Showing papers on "Outcrop published in 1994"

A latest Cretaceous hotspot and the southeasterly tilt of Britain

Abstract: The southeasterly regional dip of England and Wales is usually ascribed to Miocene movements. Loss of up to 2 km of cover from northern England, shown by apatite fission track analysis, has been related to uplift and erosion induced by a mantle hotspot, but its effects were far more widespread than considered hitherto. Outcrop patterns throughout much of England, Wales and eastern Ireland can be related to a hotspot beneath the Irish Sea. Uplift occurred within the late Maastrichtian and erosion continued into the late Palaeogene. After post-Oligocene down-faulting of the Irish sea, the modern drainage was initiated on the eroded surface. Unconformities provide the best means of dating orogenic movements; in some localities the time interval between the youngest rocks preserved beneath the unconformity and the oldest preserved above may be small enough to enable the dating of fold movements to be determined with some precision. Thus in southeastern Britain the major E-W structures of the Hampshire Basin, the Weald and the London Basin can be dated as Miocene, as rocks of Oligocene age are involved in the folding in the Hampshire Basin and Pliocene sands rest in erosional pockets of the Chalk of SE England. Because fold movements tend to affect large areas of the crust at discrete intervals of time, the southeasterly tilt of Britain, evident from the fact that a NW-SE transect from Anglesey to London crosses outcrops of rocks getting progressively younger from the Precambrian of Anglesey to the Tertiary of the London Basin, has

Outcrop Analog for Cyclic-Shelf Reservoirs, San Andres Formation of Permian Basin: Stratigraphic Framework, Permeability Distribution, Geostatistics, and Fluid-Flow Modeling

Abstract: Vertical and horizontal transects were sampled from core and outcrop of the San Andres Formation at Lawyer Canyon, Guadalupe Mountains, New Mexico, to assess permeability variation in a geologic framework of upward-shallowing carbonate cycles and to show the potential effect these variations have on viscous-dominated flow behavior in analogous reservoirs. These cycles occur in a ramp-crest facies tract, are 3-13 m (10-45 ft ) thick, and contain both vertical and lateral variation of lithofacies. Thicker cycles consist of a basal dolomudstone, which is overlain by burrowed dolopackstone, and capped by bar-flank ooid-peloid dolograinstone and bar-crest ooid dolograinstones. In vertical transects, permeability is extremely variable about the mean, yet upward-increasing trends coinciding with the succession of lithofacies typify a given cycle. Semi-variance analysis shows permeability to be uncorrelated vertically at distances greater than 5.5 m (18 ft), which is the average cycle thickness, suggesting that the cycles may equate to a fluid-flow unit in a reservoir. Semi-variance analysis of measurements collected along a horizontal transect within bar-crest dolograinstones of a single cycle shows permeability is uncorrelated at distances greater than 3.6 m (12 ft). This correlation distance appears to be controlled by alternating porous and tightly cemented zones that formed during dolomitization. Vertical and lateral variogram models were fit to the spatial parameters to generate a variety of conditionally simulated permeability fields. Fluid-flow simulations show viscous-dominated flow behavior is compartmentalized by both the individual cycles and groups of cycles. The basal dolomudstones are potential baffles to flow crossover between cycles, but poorly developed cycles (i.e., those that are mud rich and lack well-developed bar-flank and bar-crest facies) result in the greatest compartmentalization of fluid flow within a succession of cycles.

Roles of plutonism, midcrustal flow, tectonic rafting, and horizontal collapse in shaping the Miocene strain field of the Lake Mead area, Nevada and Arizona

