Carbonate Bank Research Articles

Seismic Models of a Shelf-margin Depositional Sequence: Upper San Andres Formation, Last Chance Canyon, New Mexico

ABSTRACT The seismic resolution of stratal geometries and facies distributions observed in San Andres Formation (Permian) outcrops in Last Chance Canyon, Guadalupe Mountains, New Mexico, is studied by seismic modeling of a published, detailed stratigraphic cross section. The outcrops in Last Chance Canyon are composed of two fourth-order depositional sequences: an aggrading carbonate bank (upper San Andres 3; uSA3) followed by a strongly progradational, offlapping mixed carbonate-siliciclastic succession (upper San Andres 4; uSA4). Each sequence comprises a number of subsidiary high-frequency sequences (fifth-order). Two alternative impedance models were used: Model A, in which all facies transitions are reflecting boundaries, and Model B, in which only time-significant surfaces act as reflectors and lateral facies transitions are represented by horizontal velocity gradients. The vertical-incidence modeling technique was used to compute perfectly migrated time and depth sections with different frequencies. Using a low-frequency wavelet (25 Hz), the sequence boundary separating the two fourth-order cycles (uSA3 and uSA4) is poorly imaged. Instead, one is tempted to incorrectly interpret an onlap pattern generated by a high-frequency cycle within uSA4 as this major sequence boundary. In addition, the 25 Hz runs show toplap and downlap lap-out patterns in an overly oblique fashion, obscuring true asymptotic stratal relationships. Both at 35 Hz and 50 Hz, profiles based on Model B image the genetic structure of both uSA3 and uSA4 relatively well. At 50 Hz, Model A incorrectly shows a transition from a ramp to a rimmed margin within uSA4. The 35 Hz models are qualitatively compared with a published Exxon Production Research Co. seismic line, located approximately 50 km along depositional str ke to the northeast. Model A shows an unexpected good match with the Exxon seismic line, whereas Model B comes much closer to the depositional anatomy observed in outcrop. Our results show that the resolution of stratal geometries and facies distributions in Last Chance Canyon is strongly related to carbonate-sandstone alternations and the way impedance contrasts at carbonate-sandstone transitions are represented.

Compositional and Taphonomic Variations in Modern Crinoid-Rich Sediments from the Deep-Water Margin of a Carbonate Bank

Multivariate analyses of the coarse-grained fraction (> 2 mm) of sediments accumulating in deep water (419-434 m) along the western margin of the Little Bahama Bank reneal identifiable, small-scale compositional and taphonomic variations among local subhabitats (ridge crest, slope, foreslope, base of slope, pavements and scour pit) separated by meters to tens of meters. Bulk composition varies between planktic- (crest and slope) and lithic-dominated (pavements, scour pit) sediments. Local macrobenthic skeletal components also vary significantly among subhabitats, but are commonly dominated by echinoid and crinoid material; crinoid columnals contribute 9-52% of the coarse skeletal component of 17 sediment samples considered

