of dolomite are present: (1) stratal dolomite--fine grained, tightly knit fabric, laminated to thin bedded, very slightly porous and permeable, associated with thin-bedded, mud-cracked, stromatolitic carbonate mudstone, ripple-marked carbonate grainstone, and thin evaporite-collapse layers; dolomite units are generally less than 2 ft (0.6 m) thick; magnesium carbonate content ranges irregularly from low to high; and (2) massive dolomite--fine- to coarse-grained loosely knit euhedral crystals, moderately to highly porous and permeable, replacing thick-bedded, fossiliferous carbonate grainstone; dolomite units are commonly more than 10 ft (3 m) thick and underlie prominent evaporite-solution units; magnesium carbonate content is low or high, with few intermediate values. Both stratal and massive dolomites are judged to be products of metasomatic replacement of calcium carbonate which resulted from contact with magnesium-enriched brines. Features of stratal dolomite indicate prelithification replacement of carbonate muds and grains in extensive, low-relief, supratidal and intertidal zones along the southern margin of the lagoon. Massive dolomite was created by postlithification replacement of reef-trend carbonate grainstone along the northern margin of the lagoon as a result of seepage refluxion of lagoon brines. Both dolomitization processes were part of the original depositional system, and type of dolomitization was controlled by specific depositional facies of the system.

Structure of Sigsbee Scarp, Gulf of Mexico

Abstract: A sparker profile across the Sigsbee scarp shows an interval with an anomalously high velocity which is interpreted to be salt extruded a distance of 10 km over flat-lying beds south of the scarp. The extrusion and the topographic form of the scarp are related to basinward flow of sediments and salt from under the thick shelf-slope sedimentary wedge.

Growth and Dolomitization of Silurian Reefs, St. Clair County, Michigan

Abstract: Pinnacle reefs in eastern Michigan were formed during a period of cyclic carbonate-evaporite deposition in latest Niagaran-early Salinan time. Reef sequences now have salt beds as their lateral equivalents. Restoration of evaporite sequences to their relative positions during deposition demonstrates, however, that this is the result of differential compaction of carbonate beds, rather than contemporaneous deposition of reef rock and evaporites. Compaction of carbonate mud during lithification, rather than cementation, also accounts for most porosity loss in the area. Where porosity is preserved, compaction-resistant framework is present, and the porosity is directly proportional to quantity of framework. The reef and surrounding carbonates are dolomitized. The pattern of dolomitization, quantity of dolomite in relation to bed thickness, position of the dolomite in relation to evaporite beds, and time of dolomitization suggest that dolomitization was caused by a continuous movement of large quantities of connate waters through the sediments. It cannot be shown that refluxion by highly concentrated brines during periods of evaporite deposition contributed substantially to dolomitization. Evaporite sequences overlying bodies of carbonate mud prevented the vertical escape of water during compaction. The water was forced to flow laterally to the reefs, and then vertically through them to the surface. This very large quantity of water was forced through a relatively small volume of rock in he reef area and supplied the magnesium for dolomitization. The outflow of water continued during intensely evaporitic cycles, altering evaporite deposition in the reef area, and preventing heavy brines from entering the porous reefs and destroying porosity.

Evaporite Deposition from Layered Solutions

Abstract: It is widely accepted that carbonates, especially those which originate as banks or reefs, and the evaporites with which they are closely associated represent mutually exclusive, successive phases of deposition: (1) a stage of carbonate bank or reef deposition under conditions of normal marine circulation, followed by (2) a time of sulfate deposition under conditions of restricted circulation, (3) an episode of further restriction leading to chloride precipitation, and, in some instances, ending in (4) an ultimate stage of bittern-salt accumulation. A multistage history of carbonate-evaporite deposition requires paleogeographic interpretations that appear bizarre in view of the epeiric setting of most deposits. An alternative is sought in deposition from density-layered waters with an upper layer of nearly seawater salinity and a lower layer or layers saturated with respect to one or more evaporite salts. Brine is supplied to such a system by evaporation on shelf areas surrounding a basin, and saturation is maintained by contact with salts on the basin floor. In this manner layered solutions persist for geologically significant spans of time, in spite of diffusion and mixing, while carbonate deposition continues in the upper dilute waters. Episodic exposure of the lower brine to evaporation is required for evaporite deposition. S ch exposure may be the result of displacement of the upper layer by wind, or may accompany long-period, high-amplitude, internal seiches. Layered systems and the effects of winds and internal seiches are demonstrable in present-day saline lakes; the processes involved are amenable to laboratory investigation.