Abstract: The Lake Mead area contains a well-exposed record of highly contrasting Neogene structural development that includes major magmatism and extension in its southern part and large-magnitude lateral translations (to 65 km) of amagmatic structural blocks in its northern part. These highly contrasting parts are joined at the Hamblin Bay fault, a major crustal boundary that forms the south margin of the Lake Mead fault system and the north margin of the lower Colorado River extensional corridor. Intrusive rocks adjacent to this crustal boundary on the south record major crustal rifting normal to an 350° axis. They also display steep-axis disharmonic folds that formed by post-magmatic extension-related horizontal collapse of highly weakened crust. The rocks directly north of the crustal boundary not only display large lateral translations, but they also are cut by fault systems suggestive of diverse paleostress conditions and formed into diversely oriented coeval folds collectively producing a complex Neogene deformation field that defies interpretation in terms of a uniform system of remote stresses. Because structures forming this complex deformation field range from outcrop scale to 65 km of lateral displacement and can not be uniformly partitioned according to relative age, we prefer tectonic models based on a protracted uniform dynamic process or super-posed dynamic processes to models based on time-varying states of stress. We suggest that the large lateral displacements and extreme structural complexity record synextensional rafting of structural blocks atop a flowing undermass and concomitant contraction of the zone of flowage. Structural blocks north of the main zone of flowage (the Great Basin sector of the Basin and Range) moved southward and occluded to the block south of the zone (the Colorado River extensional corridor) as a large west-ward widening wedge of upper crust was carried to the west–southwest toward California on the flowing undermass. Where occlusion is almost complete in the northern Black Mountains, north–south shortening is estimated at 55 km. The narrow zone of strong rifting, plutonism, crustal softening, and horizontal collapse that developed at the north margin of the Colorado River extensional corridor is interpreted to result from contact with the zone of crustal flowage.

Spatial Variability in Hydrologic Properties of a Volcanic Tuff

Abstract: Spatial variability of hydrologic properties was quantified for a nonwelded-to-welded ash flow tuff at Yucca Mountain, Nevada, the potential site of a high-level, nuclear waste repository. Bulk density, porosity, saturated hydraulic conductivity, and sorptivity were measured on core specimens collected from outcrops on a grid that extended vertically through the entire unit thickness and horizontally 1.3 km in the direction of ash transport from the volcanic vent. A strong, geologically determined (deterministic) vertical trend in properties was apparent that correlated with visual trends in degree of welding observed in the outcrop. The trend was accurately described by simple regression models based on stratigraphic elevation (vertical distance from the base of the unit divided by unit thickness). No significant horizontal trends in properties were detected along the length of the transect. The validity of the developed model was tested by comparing model predictions with measured porosity values from additional outcrop sections and boreholes that extended 3000 m north, 1500 m northeast, and 6000 m south of the study area. The model accurately described vertical porosity variations except for locations very close to the source caldera, where the model underpredicted porosity in the upper half of the section. The presence of deterministic geologic trends, such as those demonstrated for an ash flow unit in this study, can simplify the collection of site characterization data and the development of site-scale models.

Bed forms in submarine channels; comparison of ancient examples from Greece with studies of Recent turbidite systems

Abstract: Side-scan sonar observations have shown the presence of gravel waves, flute-like scours, and parallel erosional furrows in modern marine turbidity-current channels. Similar features are not well known from ancient rocks. Two Cenozoic sections of well-exposed turbidite channel fill in western Greece have been studied to determine why such features are difficult to recognize in the ancient geological record and which sedimentary facies are associated with large gravel bed forms. Gravel waves in ancient channel fills form in massive conglomerates with poor grading at the top of beds. They are found with sandstones of the high-density sandy turbidity current association. Series of erosional scours a few meters in size, similar to modern flute-like scours, have been mapped on the surface o debris-flow deposits in one ancient channel fill. Large gravel bed forms are difficult to recognize in the ancient geological record because of their scale and lack of distinctive features to aid lateral correlation across minor gaps in outcrop. Analogy with modern deposits and the sediment association suggest that deposition takes place in channels upslope from major sandy depocenters. The erosional features associated with modern gravel bed forms are difficult to recognize in the ancient record because of poor exposure of shales.

