Periods Of Subaerial Exposure Research Articles

Silurian evaporitic strata from New South Wales, Australia

During Silurian (Late Wenlockian to Ludlovian) time in the Cudgegong district, New South Wales, a minor regression is represented by local influx of clastic detritus and a brief period of subaerial exposure within a phase of carbonate deposition. Oolitic shoals, formed under shallow warm hypersaline marine conditions, became exposed on supratidal flats where minor dolomitization and gypsum crystallization took place. During diagenesis the gypsum was pseudomorphed by chlorite. Ooids and evaporite minerals have not been previously recorded from Silurian strata in eastern Australia but their occurrence at Cudgegong concurs with a palaeolatitude of approximately 5°N on the basis of geophysical data.

Depositional Environments and Diagenesis of Lower Pennsylvanian Caddo Formation, Stephens County, Texas: ABSTRACT

Since its discovery in 1916, the Lower Pennsylvanian Caddo limestone has been one of the most prolific producing intervals in the central Texas region. Favorable reservoir characteristics, with porosities from 9 to 24% and permeabilities from 0.1 to 1,000 md in the pay interval, have enabled many oil companies to initiate successful secondary recovery waterflood projects. Since its discovery, more than 200 million bbl of oil has been recovered from this formation in Stephens County alone. The Caddo reef trend developed during the Early Pennsylvanian, generally along the axis of the Bend flexure. Close examination of whole core and thin sections reveals the formation to be a series of small reef mounds characterized by wackestones. The wackestones consist of green algae, brachiopods, and some rugose corals, which are found on the leeward side (back reef). This sequence is commonly found in the top segment of the Caddo. Deeper into the formation the sequence grades into a packstone/wackestone, characteristic of an oceanward margin (fore reef), and is distinguished by red algae, foraminifera (fusulinids and miliolids), and crinoid hash. The basal sequence is a series of alternating lime mudstones and shales that are heavily silicified. Diagenesis is the key to reservoir rockmore » quality. Moldic porosity was created through dissolution of organisms composed of low magnesium calcite (brachiopods) and aragonite (corals) during periods of subaerial exposure of the back reef. During this period of low sea level, fore-reef sediments were partly cemented by CaCO{sub 3} and were not subaerially exposed. Therefore, moldic porosity was not created in the fore-reef sediments. Porosity in the fore reef was later created in places by dissolution of this cement. Secondary dolomite was later precipitated along pore walls, and in places it occluded porosity in the pore spaces.« less

Evolutionary Importance and Economic Potential of Mesozoic Unconformities of Levant: ABSTRACT

Six unconformities are discerned in the Mesozoic section of Israel and adjacent countries. Four unconformities (mid-Triassic, Late Triassic, Late Jurassic, and Aptian-Albian boundary) are genetically related to global sea level drops and divergence of the Tethys. The divergent processes are associated with faulting, volcanism, and clastic accumulations along the passive margins of the Arabian craton. The Late Cretaceous unconformity is associated both with global sea level drop and convergence of the Tethys. The late Middle Jurassic unconformity is related to a global eustatic low without any apparent tectonic activity in the region. All the unconformities are widely distributed and, except the one at the Jurassic-Cretaceous boundary, mark only short periods. Therefore, they serve as markers and correlation guides for the geologic evolution of the Middle East. Their economic importance can be summarized as follows. (1) The tectonic setting of the blocks overlying End_Page 255------------------------------ the unconformity plane and the duration of subaerial exposure periods associated with meteoric flushing and karst development is of prime importance for petroleum exploration. The Late Jurassic-Early Cretaceous angular unconformity brings into juxtaposition a deep Paleozoic aquifer and an overlying Mesozoic aquifer. Problems of salinity, reserves estimate, and exploitation arise. Karstic phenomena that developed during the Late Cretaceous exposure time contribute to the excellent storage capacity of the Cenomanian-Turonian aquifer. (3) Alumina-rich lateritic soils, formed under subaerial tropic conditions at the Triassic-Jurassic boundary, are used by the ceramic industry. End_of_Article - Last_Page 256------------

Depositional and Diagenetic Relationships Between Gulf of Elat (Aqaba) and Mesozoic of United States East Coast Offshore