Physical versus chemical processes of “whiting” formation in the Bahamas

Whitings are common yet problematic occurrences of highly turbid water parcels among the relatively clear waters of shallow, tropical and subtropical carbonate platforms, banks and seas (e.g. Bahamas, Belize, Gulf of Carpentaria, Persian Gulf). In this paper, existing data on Bahamian whitings are scrutinized in an attempt to identify processes which may account for whiting phenomena worldwide. An examination of the areal distribution of whitings on Great Bahama Bank shows that>90% are associated with either mud or pellet-mud sediment facies. In view of this non-random distribution, several prominent hypotheses of whiting origin are considered: a) resuspension of fine-grained sediment by bottom-feeding fish (fish-muds); b) largescale, instantaneous physicochemical precipitation events; c) large-scale carbonate precipitation incidental to planktonic algal blooms; d) bottom sediment resuspension as a result of turbulent tidal flow. Although none of the above hypotheses can be discounted completely, this paper proposes that the observed geographic and physical characteristics of whitings are consistent with predicted and observed characteristics of turbulent-flow systems; that whitings are the observable manifestation of the “bursting” cycle of turbulence production at flow boundaries. The bursting process is a primary source of turbulence emanating from flow boundaries and can generate velocity excursions which deviate from mean flow velocity by a factor of 4. Published data for mean tidal current velocities on Great Bahama Bank range from 8 to 50 cm s−1. Thus, maximum predictable velocities during the bursting process will range from 32 to 200 cm s−1, quite in excess of empirically determined threshold sand-transport velocities (<25 cm s−1). Four-fold velocity excursions during turbulent flow increase shear stress up to 4-fold and generate 16-fold increases in lift force at the sediment water interface, enhancing bottom-sediment destabilization, particle entrainment and suspension. The visible, roiling nature of whiting water parcels is believed to be the expression of the above turbulent flow processes on Great Bahama Bank. Organization of turbulent boundary-layer flow into alternating low- and high-velocity “streaks” gives rise to alternating zones of active sediment-resuspension and deposition and may account for the development of subparallel digitations frequently observed within whitings. An integrated, conceptual explanation of whiting distribution on Great Bahama Bank is proposed which considers the interaction of physical environmental energy and sediment textural characteristics across the bank-top. Despite high tidal-current, wind and wave energy flux (and sediment resuspension potential) near bank margins, whitings are rare because sediments are nearly devoid of mud-sized particles. Near Andros Island, carbonate-mud is plentiful but whitings are infrequent because deposition occurs in the tidal- and wind-energy shadow of the island. Only on the center of the bank-top is environmental energy (sediment resuspension potential) sufficiently high and carbonate-mud sufficiently abundant for whitings to originate. Evaluation of this proposed whiting generation concept requires an interdisciplinary understanding of chemical, physical, sedimentological and biological aspects of the Great Bahama Bank environment.

Lower Ordovician (Ibexian) trilobites from the Tribes Hill Formation, central Mohawk Valley, New York State

The Early Ordovician Tribes Hill Formation of east-central New York State is a sequence of peritidal to subtidal carbonates and minor shales that rests disconformably on Late Cambrian carbonates and is, in turn, succeeded disconformably by Middle Ordovician strata. More than 800 trilobites from 24 collections are assigned to six species: Bellefontia gyracantha (Raymond), Clelandia parabola (Cleland), Hystricurus ellipticus (Cleland), Hystricurus cf. Hystricurus oculilunatus Ross, Symphysurina convexa (Cleland), and Symphysurina cf. Symphysurina woosteri Ulrich. Two distinct biofacies are present: the Bellefontia Biofacies in subtidal shales with thin, storm-generated bioclastic interbeds, and the Gastropod–rostroconch Biofacies in shallow, carbonate bank lithofacies. The trilobites of the Tribes Hill Formation are assigned to a single, informal biostratigraphic unit, the Clelandia parabola Fauna, which is correlative with trilobite Zone B of the Garden City Formation of Utah and with the Bellefontia franklinense Subzone of the McKenzie Hill Formation of Oklahoma.

Porosity identification using amplitude variations with offset in Jurassic carbonate, offshore Nova Scotia

The east coast of Canada is a classic pull‐apart margin which was initiated during late Triassic time. The rifting of the seafloor continued during mid‐Jurassic time which was dominated by thick marine deposits along the ancient shelf edge. One member was an extensive carbonate bank facies called the Abenaki formation. This paper focuses on the uppermost unit, the Baccaro member.

Carbonate Bank Sedimentation in a Volcaniclastic Arc Setting: Lower Carboniferous Limestones of the Eastern Klamath Terrane, California

ABSTRACT Carboniferous volcaniclastic-arc deposits of the eastern Klamath terrane, California, include Late Visean/Namurian limestone lenses that formed as small carbonate banks. The limestone lenses, within the Bragdon and Baird formations, reach 17 m in thickness and 1.2 km in length. Slope deposits consist of argillaceous spiculitic wackestone, and bank-edge deposits include ooid grainstone, Striatifera packstone, argillaceous phylloid algal packstone, and argillaceous skeletal packstone. Bank-interior deposits include skeletal wackestone/packstone and argillaceous sandy mudstone. The limestone lenses overlie proximal deltaic deposits of thick-bedded volcaniclastic sandstone and conglomerate. Carbonate banks developed on delta lobes during intervals of minimal clastic sedimentation, ossibly related to sea-level rise and volcanic quiescence. The carbonate banks were short-lived depositional systems, and they were covered by prograding deposits of younger volcaniclastic sands.