Deposition and diagenesis of the lacustrine-fluvial Cangfanggou Group (uppermost Permian to Lower Triassic), southern Junggar Basin, NW China: a contribution from sequence stratigraphy

Abstract: The Junggar Basin in NW China contains lacustrine hydrocarbon source rocks which are among the highest quality of hydrocarbon potential in the world. Oil reservoirs in the basin are very substantial: target reservoirs span Carboniferous to Tertiary strata and include Permo-Triassic lacustrine and fluvial sandstones. The Junggar Basin was a foreland basin during the late Permian to Cenozoic, possibly with strike-slip tectonics at the southern margin during Mesozoic time. The Cangfanggou Group, as one of the major reservoirs, is well-exposed in the eastern part of the southern Junggar Basin. A measured outcrop section and a number of borehole logs coupled with resistivity logs were used to attempt sequence stratigraphic analysis. Detailed sedimentological studies on the outcrops and borehole cores have demonstrated that the Cangfanggou Group is characterized by alternating lacustrine and fluvial deposits. Four depositional sequences have been recognized. For each sequence, the basal boundary is marked by erosional truncation of fluvial channel conglomeratic sandstones in sharp contact with underlying lacustrine or floodplain mudstones. The top of each lowstand systems tract is normally overlain by the transition to lacustrine or maximum flooding surface. The transgressive systems tract is normally not identifiable at the basin margin, but was developed in the basinward area and characterized by interbedded fining-upward distal fluvial and shallow lacustrine deposits. The highstand systems tract at the basin margin is characterized by very thick floodplain mudstones or shallow lacustrine deposits, and by typical coarsening-upward parasequences of shallow lacustrine deposits in more basinward areas. Sediment input to the basin was controlled by tectonics and climate. Depositional sequences were probably controlled by fluctuating change of lake level: this was in turn controlled by climate (runoff), modified by tectonics in specific areas.

A case study of integrated hydrocarbon exploration through basalt

Abstract: A large unexplored tectonic basin with the potential for significant hydrocarbon accumulations was identified in north-central Oregon using a variety of geophysical techniques. The basin, informally named after the local town of Heppner, is covered by several thousand feet of Miocene Columbia River Basalt Group (CRBG) but is readily identified by a gravity low against the Blue Mountains Uplift. The Paleocene/Eocene Herren Formation (Pigg, 1961), which outcrops on the Blue Mountains Uplift south of the Heppner Basin, offered good source and reservoir potential. Based on lateral extent, thickness and paleocurrent structures in the Herren Formation, the unit was expected to be present in the basin. Gravity modeling produced nonunique interpretations, thus magnetotelluric (MT) information was used to constrain the CRBG thickness. Static shifts in the MT data were removed using transient electromagnetic (TEM) data before MT data inversion. After extensive experimentation, adequate seismic data were obtained for structural mapping, but the seismic data were interpretable with confidence only after MT determinations of the CRBG thickness. As a result of the favorable geologic and geophysical information, the ARCO Hanna [number sign]1 well was drilled to 9,100 ft (2,800 m) near Heppner, oregon in section 23, T2S, R27E in 1988. Geophysical wellmore » logs indicate the Clarno Formation has densities and resistivities sufficient to account for the gravity and electrical anomalies defining the prospect. Poor seismic quality was explained by the heterogeneous nature of the pre-CRBG volcanic section encountered in the well.« less

Modeling Reservoir Heterogeneity Within Outer Ramp Carbonate Facies Using an Outcrop Analog, San Andres Formation of the Permian Basin