The tectonic setting and depositional environments along the ancient margin during the Mesozoic of the East Coast of the United States may have been similar to those in the present Gulf of Elat (Aqaba). The Gulf of Elat (Aqaba) is the northern continuation of the Red Sea rift zone, where carbonates are accumulating contemporaneously with clastics under arid conditions. Clastics are deposited primarily in alluvial-fan complexes which spill out onto a narrow shelf. Carbonate deposits, including reef complexes, occur along the shelf break. In comparison, along the United States East Coast margin, Triassic to Early Jurassic rifting coincided with carbonate and clastic deposition. The Middle Jurassic and Cretaceous strata reflect the transition from synrift to postrift stages f tectonic evolution. Seismic and well data of the Jurassic and Cretaceous sections show an increase in the carbonate content approaching the United States East Coast shelf break. A discontinuous belt of carbonate buildups is inferred along the United States East Coast shelf and slope. The Gulf of Elat (Aqaba) represents an analogous model for the Early Jurassic East Coast margin of the United States. Beginning in the Middle Jurassic, the East Coast shelf-edge carbonate sequences were being deposited on a passive margin. Carbonate growth may have outpaced subsidence and may be similar stratigraphically to the Gulf of Elat (Aqaba) where carbonate deposits are prograding seaward. In the Gulf of Elat (Aqaba), calcium carbonate cementation has significantly reduced the porosity (28%) and permeability (0.01 md) of still-submerged carbonate reefs. However, Pleistocene carbonates on uplifted blocks in the adjacent onshore have undergone leaching and development of secondary porosity as a result of dissolution by meteoric fresh water. The uplifted carbonates contain high secondary porosity (60%) and permeability (10,000 md). Emergent carbonate strata exposed to percolating meteoric waters generally develop such high porosities. In the subsurface, former emergent surfaces are recognized as unconformities; among examples of Mesozoic carbonate reservoirs formed during subaerial exposure and marked by unconformities are the Casablanca (Upper Jurassic, Spain) and Golden ane (Cretaceous, Mexico) oil fields. Unconformities in the Mesozoic carbonate rocks of the United States East Coast suggest that potential carbonate reservoirs (one Jurassic and one Cretaceous) formed during periods of subaerial exposure. Hydrocarbons generated during the Early or Late Cretaceous may have moved into the reservoirs prior to deep burial, possibly inhibiting further subsurface diagenesis, including porosity and permeability changes, and preserving the reservoirs.

Quaternary Depositional Episodes, Western Spitsbergen, Norway: Aminostratigraphy and Glacial History

Quaternary events interpreted from deposits along the central west coast of Spitsbergen are subdivided and correlated by amino acid enantiomeric ratios in enclosed molluscan fossils. Deposition is characterized by relatively short episodes of rapid sedimentation associated with glacial activity and high relative sea level. Depositional episodes are separated by relatively longer periods of subaerial exposure and pedogenesis. At least five major depositional episodes, each terminated by an emergence cycle, are identified by characteristic isoleucine epimerization ratios (aIle/Ile) in Mya truncata and Hiatella arctica. Absolute age limitations are derived by calibrating the aIle/Ile ratios against radiometrically dated samples and diagnostic faunal communities. Glacial activity and sea-level change are inferred from the facies relationships within each depositional episode. The oldest recognized unit, episode H, is characterized by aIle/Ile Total ratios of 0.15 ? 0.02. At least three emergence cycles and two distinct glacial events are recorded within deposits of this episode. The absolute age is estimated to be between 300 and 1000 ka. The next younger episode, D, is represented at only one site, although reworked shells with the same ratios were also found. Total aIle/Ile ratios are ca. 0.08 and the age is estimated to be between 170 and 480 ka. Deposits of the three youngest depositional episodes, C, B, and A, are widespread throughout the study area, and are characterized by Total aIle/Ile ratios of 0.051 ?0.006, 0.033 ?+0.0022, and 0.019 ?0.001, respectively. Episode C deposits represent the last major regional glaciation of the area, an event estimated to have occurred between 90 and 260 ka ago. Episode B deposits are associated with a restricted regional glaciation estimated to have occurred between 40 and 130 ka ago. The associated fauna indicates marine conditions as warm or warmer than present. Episode A deposits are related to a latest Weichselian/earliest Holocene marine transgression/regression cycle during a period of restricted local glacier expansion, ca. 9 to 12 ka ago. These findings place constraints on the timing and extent of Spitsbergen/Barents Sea glacial events. The 18 ka BP open-ocean 6180 minimum is not associated with extensive glacierization of the Forlandsund region of western Spitsbergen, nor did outlet glaciers of the postulated Barents Sea ice sheet reach the study area in the last 40 ka or more.