Gladden Pull-Apart Basin, Ofshore Belize

The junction of the American and Caribbean plates in Belize has created a complex structural setting for oil and gas exploration. Recent seismic offshore Belize has been used to identify three structural provinces, from west to east: a shallow thrust zone, a narrow upthrown wrench faulted zone and a deeper extensional basin, named the Gladden Pull-Apart Basin. Hydrocarbon leakage from recent fault movement appears to have depleted the shallow structures to the west, but the pull-apart basin has a thick sequence of low-frequency clay-dominated sealing rocks with the potential to preserve hydrocarbon accumulations in Cretaceous carbonate banks. These buried carbonate are of the same age and depositional environment of Mexico's Golden Lane/Tabasco Reforma carbonate banks which are world class giant fields. The Belize and Mexican carbonate banks are within the same Cretaceous depositional basin, the Peten Basin. Seismic interpretations in offshore Belize have been integrated with gravity and magnetic surveys. This provides additional support for the deep extensional basin. The location of the thick Cretaceous carbonate banks is better interpreted with the integration of these three geophysical tools. Airborne geochemical surveys were used to detect the presence of oil seeps on the east and west basin margins.

Genetic modelling for banded iron-formation of the Hamersley Group, Pilbara Craton, Western Australia

The banded iron-formation (BIF) of the Hamersley Group, Pilbara Craton, Western Australia, particularly from the well studied Dales Gorge Member, is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60,000 km 2, enabling reasonable estimates for the annual input of components to the depository. In the model of this paper, varying supply of materials for the medley of mesoband types, particularly of iron and silica in the oxide BIF, can be accommodated by the interaction of two major oceanic supply systems: (1) surface currents and (2) convective upwelling from mid-oceanic ridge (MOR) or hot-spot activity, both modified by varied input of pyrochastic material. (1) The surface currents were saturated in silica and carried minimal iron due to photic precipitation, but were periodically recharged by storm mixing. Precipitation from them gave rise to the banded chert-rich horizons, including the varves, whose regular and finely laminated iron/silica distribution resulted from seasonal meteorological influences. (2) Precipitation from convection driven upwelling of high iron solution from MOR or hot-spot activity periodically overwhelmed the delicate seasonal patterns of (1) to produce the iron-dominated mesobands. A wide range of intermediate mesoband types resulted where the deep water supply was modified by varied MOR activity, or by partial blocking of upwelling waters by surface currents (such as by the present El Niño). During these periods of oxide-dominated BIF, silica was deposited from saturated solution mainly by evaporative concentration, and iron by oxidation due to photolysis and photosynthetically produced oxygen. Superimposed on these supply differences was the varying effect of fine aluminous ash from dominantly northern distal volcanic sources, changing the meteorological and depositional conditions. Occasional input of extremely fi ash during BIF precipitation produced mesoband (cm) scale variations involving increased carbonate-silicate precipitation. Sustained volcanic periods resulted in S-macroband deposition (chert-carbonate-silicate BIF, with shale), gradually returning to the dominant hematite-magnetite-chert BIF as the volcanic input waned. During volcanic periods, the normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron, and by photosynthetic production of oxygen, was modified by turbidity in the atmosphere (aerosols and dust) and in the water (colloids from reactive ash). S Surface-precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone, augmenting the iron content of the zone. Precursor ferrous carbonates and silicates were precipitated when the iron concentration of this sub-photic zone exceeded their respective solubilities. During volcanism, the increased availability of nutrients, particularly phosphorus, to surface waters increased the organic contribution despite lower light values, leading to an almost total absence of ferric iron oxides in the S macrobands (i.e. no magnetite or hematite). Cooling of warm, silica-saturated sea-water during these periods of “olcanic winter” increased the ratio of precipitation of silica to iron, which, however, was still controlled by seasonal conditions. Intermediate concentrations of organic matter, insufficient to totally convert the ferric compounds either during precipitation or diagenesis, resulted in overgrowths of magnetite on hematite, and eventually in the substantial conversion of hematite to magnetite, where higher temperatures were achieved during low-grade regional metamorphism. Changes in sea-level to explain facies changes in BIF are not required in this model, but are not excluded. The preferred conditions are for a very low oxygen to anoxic atmosphere, a much higher level of MOR activity than at present, the presence of photosynthetic plankton, the absence of si silica-secreting organisms, and a deep sea-water temperature higher than 20°C. However, none of these conditions is essential to the model. A narrow carbonate bank is postulated for part of the Fortescue River Valley area during Marra Mamba Iron Formation times (basal Hamersley Group), with BIF precipitation on either side. The reef is postulated to have grown northward becoming a major shallow-water carbonate platform on the Pilbara continent during upper Marra Mamba Iron Formation and Wittenoom Dolomite times, but ceased to play an important role in subsequent periods.