Abstract: Variably cyclic, fusulinid-rich, outer ramp facies of the Permian San Andres Formation are exposed along the Algerita escarpment, Guadalupe Mountains, New Mexico. We have used the outcrop exposures and cored wells drilled adjacent to the outcrop to assess reservoir- and interwell-scale variability of permeability as a potential analog for carbonate reservoirs in the Permian basin and elsewhere. Permeability distribution was evaluated using a field permeameter and conventional measurements on small core plugs taken along vertical and horizontal outcrop traverses and from the slim-hole cores. Permeability is related to the variable development of depositional cycles within a small-scale sequence and also to diagenetic overprinting. Geostatistical models of permeability variation, honoring the geologic and petrophysical data, were constructed and input into a waterflood simulator to understand the interactions between heterogeneity and flow. Variograms constructed from conventional plug analyses of permeability for a vertical outcrop transect contain a significant small-scale signal ("noise"), as seen in the high nugget effect (approximately 50% of the overall sample variance), and have correlation ranges of less than 4.5 m (15 ft). Different vertical variogram characteristics are displayed by cyclic and less distinctly cyclic parts of the San Andres. Variograms constructed for horizontal transect data from three distinct stratigraphic units have nearly identical properties. Overall, the ranges of correlation are short (3-3.5 m; 10-12 ft) when compared to typical interwell distances, supporting a nearly uncorrelated and highly variable permeability model. Using observed short ranges of vertical and horizontal correlation and honoring the vertical transect data, cross sectional, conditionally simulated permeability fields were generated and used in simulated waterfloods to investigate the sensitivities to an oil recovery model and overall fluid injection rate for this style of stratigraphy and cyclicity. Cyclic parts of the section are characterized by a potential for early water breakthrough and relatively high vertical sweep efficiencies. Within the less distinctly cyclic section, waterflood fronts have a fingerlike profile and vertical sweep efficiency is generally poorer.

Basics and Application of Rock-Eval/TOC Pyrolysis: an example from the uppermost Paleocene/lowermost Eocene In The Basque Basin, Western Pyrenees

Abstract: Eighteen samples from four uppermost Paleocene-lowermost Eocene outcrop sections (Ermua, Aixola, Trabakua Pass, and Zumaia) were collected in the Basque Basin for Rock

Graphic correlation and sequence stratigraphy in the Palaeogene of NW Europe

Abstract: . A sequence stratigraphic analysis of well log, seismic, and biostratigraphic data has documented a pattern of cyclic sedimentation for the Palaeogene of the Central North Sea. Previously published research has also documented cyclic sedimentation related to sea level changes. Integrating Central North Sea subsurface sections with Palaeogene outcrop from NW Europe, using sequence stratigraphic first principles and the graphic correlation method, has produced a chronostratigraphic framework for the Palaeogene of NW Europe. Northwestern Europe basins (London–Hampshire, Paris, and Belgian) have shallow marine to nonmarine environments, revealing basinward and landward fades shifts indicating sea level changes. The problem correlating NW Europe with North Sea deposits has been addressed by correlating a biostratigraphy to the deep water deposits outcropping in Denmark. Once a biostratigraphy joining the subsurface and outcrops is built, key bounding surfaces are correlated between basins. We find that: (1) sedimentation in the deep basin occurs as depositional pulses, separated by time-correlative biostratigraphic data terraces (hiatal intervals), which correspond to persistent seismic reflectors; (2) not all sequence boundaries are resolvable by graphic correlation, but the method brackets packages defined by seismic, log interpretation and biostratigraphy; and (3) correlation with outcrops reveals the true significance of the hiatal intervals.

Stratigraphic relations and U-Pb geochronology of the Upper Cretaceous upper McCoy Mountains Formation, southwestern Arizona

Abstract: A previously unrecognized angular unconformity divides the Jurassic and Cretaceous McCoy Mountains Formation into a lower and an upper unit in the Dome Rock Mountains and Livingston Hills of western Arizona. The lower unit of the McCoy Mountains Formation consists of generally fine-grained quartzose and volcaniclastic strata that were deposited after cessation of Middle Jurassic explosive volcanism. The basal contact of the lower unit is disconformable in most places, but locally it has been interpreted to be gradational with the underlying silicic volcanic rocks. The upper unit is a fining-upward sequence of quartzo-feldspathic and arkosic conglomerate and sandstone that records uplift of a northern source terrane. A tuff in the lower part of the upper unit has a U-Pb crystallization age of 79 ± 2 Ma (Late Cretaceous). Rocks of the lower unit are deformed by pre-80 Ma thrust faults of the generally southward-vergent Maria fold and thrust belt, which bounds the outcrop belt of the McCoy Mountains Formation on the north. The upper unit is exposed only south of the fold and thrust belt. We interpret the intraformation unconformity in the McCoy Mountains Formation to have developed where rocks of the lower unit were deformed adjacent to the southern margin of the Maria fold and thrust belt. The upper unit of the formation is interpreted as a foreland-basin deposit that was shed southward from the actively rising and deforming fold and thrust belt. The apparent absence of an equivalent unconformity in the McCoy Mountains Formation in adjacent California is presumably a consequence of the observed westward divergence of the outcrop belt from the fold and thrust belt. Continued southward shortening deformed the entire formation under greenschist- and, locally, amphibolite-facies conditions soon after the upper unit was deposited. Tectonic burial beneath the north-vergent Mule Mountains thrust system in the latest Late Cretaceous (∼70 Ma) marked the end of Mesozoic contractile deformation in the area.