Depositional Environments and Diagenesis of Frobisher-Alida Interval, Madison Group (Mississippian, North Dakota Williston Basin: ABSTRACT

The Frobisher-Alida interval is a well log-marker-defined interval, not lithologically based, which extends across middle and upper Mission Canyon carbonates into lower Charles evaporite from the basin center toward the eastern basin margin. The interval represents a regressive sequence with superimposed minor transgressive pulses. A wide spectra of depositional and diagenetic environments with associated fabrics and porosities are represented in these carbonates. Basin-margin carbonates are dominated by supratidal sediments with lesser amounts of intertidal and subtidal sediments. Major fabric types of important producing zones consist of major developments of hypersaline, and minor development of caliche and vadose, ooid and pisolite wackestones to grainstones. Intercalated with these are fenestral mudstones to laminated dolomudstones, gastropod mudstones, and evaporite mush dolomudstones. In addition, desiccation cracks and minor occurrences of karst breccias and laminated crusts indicate periods of subaerial exposure. Porosities associated with the supratidal-subaerial realm include well-developed interparticle, fenestral, vuggy, and intercrystal types, and in places are filled with clear spar and/or void-filling clear anhydrite cement. > Basinward, subtidal sediments dominate the interval. These consist of cyclic, shallow open-marine bioclastic mudstones to packstones-grainstones and, partly restricted marine burrowed peloidal mudstones to bioclastic wackestones. Upsection, non-cyclic burrowed bioclastic peloidal mudstones to wackestones indicate increasingly restrictive conditions. Interparticle porosity is exhibited in the bioclastic grainstones. Intercrystal porosities are developed from selective dolomitization of burrows and mudstones. End_of_Article - Last_Page 614------------

Tidal Deposition in the Basal Upper Cambrian Mt. Simon Formation in Wisconsin

The Upper Cambrian Mt. Simon Formation (0-65 m thick) is a basal quartz arenite exposed in westcentral Wisconsin. A detailed field investigation of the physical and biogenic sedimentary structures of the Mt. Simon has led to the recognition of three distinct lithofacies. The lower one unconformably overlies Precambrian basement rocks. It consists of medium- to very large-scale sets of tabular and trough cross-bedded, medium- to very coarse-grained sandstone and pebbly sandstone with minor intercalated horizontal beds of very fine- to medium-grained sandstone, siltstone, and shale. Sparse examples of Skolithos and Arenicolites are present. This facies consists of a very thin sequence of possible braided-fluvial and marine foreshore deposits, overlain by probable marine shoreface and tidal channel deposits. Much of the facies seems to represent shallow subtidal deposition in a relatively high-energy regime. The middle lithofacies consists of two distinctly different subfacies, which probably were deposited in a low tidal flat setting. The higher-energy subfacies consists of small- to medium-scale sets of tabular and trough crossbedded, fine- to coarse-grained sandstones containing distinct zones dominated by Skolithos and Arenicolites . This subfacies probably represents deposition in meandering tidal channels. The lower-energy subfacies consists of thinbedded, horizontally-laminated and ripple cross-laminated, very fine- to medium-grained sandstone, siltstone, and shale, with common specimens of Cruziana, Rusophycus, and Planolites . This subfacies probably represents deposition on lower-energy tidal flats adjacent to the tidal channels. The upper lithofacies consists predominantly of structureless, densely bioturbated, very fine- to coarse-grained sandstone containing abundant specimens of Skolithos . The upper few meters of the facies consists of small- to medium-scale sets of trough cross-bedded, very fine- to coarse-grained sandstone with layers of disarticulated valves of the brachiopod Obolus . The upper facies probably represents deposition on tidal flats, perhaps in a midtidal flat setting, characterized by slower sedimentation rates, a correspondingly higher degree of bioturbation, persistent reworking of shelled macrobenthos, and periodic subaerial exposure. The Mt. Simon Formation is interpreted as a largely progradational (regressive), shoaling- and fining-upward tidal sequence. A marine interpretation is supported by the widespread occurrence of marine trace fossils within this unit. Evidence for a tidal origin is seen in the presence of unimodal cross-strata associated with reactivation surfaces, compound cross-strata, numerous scour and truncation surfaces lined with intraformational conglomerates, common clay drape laminae separating sets of cross-strata, interference and flat-topped ripple marks, and desiccation cracks. Sedimentation continued without apparent interruption as the overlying Eau Claire Formation was deposited. also under tidal influence. Recent reinterpretations of other basal Cambrian cratonic quartz arenites, together with this new interpretation for the Mt. Simon Formation, suggest that the long-held concept of basal transgressive sandstones deposited as blankets across the craton may be too simplistic, for deposition in braided-fluvial, marginal marine (tidal flat-tidal channel), and marine foreshore and shoreface environments seems indicated.