Gas Potential of New Albany Shale (Devonian-Mississippian) in the Illinois Basin

A study to update and evaluate publicly available data relating to present and potential gas production from New Albany Shale in the Illinois basin was conducted cooperatively by the Indiana. Illinois, and Kentucky geological surveys (Illinois Basin Consortium), and was partially funded by the Gas Research Institute. Deliverables included a plate of stratigraphic cross sections and six basin-wide maps at a scale of 1:1,000,000. The New Albany Shale is an organic-rich brownish black shale present throughout the Illinois basin. Gas potential of the New Albany Shale may be great because it contains an estimated 86 tcf of natural gas and has produced modest volumes since 1858 from more than 60 fields, mostly in the southeastern part of the basin. Reservoir beds include organic-rich shales of the Grassy Creek (Shale), Clegg Creek, and Blocher (Shale) members. Limited geologic and carbon isotope data indicate that the gas is indigenous and thermogenic. T[sub max] data suggest that the gas generation begins at R[sub o] values of 0.53% and may begin at R[sub 0] values as low as 0.41% in some beds. New Albany Shale reservoirs contain both free gas in open-pore space and gas adsorbed on clay and kerogen surfaces. Natural fracturing ismore » essential for effective reservoir permeability. Fractures are most common near structures such as faults, flexures, and buried carbonate banks. Based on limited data, fractures and joints have preferred orientations of 45-225[degrees] and 135-315[degrees]. Commercial production requires well stimulation to connect the well bore with the natural fracture system and to prop open pressure-sensitive near-borehole fractures. Current stimulations employ hydraulic fracture treatments using nitrogen and foam, with sand as a propping agent.« less

The marine paleogene of the tremp region (NE Spain)-depositional sequences, facies history, biostratigraphy and controlling factors

The marine Paleogene of the Tremp Basin in the Central Southern Pyrenees corresponds to four depositional sequences which are related to global eustatic third order cycles (Tejas A 2.3–2.6). Associated transgressive and downlap surfaces coincide with boundaries of biozones. Lowstand systems tracts consist of estuarine and braid delta systems. Transgressive and highstand systems tracts are composed of carbonate banks and reefs.

Deformation of a foreland carbonate thrust system, Sawtooth Range, Montana

An analysis of mesoscale faulting associated with thrust-belt development was completed along a transect crossing several foreland thrust sheets of the central Sawtooth Range, Montana. At a regional scale, a balanced cross section of the central Sawtooth Range, constrained by well, seismic, and surface data, indicates a minimum of 60% horizontal shortening of the upper Paleozoic carbonate bank sequence, accommodated by a forward-developing thrust system. Subsurface data delineates a 4° westward-dipping Precambrian basement and westward thickening of the Precambrian Belt Supergroup. Major decollements within the central Sawtooth Range are present at the base of the Devonian Jefferson Formation, in the lower Mississippian Allan Mountain Limestone, and in the Cretaceous Colorado Group. Within the Diversion, French, Norwegian, and Beaver thrust sheets of the central Sawtooth Range, deformation has been partitioned into arrays of meter- to decimetersized faults that constitute brittle deformation zones (BDZs). BDZ thickness is directly related to fault displacement but is not related to fault trajectories or subsequent foreland imbrication. The ratio of BDZ thickness to displacement is nearly constant, with BDZ thicknesses ranging from 65 m to 200 m for faults whose displacements range from 2 km to 5 km. The BDZs of each thrust sheet can be separated into three regions defined by differences in fault diversity, fault intensity, and fault kinematics. Proportionally, the locations of boundaries between regions within the BDZs are remarkably constant for each of the four thrust sheets studied, indicating that the BDZs widen uniformly during thrust sheet emplacement. BDZ bases are characterized by an interlocking network of faults that exhibit great diversity and accommodate mesoscale cataclastic flow. BDZ bases are dominated by oblique-slip faults, whereas the middle and upper regions of BDZs are dominantly dip- or strike-slip faults. In all regions of a BDZ, localized zones of contractional, extensional, or more commonly, transport-parallel strike-slip faults bounded by bedding-plane detachments are present. Fault length/ m2 of outcrop decreases logarithmically to zero from a maximum of 4 m/m2 across the French and Norwegian BDZs. Zones of intense faulting alter this general pattern of decreasing fault intensity in the Diversion and Beaver thrust sheets.