Seismic modeling of Tertiary sandstone clinothems, Spitsbergen

Abstract: Two-dimensional seismic modelling has been undertaken on an overall progradational succession of sloping mudstone and sandstone units from the Palaeogene of Spitsbergen. The modelling shows that the main geometric features of the section would be resolved at 1500 m depth (with frequencies below 60 Hz, which is common in seismic data at these depths). However, interference between the base and top of lithological units gives lateral amplitude variations and discrepancies between the seismic image and the geometrical model. This is particularly prominent in low-frequency models. Terminations of reflectors, resembling toplap and onlap, may be interpreted, but are artefacts of the general convergence of lithological units present in the geometrical model. The geological section causes a seismic pattern resembling sigmoid progradational seismic facies. Two-dimensional seismic modelling is an efficient tool in bridging the gap between outcrop observations and subsurface data. Hence, modelled outcrop sections are important as reference points' for improved seismic stratigraphic interpretation.

The Triassic-Jurassic Boundary in the Sverdrup Basin, Arctic Canada

Abstract: The Triassic-Jurassic (T-J) boundary has been located within the strata of the Sverdrup Basin through palynological analysis of both outcrop and subsurface sections. In the east the boundary occurs within the Fosheim Member of the Heiberg Formation, which consists mainly of delta-plain strata. In the west the boundary occurs within the marine shales and siltstones of the Grosvenor Island Formation. In all studied sections the boundary appears to be an unconformity which separates Rhaetian strata from Hettangian strata. The boundary is one of a number of low-order sequence boundaries which punctuate the Mesozoic succession of the basin with a frequency of 5-8 m.y. These data support the interpretation that the T-J boundary coincides with a global sequence boundary. Palynological analysis indicates that the spore assemblage changes gradually throughout the Norian and Rhaetian and that a significant and relatively abrupt change occurs across the T-J boundary. The boundary is marked by the termination of common Norian-Rhaetian palynomorphs and by an influx of Jurassic bisaccates and the first appearance of typical Early Jurassic forms. These data support the hypothesis of a mass extinction at or very near the boundary, but the lack of detailed sample control precludes an interpretation of the suddenness (a few versus hundreds of thousands of years) of the extinction. One explanation for the changes in the lithologic and paleontologic characteristics across the T-J boundary is that they were caused by short term environmental stresses (large bolide impact) which were superimposed on longer term stresses and base-level shifts associated with terrestrially-driven tectonic and eustatic changes.

Depositional history of the southeastern outcrop belt of the Chuckanut Formation: implications for the Darrington – Devil's Mountain and Straight Creek fault zones, Washington (U.S.A.)

Abstract: The southeastern outcrop belt of the Eocene Chuckanut Formation contains the erosional remnants of a larger depositional system. In the study area, the Chuckanut Formation can be split into four units based upon differences in age, lithology, sedimentology, paleocurrents, and provenance relationships. The Coal Mountain unit (Early Eocene) represents a southwest-flowing fluvial system that shows no evidence for fault control of drainage. The overlying Higgins Mountain unit (early Middle Eocene) represents a northeast-flowing fluvial system east of the Devil's Mountain fault zone (DMFZ), with lithologies derived from western source areas. The Sperry Peak unit (early Middle Eocene) represents a fluvial system with a wide dispersion of paleocurrent azimuths and a possible mixture of sediment source areas. We believe the Sperry Peak unit was deposited in a fault-wedge graben at the junction of the DMFZ and Straight Creek fault zone (SCFZ), with sediment sources from both east and west. The Grade Creek unit (ag...