Effect of Sea Level Change on Shelf-Slope Boundary: ABSTRACT

Eustatic sea level changes cause alternating periods of subaerial exposure, flooding, or progradation at the shelf-slope boundary. Eustatic falls of sea level that are more rapid than subsidence cause subaerial exposure and canyon cutting. Rises of eustatic sea level coupled with subsidence commonly cause flooding of the shelf margin, marine transgression, and sediment starvation of the middle and outer shelf. Stillstands or slow falls of eustatic sea level that are less than the rate of subsidence commonly cause marine regressions where, in many places, the shoreline progrades out to the shelf-slope boundary. The effect of eustatic sea level changes on the shelf-slope boundary is readily observable on seismic data despite changes in the rate of subsidence and rate and type of deposition. Seismic and well data from offshore northwest Africa are used to demonstrate these relations. In this area, high subsidence rates followed the opening of the Atlantic in the Early Jurassic and gradually changed to very slow subsidence rates in the Tertiary. Depositional rates increased throughout the Jurassic, reaching a maximum in the Early Cretaceous. Rates of deposition were very low in the Late Cretaceous and Early Tertiary and increased again in the late Tertiary. Sediment type changed from primarily carbonate in the Jurassic to sands, silts, and shales in the Cretaceous and Tertiary. This and other examples demonstrate that sea level change is the major factor affecting the shelf-slope boundary in different tectonic settings. The only known cause for the high rates of sea level change determined in these studies is glaciation. Such studies indicate major phases of glaciation occurred periodically throughout the Phanerozoic. Timing of possible glacial periods is shown for the Jurassic and Tertiary. End_of_Article - Last_Page 1003------------

Depositional Environments of Lacustrine Limestones and Their Relation to Major Coal Seams, Upper Pennsylvanian of West Virginia: ABSTRACT

In the Monongahela Group, lacustrine limestones are intimately associated with important coal deposits as evidenced by (1) their common biogenic origin within embayments between clastic deltaic lobes, (2) the interbedding of thick limestones and minable coal seams, and (3) their paleogeographic proximity. A preliminary study of the lower Monongahela Group in the northern coal basin of West Virginia shows that the seemingly monotonous limestones are actually variable in terms of both paleontology and petrology. Freshwater fossils in these beds include micro- and macro-invertebrates (darwinulid and cypridid ostradoces, mollusks, and abundant Spirorbis), vertebrates (fragments of amphibians and fish), and megaplants, and calcareous algae. These rocks are classified as finely laminated micrite, pelmicrite, and intramicrite. Sedimentary structures such as mud cracks, desiccation chips, caliche, and bird's-eye all indicate frequent drying. Water-level fluctuations that commonly affected the lakes, as interpreted from limestones, also affected nearby coal swamps in terms of plant types, water chemistry, and degradation rates. These then determined coal lithotype and ash content. During the periods of subaerial exposure, rare gypsum crystals were precipitated interstitially in the carbonate mud, and the concentration of minerals like gypsum may be one source of sulfur for pyrite in adjacent high-sulfur coals. Hence, the understanding of depositional environments in the lacustrine limestones provides further insights into details of coal formation. End_of_Article - Last_Page 800------------

Comparison of Shallow-Marine Shelf Carbonate Mounds of Fort Payne Formation (Lower Mississippian) of Tennessee with Waulsortian Mounds of Western Europe: ABSTRACT

Although Fort Payne and Waulsortian carbonate mounds are similar, differences in sedimentary and diagenetic environments account for the development of porosity, permeability, and emplacement of petroleum in producing Fort Payne mounds. Waulsortian-type mounds, including the Fort Payne mounds, are characterized by: (1) similar mound morphology and alignment of mound trends, (2) distinctive microfacies and limited faunal diversity, and (3) limited stratigraphic and widespread geographic occurrence. European Waulsortian mounds, exposed on the surface, range up to 300 m thick, are not generally porous, and contain little evidence of hydrocarbons. They occur on relatively rapidly subsiding shelf margins, at intermediate water depths, and were not subaerially exposed during buildup. Fort Payne limestone mounds are low relief features (30 m thick) with initial dips less than 1°, containing primary and secondary porosity in bryozoan grainstones in multiple zones within dominant mud-supported microfacies. Mounds formed on a shallow slowly subsiding shelf (ramp) during transgression. Variation in rate of subsidence or eustatic sea-level changes are recorded either by minor zones bearing normal marine fauna or by zones bearing shallow-marine dolomite and evaporites. Periodic subaerial exposure and descending, dissolving, meteoric water probably enhanced secondary porosity development. Minor, but complex, synsedimentary solution collapse, fracturing, and brecciation during mound compaction occurred above evacuated evaporites, solution seams, and stromatactis cavit es. After burial by Warsaw Formation clastics, subsurface (mesogenetic) diagenesis is documented by postlithification compaction features including stylolites, microstylolites, and multiple-fractured breccias. Pore-fill spar occlusion of some porosity, and emplacement of petroleum in permeable reservoirs were late mesogenetic events. End_of_Article - Last_Page 743------------