Triassic stratigraphy in the island of Hydra (Greece)

The Triassic sedimentary succession, cropping our in the island of Hydra, starts with a few metres of quartzarenites (Aghios Nikolaos Fm., Scythian) vertically making transition to a thick carbonate unit (Eros Lmst., Spathian-Pelsonian) subdivided into three lithozones and one member. During Pelsonian an extensional tectonic phase dissects the Eros carbonate platform. Its consequent downwarping results in the deposition of a pelagic sequence spanning the Pelsonian-Late Triassic time interval. This succession generally consists of few metres of nodular limestones (Han Bulog Lmst., Late Pelsonian-Early Ladinian) associated with green tuffs, followed by few hundred metres of cherty limestones (Adhami Lmst.). In the eastern part of the island a thick carbonate bank (Pantokrator Lmst.) develops, spanning the lllyrian to Late Triassic time interval. An overlying pelagic sequence, Jurassic in age, marks the downwarping of the Pantokrator carbonate platform. A typical passive continental margin succession is thus recorded in the Triassic of Hydra, suggesting its affinity with the Subpelagonian domain.

Demise of the middle Paleozoic crinoid fauna: gradual or mass extinction?

There are essentially three major crinoid faunas of the Paleozoic: 1) the lower Paleozoic (Ordovician) fauna dominated by disparids and diplobathrid camerates; 2) the middle Paleozoic (Silurian-middle Mississippian) fauna dominated by monobathrid camerates, cladids, and flexibles; and 3) the upper Paleozoic (middle Mississippian-Permian) fauna dominated by cladids. Change from the middle Paleozoic fauna to the late Paleozoic fauna, at or near the Osagean-Meramecian boundary, was characterized by Laudon (1948) as “one of the most remarkable faunal breaks in the entire Paleozoic era”. The monobathrids that had reached their zenith in the Osagean (Tournaisian-Visean) became a very minor component of late Paleozoic faunas. Conventional thinking has implied that a mass extinction of crinoids occurred at the Osagean-Meramecian boundary.We have biostratigraphically subdivided the late Osagean and early Meramecian into four zones (times A-D, oldest to youngest, all within the Gnathodustexanus zone) in order to study the origination and extinction of all crinoid species during the changeover from the middle to upper Paleozoic faunas. Rather than a mass extinction, a monotonic turnover of species fits a pattern of gradual extinction as extinctions outpaced originations. Data are based on 216 species (taxonomically updated) from 69 localities and are as follows:The pattern of originations and extinctions for monobathrids is very similar to the pattern for all crinoid species.The above data have maximum diversity during Time B of the late Osagean followed by declining diversity in times C and D into the early Meramecian. Late Meramecian crinoid faunas (after time D) were less diverse with a maximum of about 45 species. The gradual decline of crinoids across the Osagean-Meramecian boundary cannot be explained as sampling bias (Signor-Lipps effect), because ranges of crinoids thought to be extinct at the boundary are extended into the early Meramecian as well as disappearing at various tinles prior to the boundary.The decline in diversity and the monotonic turnover of species are hypothesized to be the result of habitat reduction as the Eastern Interior Basin of North America was gradually infilled with clastic sediments at the end of the Acadian Orogeny. Most notable in this regard was the smothering of the Keokuk Limestone carbonate bank. Changes in sea level also contributed to habitat reduction. Lowering of sea level at the end of the Osagean caused restriction of open marine environments. A transgression in the early Meramecian was followed by shoaling and restriction during deposition of the late Meramecian Salem and St. Louis limestones.

Relationship between different types of evaporitic deposits, and the occurrence of organic-rich layers (potential source-rocks)