Field guide to Cretaceous-tertiary boundary sections in northeastern Mexico

Abstract: This guide was prepared for the field trip to the KT elastic sequence of northeastern Mexico, 5-8 February 1994, in conjunction with the Conference on New Developments Regarding the KT Event and Other Catastrophes in Earth History, held in Houston, Texas. The four-day excursion offers an invaluable opportunity to visit three key outcrops: Arroyo El Mimbral, La Lajilla, and El Pinon. These and other outcrops of this sequence have recently been interpreted as tsunami deposits produced by the meteorite impact event that produced the 200 to 300-km Chicxulub basin in Yucatan, and distributed ejecta around the world approximately 65 m.y. ago that today is recorded as a thin clay layer found at the K/T boundary. The impact tsunami interpretation for these rocks has not gone unchallenged, and others examining the outcrops arrive at quite different conclusions: not tsunami deposits but turbidites; not KT at all but 'upper Cretaceous.' Indeed, it is in hopes of resolving this debate through field discussion, outcrop evaluation, and sampling that led the organizers of the conference to sanction this field trip. This field guide provides participants with background information on the KT clastic sequence outcrops and is divided into two sections. The first section provides regional and logistical context for the outcrops and a description of the clastic sequence. The second section presents three representative interpretations of the outcrops by their advocates. There is clearly no way that these models can be reconciled and so two, if not all three, must be fundamentally wrong. Readers of this guide should keep in mind that many basic outcrop observations that these models are based upon remain unresolved. While great measures were taken to ensure that the information in the description section was as objective as possible, many observations are rooted in interpretations and the emphasis placed on certain observations depends to some degree upon the perspective of the author.

Information derived from soil maps: Areal distribution of bedrock landslide distribution and slope steepness

Abstract: Soil maps can be used to generate maps showing the areal distribution of bedrock. This technique is especially useful in heavily vegetated areas where weathering is intense and outcrops sparse. Broad lithologic categories that are readily distinguished include: diabase/basalt, sandstone, shale, limestone, conglomerate, and hornfels. Using soil maps is not a substitute for field work, but is a valuable tool to aid in making geologic maps. Soil maps can also be used to produce derivative maps showing slope steepness and landslide distribution.

Laminaset geometries in fine grained shallow marine sequences: core data from the Rannoch Formation (North Sea) and outcrop data from the Kennilworth Member (Utah, USA) and the Bencliff Grit (Dorset, UK)

Abstract: Sandbodies from storm-dominated marine and marginal marine environments commonly contain intervals of laminated fine sandstones. A characteristic of such lamination is the presence of low angle cross lamination. In order to model correctly the effects of such lamination on a waterflood of an oil-bearing shoreface sequence it was necessary to quantify the geometry of the laminaset elements. This challenge has been greatly complicated by the lack of outcrop of the formation of interest. The Middle Jurassic Rannoch Formation of the North Sea only occurs in the subsurface where it is not possible in core to measure the aspect ratio of laminasets directly. In this study, the laminaset geometry data that can be obtained from core (e.g. apparent set thicknesses) were collected for the Rannoch Formation. These data were compared with similar data from potential outcrop analogues in (1) the Cretaceous Kennilworth Member of the Blackhawk Formation in Utah, USA and (2) the Upper Jurassic Bencliff Grit from the Dorset coast, UK. A quantitative analysis of laminaset geometries has been used to compare subsurface core with potential outcrop analogues. The Rannoch Formation core is characterized by numerous low angle truncations. We have measured these features in two wells (means of 7.2° and 12.1°). Mean apparent set thicknesses were 0.24 and 0.19 m. In the outcrop sections studied, truncation angles ranged from 9.6° to 13.4° and mean set thicknesses from 0.24 to 0.34 m. Mean bounding surface dips of 5.8° and 8.6°, and mean laminaset lengths of 2.3 and 4.1 m were also measured. directly in the field and by using photomosaics. On the basis of this comparison, the Kennilworth Member in Utah was found to be the most suitable and the geometries (i.e. aspect ratios) measured there were used to generate an appropriate geometry of Rannoch laminaset geometries for use in engineering studies: laminaset length, 2.0 m; laminaset thickness, 0.2 m.