Quaternary rivers on the New Jersey shelf: Relation of seafloor to buried valleys

The Quaternary evolution of the stream net on the New Jersey shelf has been interpreted on the basis of bathymetric maps and also by means of seismic profiling, with somewhat different results. Maps show the most recent positions of seafloor shelf valleys, but these valleys may have been created by retreating estuary mouths rather than by subaerial stream erosion. Seismic profiles reveal buried valleys of subaerial fluvial origin, which may follow courses that diverge markedly from the trends of associated seafloor valleys. Shelf valleys must be understood in the context of erosional shoreface retreat, a process that largely remade the shelf surface during successive Quaternary transgressions. Most shelf landforms are marine and post-transgressional in origin, having been formed at the foot of the shoreface. Only very large and deeply incised subaerial landforms survive the shoreface-retreat process. The marine landforms that tend to replace or bury subaerial river valleys include shelf valleys created by estuary-mouth scour, shoal-retreat massifs, and shelf deltas. Three distinct shelf valley sets, including both seafloor and buried valleys, are attributable to the ancestral Delaware, Great Egg, and Hudson Rivers, respectively. Individual valleys within valley sets may follow markedly divergent paths. In the case of the Hudson, the estuary retreated up a deeply incised river valley and was confined by it; the shelf valley is a river valley only partially filled by estuarine deposits. In the case of the other two rivers, the estuary mouths became largely decoupled from the underlying river valleys during the transgression, and their retreat paths do not everywhere overlie the buried channels. In each valley set, divergent buried valleys apparently belong to periods of subaerial exposure of the shelf that occurred earlier in Holocene time.

Porosity in Giant Gas Field, Ellenburger Formation, Puckett Field, Pecos County, Texas: ABSTRACT

The Lower Ordovician Puckett field has produced nearly 2.5 Tcf of gas from the Ellenburger Dolomite since the discovery of the field in 1952 by Phillips Petroleum Co. Production is from a depth interval of approximately 12,000 to 15,000 ft (3,600 to 4,500 m), and the estimated ultimate recovery is 3.3 Tcf. The Ellenburger facies are interpreted to have been deposited in several major environmental settings--subtidal, intertidal-channel belt, and supratidal. Subtidal deposition is represented by burrowed, irregularly laminated mudstones and wackestones and by oolitic grainstones. In the intertidal-channel belt, intraclastic packstones and stromatolitic boundstones accumulated. Laminated mudstones and algal-laminated mudstones were deposited on the supratidal flats in which desiccation produced mud cracks and thin layers of flat-pebble conglomerates. During Ellenburger sedimentation there were many periods of subaerial exposure which resulted in formation of soil zones and karst terranes as deep as 20 ft (6 m). Solution collapse produced thick brecciated zones. Maximum porosity in the reservoir is 12% and the greatest permeability is 117 md. Porosity originated dominantly from tectonic and karst fractures and karst vugs. The generally low porosity is locally enhanced by intercrystalline, moldic, and interparticle porosity. The greatest porosity and permeability is commonly in the facies of the supratidal and intertidal environments most affected by tectonic fractures and by soil and karst development. End_of_Article - Last_Page 488------------