Close relationships between evaporites and hydrocarbons are well known. Three major types of evaporitic deposits can be distinguished on a basin scale. The hydrocarbon potential of each of them is examined. I) In shelf (epeoric seas can be deep basins) evaporites, where salinity and depositional segregation are synchronous and lateral, the water depth must have been shallow and hence unsuitable for the formation of stratified water bodies. Such deposits generally have a low organic content, with very few associated reefs. II) In basin-center evaporitestats deposition results from successive stages of salinity increasing through time. Limestones form on high areas where the total formation accumulation is comparatively low; simultaneously, within the basin thin organic-rich layers form. Calcium sulfate forms in areas where the depositional sequence has a intermediate thickness and also may be contemporaneous with organic-rich beds in low-lying zones. Finally, halite fills the relatively deep and low-lying central areas, but may then overstep the basin margins. In these successive stages vertical salinity gradients are superposed (stratified systems)-at least during limestone and sulfate deposition. On a map, these facies show all apparent centripetally increasing salinity gradient. When located in a carbonate setting, basin-center evaporites are commonly associated with reefs which may have thrived in a ring-like marginal belt (eg. Silurian Salina, Michigan). Because of the occurrence of thin organic-rich layers, in low areas, and the occurrence of reefs with a salt seal, these basins are often rich in hydrocarbons. III) The last type of evaporitic deposits is represented by basin-margin evaporites. In these, the basin center remains “starved” — even “insatiable” — for a long time. Sedimentation remains marine, commonly characterized by the exclusive occurrence of planktonin and nektonic biota and by a more or less constant elevated organic content. Around the basin, carbonate banks or reefs develop and grade landward through lagoonal facies to terrestrial clastics on the marginal platforms, which are sometimes very extensive. Such deposits show an centrifugal salinity gradient, in contrast to the gradient of basin-center evaporite deposits. The paucity of the most soluble salts is significant and involves a deep brine outflow from evaporitic “annexes” (small peripheral shallow pans) located on back-reef shelves. The flow of water towards these back-reef evaporitic sub-basins results in a permanent nutrient input to the entire basin. The brine outfow enhances. the perenniality of the stratified system. Because of this balance and the system’s perenniality, the organic productivity of such basins can be very high. The deposition of basin margin evaporites with syncheonous organic-rich deposits in the basin center takes place over a long period of time, while the infilling of the central trough by basin-center evaporites can be very rapid and can result in the disappearance of the basin. Shelf evaporites overlapping previous basin and marginal shelves can represent the beginning of a new and independent or separate sedimentary cycle.

AN ISOLATED CARBONATE BANK IN THE ZECHSTEIN MAIN DOLOMITE BASIN, WESTERN POLAND

Isolated carbonate banks in starved basins are very attractive targets for hydrocarbon prospecting. In the Zechstein (Late Permian) Main Dolomite (Ca2) Basin of Western Poland, six such banks or isolated platforms have been penetrated by wells, and gas and/or oil has been found in all of them. The largest is the Chartów Platform. which lies a few tens of kilometres from the edge of the carbonate shelf and shows well‐defined facies zonation, Three shallowing‐upward cycles may be distinguished in the bank‐edge sands, while the central (lagoonal) region and lower slope show a single deepening‐upward sequence. Local rapid subsidence, usually balanced by the rate of deposition, is inferred for the edge of the Chartów Platform: this subsidence was not balanced in other zones. The driving force for increased local subsidence may have been related to subtle block movements, and the break‐up of the P21 (Werra) evaporite platform. The rate of increased subsidence fluctuated: pulses of relatively rapid subsidence disturb the sequence, which was controlled by sea‐level fluctuations. Differential subsidence and sea‐level changes are common during the evolution of evaporite basins, and isolated platforms similar to the Chartów structure may be common in other evaporite basins, where isolated banks are located between the central and marginal areas.

SEDIMENTARY BASINS OF THE NE CARIBBEAN PLATE BOUNDARY ZONE AND THEIR PETROLEUM POTENTIAL

Sedimentary basins in the NE Caribbean Plate Boundary Zone are associated with the evolution of a Cretaceous to Early Tertiary island arc, and with carbonate banks which formed following cessation of volcanism in the Eocene to Oligocene. Neogene sedimentary basins are typically present where crustal blocks in the plate boundary zone have undergone divergence, with or without rotation. Cretaceous through Oligocene basins may also have formed during divergence between crustal blocks, although this is difficult to prove owing to limited exposure and subsequent tectonic deformation. Other basins are associated with localized overthrusting along the plate boundaries.