Stratigraphy, Structure, and Origin of Siwa Depression, Western Desert of Egypt

Abstract: Marine Miocene sedimentation on the broad platform of the northern Western Desert of Egypt was controlled tectonically by slight oscillations and local small undulations and warps of the sea bottom. Sedimentation ceased in post-middle Miocene times when intensified tectonism uplifted the whole platform to form El Diffa Plateau of the present Western Desert. Three lithostratigraphic units--a lower one belonging to the Moghra Formation and two upper units belonging to the Marmarica Formation--are delineated in the Miocene succession exposed across most of the northwestern part of the Western Desert of Egypt. All three units are marine, and generally they are a lower coquina-marlshale member, a middle chalky member, and an upper fossiliferous limestone-chalk member. In the S wa Depression area of the Western Desert, the constant thickness of the middle member helps to define the structural configuration of the depression, which is an elongate asymmetric and irregular basinlike syncline. This gentle syncline has a west-east extension of more than 80 km (50 mi) and forms the middle part of the great geomorphic arc of the Jaghbub-Siwa-Qattara depressions of the northern Western Desert. It is suggested that during post-middle Miocene uplift, hinge faulting and rejuvenation of old faults occurred in the basement underlying the Miocene blanket sediments. This resulted in sags and downwarps of the overlying Miocene beds and finally produced large, tectonically controlled surface depressions like that of Siwa. In the exposed Miocene succession around Siwa Town, along the main trough of the Siwa syncline, the higher beds with relatively lighter overburden are much more deformed than the beds underlying them. This deformation is ascribed to internal stresses generated within a localized core of the irregular Siwa syncline by the squeezing and upward buckling of the freer part of the downwarped sediments, possibly above or along a small decollement. In the wet post-Miocene climate, the morphotectonic depression of Siwa and its associated tectonically induced jointing and other surface fractures immediately captured and localized the surface drainage. Water erosion within the chemically vulnerable Miocene sediments of the initial depression proceeded rapidly and greatly enlarged it. There is conflict of opinion concerning the agents and mechanisms responsible for the creation of the Siwa and other large oasis depressions in the Western Desert; however, this study proves that initial tectonism and subsequent ancient water erosion were the most important factors. Wind deflation had a relatively late and minor role. Exsudation is likewise unimportant except perhaps in the late dry stages.

Deep-Sea Drilling into the Challenger Knoll, Central Gulf of Mexico

Abstract: During the recent pioneering cruise of a deep-sea coring project, a series of holes was drilled into the deep floor of the Gulf of Mexico and the western North Atlantic. One of the holes, in the Sigsbee Knolls area of the central Gulf of Mexico, encountered salt-dome caprock saturated with oil, gas, and sulfur. The objectives of this coring project are scientific and the discovery of petroleum was not an intended part of the program, even though the possibility had been recognized. The results of drilling into the Challenger Knoll indicate that the Sigsbee Knolls and adjacent buried domes are diapirs derived from a mid-Mesozoic salt deposit, comparable in age and lithology with other major evaporite sequences within the Gulf of Mexico area. Marine geophysical data show that these diapirs are part of a belt which extends southwest into the saline basin of the Isthmus of Tehuantepec. Cores of caprock, recovered from more than 12,000 ft below sea level, contained a typical salt-dome caprock mineral assemblage of calcite, gypsum, and free sulfur (up to 19 percent). The petroleum is sulfur-rich and of low gravity, and geochemical data indicate that these hydrocarbons are immature, of fairly young age, and derived from marine organic material. Residual mineral assemblages--including quartz crystals--are comparable with those found in caprock elsewhere in the Gulf of Mexico region; similar quartz crystals are common in the mid-Mesozoic Louann Salt. All sediments above the caprock are pelagic clays and calcareous organic oozes of late Tertiary age (late Miocene and younger). The presence of these sediment types indicates that the Challenger Knoll has been topographically above the surrounding sea floor at least since late Miocene time. In contrast, the turbidite-rich sequence cored beneath the adjacent abyssal plain shows that pelagic sediments constitute only a fourth to less than half the total late Miocene and younger section. The late Cenozoic pelagic deposits of Challenger Knoll and the turbidites appear to reflect major geologic events and significant changes in sediment provenance within the land area surrounding the Gulf of Mexico. The regional problem which probably will be debated most earnestly as a result of the Challenger Knoll coring is whether the salt underlying the knoll was deposited originally in a deep-sea environment, or on a crust which once was much shallower. This problem is of considerable geologic importance, and it can now be approached with a background of firm data.