Porosity Examples from IAB Field, Coke County, Texas: ABSTRACT

IAB field produces oil from an Upper Pennsylvanian (Cisco-Canyon) carbonate buildup situated on the western edge of the Eastern shelf. The reservoir is approximately 6 mi (9 km) long, 2 mi (3 km) wide, and 300 to 1,000 ft (100 to 300 m) thick. The upper surface of this buildup is irregular with several pinnacles, probably caused by subaerial erosion that occurred during drops in sea level. From examination of cores, 10 lithofacies have been identified and divided according to texture, sediment types, fossils, and mineralogic composition. Facies recognized are subtidal normal-marine deposits, ranging from relatively quiet-water skeletal wackestones to relatively high-energy, shallow-water oolitic grainstones. Porosity consists of both primary and secondary types and is controlled by depositional fabric, dolomitization, and freshwater diagenesis. Five main porosity types are (1) primary interparticle and (2) primary intraparticle porosity, both in grain carbonate rocks, (3) secondary leached grain and moldic porosity in grain carbonate rocks, created during a drop in sea level, (4) secondary vug porosity in muddy or very fine-grained carbonate rocks, also created during a drop in sea level, and (5) secondary intercrystalline and vug porosity in dolomites. Muddy carbonate beds had low initial effective porosities because of poor sorting, whereas grain-rich carbonate beds had high initial effective porosities owing to good sorting. Following deposition, sea level probably dropped, and the buildup was exposed to freshwater vadose and phreatic diagenesis. Solution of various grains and simultaneous precipitation of equant calcite cements produced secondary leached porosity and reduced primary interparticle and intraparticle porosity. Porosity preserved during this period of subaerial exposure and freshwater diagenesis was probably later reduced during deeper burial by additional cementation and compaction. End_of_Article - Last_Page 457------------

Porosity Preservation and Early Freshwater Diagenesis of Marine Carbonate Sands: ABSTRACT

Observations from Holocene and Pleistocene limestones of south Florida and the Bahamas provide the basis for a general outline of freshwater alteration of marine, primarily aragonitic, sands. Porosity and mineralogic data suggest that metastable carbonate recrystallization takes place before significant porosity loss. The outline proposes the following main points: (1) porosity is only slightly modified during mineralogic stabilization and early cementation; (2) secondary porosity development during early cementation preserves overall porosity; (3) early cements formed during metastablephase recrystallization are almost entirely autochthonous on a reservoir scale. The time of stabilization may be as short as 104 years but may be slowed for long periods (e.g., b salt water intrusion, dry vadose conditions, stagnant water). Major porosity reduction occurs after stabilization and takes longer periods of time > 106 years (e.g., extended period of subaerial exposure, burial diagenesis). Our observations of the effects of early freshwater diagenesis underscore the importance of later diagenetic events in porosity reduction of limestones. This outline suggests that reservoir limestones developed through early freshwater diagenesis of aragonitic sands should be characterized by high porosity, most of which is secondary. Conversely, high-porosity reservoirs with considerable amounts of primary pore space have either escaped pervasive freshwater diagenesis or are developed in sediments that were originally calcite. End_of_Article - Last_Page 460------------

Evolution of Yucca Mound Complex, Late Pennsylvanian Phylloid-Algal Buildup, Sacramento Mountains, New Mexico

The Late Pennsylvanian Yucca Mound complex is a carbonate buildup on the western side of the Sacramento shelf facing the Oro Grande basin, in south-central New Mexico. This complex is an accumulation of algal and foraminiferal boundstones and phylloid-algal packstones and wackestones which have been subjected to several periods of subaerial exposure. The growth history of this organic complex is related closely to late Paleozoic cyclic sedimentation in a tectonically active area, and is thought to be the result of progressive offlapping growth stages down the western flank of the active La Luz anticline. The oldest beds exposed in the complex (plumose masses of algal and foraminiferal boundstone) formed within wave base on the crest of an earlier organic buildup. The crest of the older buildup was subaerially exposed while a thick accumulation of carbonate mud replete with the phylloid algae, Ivanovia, started forming against its western (seaward) flank. As sea level rose and submerged the earlier mound, the phylloid-algal accumulation kept pace with sea-level rise and onlapped the older mound, despite a few brief interruptions. When the phylloid algae built up into wave base, and attained its climax stage of evolution, a typical shallow-water facies developed. This distinctive rock type on the preserved, rounded mound top contains abundant forams, both encrusting and mobile chambered forms, together with fish remains and conodonts. A bedded detrital flank facies, composed of broken phylloid-algal material, and concentrated remains of forams, fish, and conodonts that inhabited the upper mound, is present on the shelfward (lee) side of the complex. Abundant, algally coated skeletal grains characterize the flank facies, which is easily recognizable elsewhere within the area, mainly in mound-flanking beds. When sea level dropped, Yucca Mound growth ceased, and smaller boundstone accumulations were formed down the offshore flank of the main mound complex. The varied growth history of this particular organic buildup demonstrates just how much opportunity existed for subaerial and littoral diagenetic processes to act on and to modify the varied rock facies common in late Paleozoic phylloid-algal buildups.