Gravity survey of the Mount Toondina impact structure, South Australia

Abstract: Gravity and seismic reflection data, together with geologic mapping, indicate that the Mount Toondina feature in South Australia is best interpreted as an eroded 4-km-diameter impact structure consisting of a ring structural depression surrounding a pronounced central uplift. Beds at the center of the structure within the central uplift have been raised as much as 200 m from depth and deformed by convergent flow. Seismic reflection data indicate that deformation extends to depths of only approximately = 800 m; at greater depths the reflectors are nearly flat lying, indicating little or no deformation. Gravity data show residual anomalies of +1.0 mGal coincident with the central uplift and a -0.5 Mgal low associated with the ring structural depression. Modeling of the gravity data indicates that relatively high-density material occurs within the central uplift, whereas the ring depression is filled with low-density material. The deformation at Mount Toondina is typical of a complex impact crater; the 4-km diameter is consistent with the expected threshold size for complex craters formed in weak to moderate strength sedimentary rocks.

Water-Rock Interaction During Meteoric Flushing of a Limestone: Implications for Porosity Development in Karstified Petroleum Reservoirs

Abstract: The Lincolnshire Limestone, comprising a succession of Jurassic wackestones, packstones, and oolitic grainstones, forms an important carbonate aquifer in eastern England. Meteoric waters enter at outcrop and penetrate between confining strata at least 25 km down-dip. This water dissolves and interacts with the limestone, and even water samples collected at or near outcrop are calcite-saturated. Net limestone dissolution is thus a process that is most dominant in the near-surface environment. Water samples taken at increasing distances from outcrop have increasing Sr and Mg contents, and 13C values of dissolved bicarbonate increase from -15 to -8 (PDB), while 87Sr/86 falls from 0.7082 to 0.7077. These data are interpreted to be the result of a coupled carbonate dissolution-reprecipitation process. Modeling of the C and Sr isotopic data indicates that primary Jurassic carbonate (ooids, micrite, fossils) is dissolving preferentially over burial cements. Isotopically light bulk-rock carbon near joint surfaces suggests that reprecipitation of calcite in the form of cement could be concentrated preferentially in and near joints. The Lincolnshire Limestone may be used as an analogue for karstified petroleum reservoirs, specifically those which have been buried and lost their unstable carbonate minerals (aragonite, high-Mg calcite) prior to uplift and karstification. The present water ch mical data suggest that, in such reservoirs, influx of meteoric water at an unconformity creates porosity and enhances permeability through limestone dissolution, but this may be concentrated close (tens of meters) to the unconformity. This dissolution also promotes surface erosion, which limits the thickness of preserved karstified limestone. The dissolution-reprecipitation process that occurs at greater distances from outcrop has effects that are far more widespread (lens of kilometers), leading to an increase in microporosity (e.g., micrite leaching) and a possible reduction in joint or fracture porosity. Such factors should be taken into account when exploring for, and appraising, karstified petroleum reservoirs.

Age and depositional environment of the Rock River coal basin, Yukon Territory, Canada

Abstract: Poorly exposed Late Eocene strata in the Rock River basin, 115 km northeast of Watson Lake, accumulated in an intermontane valley with a geometry and history controlled by subsidence associated with the Rock River Fault. The sequence, as seen in one outcrop and five borehole sections, is dominated by drab mudrocks with minor sandstones and some thick lenses of coal. The mudrocks accumulated in floodplain marsh and pond settings associated with a low-gradient, possibly anastomosed, fluvial system. River banks were stable owing to the abundance of plant roots in the channel walls. Although channel sandstone and conglomerate were not identified in the core, the abundance of coarsening- and fining-upwards sets of sandstone of splay origin indicates pronounced levee development. Woody coals accumulated in areas well away from the main channel, in a series of elongate forested swamps, which were periodically inundated by flood water.The overall palynological assemblage is typical of the Eocene and Early Oligoce...