High Production and Highstand Shedding from Deeply Submerged Carbonate Banks, Northern Nicaragua Rise

ABSTRACT Because the shallow isolated carbonate banks on the northern Nicaragua Rise, the Nicaragua/Honduras and southern Jamaica carbonate shelves, and many other modern carbonate banks worldwide, are covered by an average of 20 to 30 m of water and by a thin blanket of coarse carbonate sediments, other carbonate sedimentologists have considered these banks good examples of incipiently drowned, or even drowned carbonate banks. However, based on recent research on the northern Nicaragua Rise, we can demonstrate that these banks currently are healthy producers of large volumes of periplatform sediments (fine aragonite and magnesian calcite), which are exported almost entirely to the deep surrounding slopes. These sediments, which were deposited during the past 9000-10,000 years, form peripl tform wedges on the middle and upper slopes, which are seen clearly on 3.5 kHz seismic profiles. Radiocarbon ages of the wedge surface sediment range between 230 and 610 YBP and provide clear evidence for the contemporaneous production of sediments on the shallow bank and shelf and their instantaneous export to the upper slopes. For the last 5000 years, sedimentation rates ranged from 2000 mm/ky off Pedro Bank to 1300 mm/ky off Jamaica. These rates are somewhat lower than, but of the same order of, magnitude as sedimentation rates on the western (leeward) slopes of Great Bahama Bank. Deposition of the metastable carbonate sediments discussed in this paper occurred as soon as sea level rose to flood the bank and shelf which flank Walton Basin, following the last glacial lowstand. They com rise a wedge-shaped package of transgressive and highstand sediments on the upper and middle slopes of Pedro Bank and the southern Jamaica shelf.

The Subsurface Upper Frio Formation of Southeastern Louisiana and Correlations to the Paynes Hammock and Catahoula Formations in East-Central Mississippi

The Subsurface Upper Frio Formation of Southeastern Louisiana and Correlations to the Paynes Hammock and Catahoula Formations in East-Central Mississippi

Depositional Controls and Environments of the Upper Frio Formation: Southeastern Louisiana Subsurface

The subsurface upper Frio Formation (late Oligocene) consists of interbedded carbonate and siliciclastic facies. Synthesis of induction-electric logs and fragmented conventional core data show that resistive, low porosity carbonates are key to understanding environment of deposition, sandstone facies geometry, and hydrocarbon distribution. The southeastern Louisiana Oligocene shelf received little clastic influx during Frio Formation time. Periodically, the resistive, carbonate facies formed a semi-continuous lineation of banks at the paleo shelf-edge at or about 30N latitude. The banks acted as barriers to siliciclastic deposition with sediments being deflected westward behind the banks and funnelled basinward through breaks in the banks at the shelf edge. Petrographic study of the carbonates commonly reveals lepidocyclinid-nummulitid biomicrites that show signs of early porosity occlusion and minor burial diagenesis. They range in composition from pure limestone to impure, iron-rich dolomitic sandstones. Several cycles of carbonate-shale-sandstone are noted in the upper Frio Formation driven by relative sea-level change. Carbonate banks grew during transgressive periods and were periodically inhibited during early regressive siltation events or water-deepening below the photic range. Subsequent lowering of sea-level initiating progradational pulses of fine- to medium-grained sands that were funnelled around carbonate banks. Sandstone distribution patterns imply funnelling aspects and paleo-bathymetric bank reliefmore » while sand lenses within shales imply progradation. The nex sea-level rise initiated conditions advocating renewed bank growth as depicted by carbonate facies overlying sandstones.« less

Geological Mounds and Their Seismic Expression

Mound geometry (convex upward structure developed above a subhorizontal surface) is common in many geological environments and frequently observed in 2-dimensions on seismic sections. Seismic mounds are typically associated with deep-water clastic sediments, e.g. submarine fans and slumps, and with a variety of carbonate depositional settings, e.g., reefs and banks, but also exist in other depositional settings. Recognition will be dependent on mound dimension, velocity contrast, amplitude strength, and the resolution of the seismic data. Since mounds can represent an important exploration target and recognition of porous, hydrocarbon-bearing section is all-important, careful restitution of the original depositional morphology from the seismic data is required. Details of present velocity distribution are critical, along with a realistic concept of any post-depositional modification, such as compaction, which may have taken place during burial. Where differential compaction is taking place, for example between sand and shale, seismic expression of morphology will be continually modified during progressive burial. Analysis of structure at the top and base of the mound can provide support for lithological interpretation based on other criteria, such as seismic facies analysis based on internal and external reflections. Modeling, using parameters from mounds in a variety of known depositional settings, illustrates many ofmore » the interpretational problems associated with seismic mounds and provides some objective criteria for analysis of mound morphology. Comparison is made with real data, principally from northwest Europe and North America.« less