Rudistid Fringing Reefs of Cretaceous Shallow Carbonate Platform of Israel

Rudistid reefs are well developed in Albian-Turonian strata in Israel. These strata are divided laterally into shallow-shelf carbonate rocks (the Judea Limestone) deposited on the Arabian craton and fine-detrital carbonate deposits (the Talme Yafe Formation) on its margins. The main rudistid belts, developed during three depositional cycles, fringed the outer margins of the shelf platform. The morphologic features of the reef belts such as their biohermal shape, limited lateral extension, thick vertical continuity (135-350 m), and steep inclined fore-reef beds (25-35°) indicate the rudistid colonies developed into topographic wave-resistant structures on a steep sloping bottom facing the open sea. In contrast to recent coral reefs, the rudistid reefs are almost completely barren of encrusting organisms. Early submarine cementation is indicated in some void fillings. The stability of rudistid reefs was achieved mainly through the simultaneous growth of individual organisms in crowded interlocking and mutually cemented populations and was less the result of attachment to a firm substrate. Ecologic adaptation to environmental conditions such as depth, energy regime, salinity of the water body, and periodic subaerial exposure gave rise to lateral zonation of the rudistids in two different stratigraphic populations. Albian-Cenomanian Eoradiolites and Turonian Ichtiosarcolites formed the main reef belts fringing the shallow and hypersaline platforms, whereas Albian-Cenomanian Caprina and Caprinula and Turonian Radiolites and Vaccinites formed small patch reefs in deeper water of normal salinity.

Post-Depositional Subaerial Weathering Effects on the Mineralogy of an Upper Cretaceous Sand in Southeastern United States

ABSTRACT The Upper Cretaceous Cusseta Sand, which crops out in east-central Alabama and west-central Georgia, occurs frequently as cuestas. The lithologies range from a clayey sand to a fine- to a coarse-grained, moderately well sorted sand. Because of their topographic position above the water table, their high permeability, and an extensive period of subaerial exposure, these sediments have been subjected to intensive leaching. Compositions and distributions were determined for heavy and light minerals, and clay minerals from channel samples at five selected outcrop profiles. In the unweathered lower portion of each outcrop profile, the dominant nonopaque heavy minerals are rutile, tourmaline, zircon, staurolite, kyanite, garnet, epidote, and sillimanite. Ilmenite is the most prominent opaque heavy mineral. Among the light minerals, potassium feldspar accounts for as much as 6% of the sand fraction. The predominant clay minerals are montmorillonite and kaolinite in order of importance. In all of the sections, the following changes take place from bottom to top: (1) reduction in the percentages of feldspars and some heavy minerals, particularly garnet; (2) ilmenite is partly altered to leucoxene; (3) the X-ray diffraction peak for the 17 A montmorillonite shortens and broadens. These changes in mineralogy are not the result of changes in provenance, changes in source-area climate, or abrasion during transport, but are attributed to intense post-depositional subaerial weathering. In the light of post-depositional subaerial weathering and intrastratal solution re-evaluation of the significance of mineral suites from Cretaceous and Tertiary sediments in the eastern and southeastern United States is required for interpretations of their provenance and reconstruction of the paleoclimates.

Early Cretaceous Stuart City Shelf Margin of South Texas: Its Depositional and Diagenetic Environments and Their Relation to Porosity: ABSTRACT