Floral Zones and Correlations of Cretaceous Kukpowruk and Corwin Formations, Northwestern Alaska

Abstract: Cretaceous fossils, mainly plant megafossils, were collected from 140 localities in the Kukpowruk River-Corwin Bluff areas of northwestern Alaska. Mollusks, from 13 marine interbeds in the Kukpowruk Formation of the Kukpowruk River area, correlate with middle to late? Albian faunas of the Chandler-Colville area about 300 mi east. Correlations by plant megafossil zones suggest an early Albian to possibly Cenomanian age for the Kukpowruk and Corwin Formations of northwestern Alaska. Floral records in the type section of the Corwin Formation show three sequential stages: (1) an oldest stage, equivalent largely to the widespread marine Torok Formation; (2) a middle stage, found also in the Kukpowruk Formation and the lower part of the Corwin Formation in the Kukpowruk River area, and in the Tuktu Formation of the Chandler-Colville areas; (3) a youngest stage in the bentonitic clay member of the Corwin type section, correlative with medial Corwin units in the Kukpowruk River area, and with a floral zone in the lower part of the Killik Tongue (Chandler Formation) in the Chandler-Colville areas. Plants in upper Corwin beds along Kukpowruk River represent a floral zone younger than the zones in the type section at Corwin Bluff. The floral zone in Chapman and Sable's upper sandstone member in the Corwin type section repeats a zone found in older beds of the type section. The upper sandstone member of the Corwin type section is thus a duplication of older beds, rather than a younger unit containing a later and continuing stage of floral development.

Evaporite-Solution Brecciation and Devonian Carbonate Reservoir Porosity in Western Canada

Abstract: The presence of "primary porosity" in ancient reef or bank sequences is generally assumed. Evaporite solution porosity in exposed Devonian reef-bank complexes of western Canada, however, is important. Detailed investigations are likely to show that such solution porosity is more general in the laterally equivalent subsurface petroleum reservoirs than the literature suggests. Much of the so-called primary porosity in reef reservoirs may be evaporite solution porosity.

Stratigraphic and Petrographic Analysis of Upper Jurassic-Lower Cretaceous Morrison and Kootenai Formations, Southwest Montana

Abstract: Sedimentary regimen, effects of Nevadan orogenesis, development of elastic-wedge configuration, sediment composition and textures, source areas, paleotectonic synthesis, time-stratigraphic cross-section

New Evidence of Transcurrent Movements in Red Sea Area and Petroleum Implications

Abstract: New evidence, derived from the displaced positions of two pairs of prominent shear zones--Abu Masarib (Arabia)-Duwi (Africa) and Wadi Alhamd (Arabia)-Wadi Hafafit (Africa)-indicates that Arabia has moved northward about 150 km relative to Africa along the Red Sea rift. The displacement of the shear zones was observed initially in photographs of the Red Sea area taken during Gemini earth-orbiting missions and has been substantiated subsequently by data derived from the geologic literature. The movement also must have displaced the basement block in northwestern Arabia (Maqna block) from its original position at the southern end of the Gulf of Suez to its present position at the Gulf of Aqaba. Hence it is evident that Maqna block and the adjacent offshore area south of Sinai Peninsula were part of a structural unit (Greater Gulf of Suez basin) and thus are similar in sedimentary and tectonic history and petroleum potential. Gemini pictures show that the Red Sea structures are dominated by five major fissure systems; many show evidence of being transcurrent. Certain fissures trending WNW-ESE appear to bear a spatial relation to most oil occurrences in the Gulf of Suez.

A Modern Evaporite Deposit in Mexico

Abstract: Marine evaporites are being deposited in the Ojo de Liebre Lagoon area, Baja California, in shallow diked basins flooded by high tides from an adjacent lagoon. Aragonite is deposited in the brine pans where daytime pH is high because of photosynthesis. Halite is deposited when wind transports brine into inner basins, and most of the gypsum is intermediate in location. Evaporites and brine support a very large standing stock of organisms.