The Stuart City trend, South Texas, represents a climax biogenic development along the Early Cretaceous (late Aptian, Albian, and early Cenomanian) shelf margin. Landward of this trend, a wide variety of shallow-water shelf carbonate sediments accumulated on a broad, relatively flat platform. Seaward, the entire section consists of dark planktonic foraminifer-bearing argillaceous carbonate sediments. The sediments of the Stuart City trend make up the Stuart City limestone, which attains a total thickness of 2,000 to 2,500 ft. Time-equivalent rocks which crop out in central Texas are the Glen Rose and Edwards Formations. Between 1954 and 1961 many wells ranging in depth from 11,000 to 20,000 ft were drilled with the Stuart City Formation as their final objective. Of the 19 wells from which cores were obtained for this study, 12 were considered gas wells with initial production ranging from 1.5 to 36.5 MMCFGD. Six of these wells still produce gas. Depositional facies and environments and their relation to the diagenesis and porosity development provide a model for further hydrocarbon exploration along the Stuart City and the deeper Sligo trends. The Stuart City carbonate rocks have been assigned to five major environments of deposition: shelf lagoon, shelf margin, upper shelf slope, lower shelf slope, and open marine. The shelf-lagoon facies include miliolid wackestone, mollusk wackestone, toucasid wackestone, and mollusk-miliolid grainstone. These facies accumulated in generally low-energy condition in water depths from 0 to 20 ft. In contrast, the narrow band of shelf-margin carbonate rocks is made up of coral-caprinid boundstone, requienid boundstone, and rudist grainstone, all of which accumulated in moderate to high-energy conditions and in less than 15 ft of water as a complex of reefs, banks, bars, and islands. Seaward of the shelf margin, the upper shelf-slope environment comprises the caprinid-coral wackestone and co al-stromatoporoid boundstone facies, the lower shelf slope, the intraclast-grainstone, echinoid-packstone, and echinoid-mollusk-wackestone facies. Farther seaward in water depths greater than 60 ft, the open-marine environment is represented by the planktonic-foraminifer wackestone basins. Porosities in the carbonate rocks of the Stuart City trend are divisible into two main types, those which are fabric related and End_Page 2206------------------------------ those which are nonfabric related. Primary fabric-related types consist of intraparticle and interparticle. Primary intraparticle porosity, openings within the body chambers of the rudists, is present in the caprinid-coral wackestone, coral-caprinid boundstone, and requienid boundstone. Primary interparticle porosity was originally very high (greater than 30 percent) in the rudist grainstone facies but cementation soon after deposition--submarine, phreatic, and meteoric--reduced the porosity to less than 10 percent, and late subsurface cementation filled the remaining porosity. Primary interparticle porosity now is present only in a few very thin intervals. Secondary fabric related porosity consists of solution-enlarged interparticle and moldic. Both occur in the boundstone and grainstone facies but in very thin restricted units. The poor development of solution enlarged interparticle and moldic porosity reflects the minor role that subaerial exposure played during the development of Stuart City trend. Nonfabric related porosity consists of vertical fractures. This type of porosity is present in abundance in several studied wells in the form of open, nonlined, vertical fractures. The low porosities along the Stuart City Trend are the result of two processes--(1) lack of significant periods of subaerial exposure for development of secondary porosity types and (2) massive cementation which destroyed primary porosity. Further exploration along this trend should be aimed at identifying areas which may have been exposed soon after deposition and developed secondary porosity or areas which subsided more rapidly and have preserved primary porosity. End_of_Article - Last_Page 2207------------

Carbonate Facies in Ordovician of Northern Arkansas

The Ordovician carbonate sequence in northern Arkansas consists, in ascending order, of the Cotter, Everton, Joachim, Plattin, Kimmswick, and Fernvale formations. A comparison of the rock types, sedimentary structures, and faunal assemblages of these rocks with those of recent carbonate sediments indicates that these ancient rocks may be subdivided into lithofacies that are similar to modern supratidal, intertidal, and subtidal facies. Further comparison of these lithofacies with an epeiric sedimentation model shows that (1) the Cotter Dolomite, Joachim Dolomite, and Plattin Limestone were deposited chiefly in supratidal and intertidal environments, (2) the Everton Formation was deposited chiefly in an intertidal environment, and (3) the Kimmswick and Fernvale Limestones ere deposited in protected to open subtidal environments, above wave base. Two major unconformities in the sequence, each overlain by a sandstone (Kings River Sandstone Member of the Everton Formation and St. Peter Sandstone), probably are the result of the widespread epeirogenic movements that initiated development of the sub-Tippecanoe unconformity. Earlier workers have reported paraconformities and microdisconformities within the post-St. Peter sequence that supposedly represent periods of subaerial exposure and erosion. Our premise is that these surfaces were integral to the sedimentary environments and were, in fact, developed by shifts in laterally adjacent, coexisting facies; these shifts were not related to uplift or to subaerial exposure of preexisting rocks.

Biogenic and Sedimentary Features Associated with Jurassic Carbonate Tidal Channel, West Pakistan: ABSTRACT

Exposures of steeply dipping Jurassic limestones in the Surghar Range, West Pakistan, allow detailed observations of various biogenic and sedimentary features. Thin-bedded dolomites, desiccation cracks, scour-and-fill structures, and shallow-water benthonic Foraminifera, within a dominant lithology of pellet carbonate grainstones and mudstones, indicate very shallow carbonate shelf conditions at the time of deposition. Periods of subaerial exposure, and subsequent lithification are indicated by the presence of oriented bivalve borings, both on surfaces which have been desiccated and on one surface which has been corraded by tidal currents. The latter exposure surface contains pebbles of identical lithology within the reticular gullies. Encrusting oysters are common. Subma ine cementation of the surfaces is discounted because of the absence of inverted borings, superimposed bored surfaces, and extensively bioturbated sediment. End_of_Article - Last_Page 366------------