Newly Discovered Cenozoic Basins, Bering Sea Shelf, Alaska

Abstract: Recent geophysical studies over the Bering shelf have outlined the basic structure of three previously unknown Cenozoic basins, Norton basin and Pribilof and Zhemchug depressions. The upper part of the crust beneath the Bering shelf consists of a stratified sequence of generally undeformed Cenozoic deposits overlying a basement of Mesozoic sedimentary, volcanic, and plutonic rocks, and older Paleozoic sedimentary and metamorphic units. The average thickness of the Cenozoic deposits is approximately 1.0 km. However, they are more than twice that thick where the basement has been downfolded or downfaulted to form the major intrashelf basins (such as Bristol, Anadyr, and Norton basins) that typically underlie major bays and gulfs, and the narrower, fault-controlled outer shelf basins (such as Pribilof and Zhemchug depressions) that trend parallel with the continental margin. Norton basin underlies the northern part of the Bering shelf north of St. Lawrence Island. It is an east-west trough filled with terrestrial and neritic deposits of Paleogene age and younger, as much as 2.0 km thick. Minor folds and offsets in this fill are present along the flanks of the basin. Pribilof and Zhemchug depressions underlie the outer shelf and the northeast-trending canyon heads of Pribilof and Zhemchug Submarine Canyons, respectively. These depressions in the basement rocks are grabens and/or half grabens, and are filled with stratified neritic deposits of Paleogene age and younger, as much as 2.0 km thick. The Cenozoic fill is deformed above faults that offset the basement and the sea floor near the Pribilof Islands.

Structural Analysis of Fractures in Cores from Saticoy Field, Ventura County, California

Abstract: The orientation and relative positions of subsurface faults are inferred from analysis of macro- and microfractures in cores from the Saticoy field, Ventura County, California. The conclusions are reached from study of 1,044 macrofractures (joints) in 4,168 ft of core from 28 wells and from the orientations and relative abundances of microfractures (unhealed fractures in quartz grains) studied in thin sections from cores at several depths in each of six wells. The conclusions are checked by comparison with the known structure of the Saticoy field. In the main, four macrofracture sets exist along the length of the field. Their "real" rather than observed relative development is assessed upon consideration of the problems of determining the orientation of macrofractures in core samples. In the absence of offset criteria, an interpretation of the fracture data is made solely on geometric grounds and with the geologic knowledge available before the field discovery well was drilled. It is concluded that two of the fracture sets parallel, and therefore define, the orientations of two reverse faults. The microfractures are shown statistically to be parallel with the macrofractures. In addition, information on the abundance of microfractures is presented for 31 sample locations in six holes that are deviated both toward and away from known faults. The abundance of microfractures increases with proximity to the faults and is essentially independent of the depth of burial. The strike, dip, and position relative to drill holes of the faults predicted from the fracture data agree with the known subsurface structure.

Cretaceous floras of chandler-colville region, alaska-stratigraphy and preliminary floristics

Abstract: Field studies of the stratigraphy of Cretaceous plant-bearing beds were undertaken in the Chandler and Colville Rivers region of northern Alaska during the summers of 1964 and 1966. Florules were discovered and sampled at 55 localities. Their superpositional relations and their relation to fossiliferous marine interbeds were determined in the field. The lower part of the section (about 4,700 ft) contains the Nanushuk Group of middle Albian to early Cenomanian age. The upper part (about 5,100 ft) contains the Colville Group of Turonian to Maestrichtian? age, which lies with angular unconformity on the Nanushuk Group. Invertebrate megafossils include species of ammonites, of Inoceramus, and of other mollusks and provide direct field evidence for referring floral records to uropean stages. Plant megafossils from nonmarine units include ferns, ginkgophytes, cycadophytes, conifers, and angiosperms. The numerical importance and the variety of each major plant taxon differ at different stratigraphic levels. Also noted is a striking change in floristics from bottom to top of local sections, similar to the floral records found earlier in the Kuk River area 200 mi northwest. Seven biostratigraphic units (plant megafossil zones) are distinguished, of which five were recognized previously by the author in the Kuk River section. Several of the floral zones appear to be present also in the Soviet Arctic 1,500 and 2,000 mi west, and in Greenland 2,500 mi east, all at approximately the same latitude. The floral zones in other Arctic regions seem to have the same sequential relations as in northern Alaska, suggesting that such zones may be holarctic in extent. It appears probable that the sequential floral records of northern Alaska, with their direct marine interties, may serve as reliable references for correlations of plant-bearing nonmarine deposits of similar age throughout Arctic regions.