High-energy Shelf Research Articles

Repeated enrichment of trace metals and organic carbon on an Eocene high-energy shelf caused by anoxia and reworking

Abstract Petroleum source rocks are strongly enriched in organic carbon (OC), and their trace metal (TM) contents often reach low-grade ore levels. The mechanisms leading to these coenrichments are important for understanding how extreme environmental conditions support the formation of natural resources. We therefore studied organic-rich Eocene marls and limestones (oil shale) from the central Jordan Amzaq-Hazra subbasin, part of a Cretaceous–Paleogene shelf system along the southern Neo-Tethys margin. Geochemical analyses on two cores show highly dynamic depositional conditions, consistent with sedimentological and micropaleontological observations. Maximum and average contents, respectively, in OC (∼26 and ∼10 wt%), sulfur (∼7 and ∼2.4 wt%), phosphorus (∼10 and ∼2 wt%), molybdenum (>400 and ∼130 ppm), chromium (>500 and ∼350 ppm), vanadium (>1600 and ∼550 ppm) and zinc (>3800 and ∼900 ppm) are exceptional, in particular without any indication of hydrothermal or epigenetic processes. We propose a combination of two processes: physical reworking of OC- and metal-rich material from locally exposed Cretaceous–Paleogene sediments (as supported by reworked nannofossils), and high marine productivity fueled by chemical remobilization of nutrients and metals on land that sustained anoxic-sulfidic conditions. Burial of high-quality organic matter (hydrogen index 600–700 mgHC/gOC) was related to strongly reducing conditions, punctuated by only short-lived oxygenation events, and to excess H2S, promoting organic matter sulfurization. These processes likely caused the OC and TM coenrichments in a high-energy shallow-marine setting that contradicts common models for black shale formation, but may explain similar geochemical patterns in other black shales.

Urchins on the edge: an echinoid fauna with a mixed environmental signal from the Eocene of Jamaica

An echinoid fauna from the Early–Middle Eocene of Jamaica has yielded six species of echinoid: the phymosomatoid Acanthechinus peloria (Arnold and Clark); the oligopygoid Oligopygus sp.; the clypeasteroids Fibularia jacksoni Hawkins and Neolaganum sp.; and the spatangoids Eupatagus alatus Arnold and Clark and Eupatagus sp. cf. E. clevei (Cotteau). Eupatagus alatus has a relatively low test, is broadest posterior of centre, and lacks an anterior sulcus; Eupatagus sp. cf. clevei is relatively higher and more inflated, more parallel-sided, and blunter anteriorly with an anterior sulcus. The discovery of F. jacksoni and Neolaganum sp. together with Oligopygus sp. is unexpected, as these species commonly have different ecological niches in the Eocene of Jamaica. In previous examples described from the Eocene succession of the island, oligopygoids favoured high-energy shelf edge settings, whereas Fibularia and neolaganids prefered low-energy lagoonal settings. The assemblage as a whole likely represents an Early–Mid Eocene echinoid fauna inhabiting a shallow island shelf sea, in a transition between shelf edge and lagoonal setting.

Character, distribution, and ecological significance of storm wave-induced scour in Rhode Island Sound, USA

Multibeam bathymetry, collected during NOAA hydrographic surveys in 2008 and 2009, is coupled with USGS data from sampling and photographic stations to map the seabed morphology and composition of Rhode Island Sound along the US Atlantic coast, and to provide information on sediment transport and benthic habitats. Patchworks of scour depressions cover large areas on seaward-facing slopes and bathymetric highs in the sound. These depressions average 0.5–0.8 m deep and occur in water depths reaching as much as 42 m. They have relatively steep well-defined sides and coarser-grained floors, and vary strongly in shape, size, and configuration. Some individual scour depressions have apparently expanded to combine with adjacent depressions, forming larger eroded areas that commonly contain outliers of the original seafloor sediments. Where cobbles and scattered boulders are present on the depression floors, the muddy Holocene sands have been completely removed and the winnowed relict Pleistocene deposits exposed. Low tidal-current velocities and the lack of obstacle marks suggest that bidirectional tidal currents alone are not capable of forming these features. These depressions are formed and maintained under high-energy shelf conditions owing to repetitive cyclic loading imposed by high-amplitude, long-period, storm-driven waves that reduce the effective shear strength of the sediment, cause resuspension, and expose the suspended sediments to erosion by wind-driven and tidal currents. Because epifauna dominate on gravel floors of the depressions and infauna are prevalent in the finer-grained Holocene deposits, it is concluded that the resultant close juxtaposition of silty sand-, sand-, and gravel-dependent communities promotes regional faunal complexity. These findings expand on earlier interpretations, documenting how storm wave-induced scour produces sorted bedforms that control much of the benthic geologic and biologic diversity in Rhode Island Sound.

Chemical weathering and provenance evolution of Holocene–Recent sediments from the Western Indus Shelf, Northern Arabian Sea inferred from physical and mineralogical properties

We present a multi-proxy mineral record based on X-ray diffraction and diffuse reflectance spectrophotometry analysis for two cores from the western Indus Shelf in order to reconstruct changing weathering intensities, sediment transport, and provenance variations since 13ka. Core Indus-10 is located northwest of the Indus Canyon and exhibits fluctuations in smectite/(illite+chlorite) ratios that correlate with monsoon intensity. Higher smectite/(illite+chlorite) and lower illite crystallinity, normally associated with stronger weathering, peaked during the Early–Mid Holocene, the period of maximum summer monsoon. Hematite/goethite and magnetic susceptibility do not show clear co-variation, although they both increase at Indus-10 after 10ka, as the monsoon weakened. At Indus-23, located on a clinoform just west of the canyon, hematite/goethite increased during a period of monsoon strengthening from 10 to 8ka, consistent with increased seasonality and/or reworking of sediment deposited prior to or during the glacial maximum. After 2ka terrigenous sediment accumulation rates in both cores increased together with redness and hematite/goethite, which we attribute to widespread cultivation of the floodplain triggering reworking, especially after 200years ago. Over Holocene timescales sediment composition and mineralogy in two localities on the high-energy shelf were controlled by varying degrees of reworking, as well as climatically modulated chemical weathering.

The Shah Kuh Formation, a latest Barremian – Early Aptian carbonate platform of Central Iran (Khur area, Yazd Block)

The Shah Kuh Formation of the Khur area (Central Iran) consists of predominantly micritic, thick-bedded shallow-water carbonates, which are rich in orbitolinid foraminifera and rudists. It represents a late(est) Barremian – Early Aptian carbonate platform and overlies Upper Jurassic – Barremian continental and marginal marine sediments (Chah Palang and Noqreh formations); it is overlain by basinal deposits of the Upper Aptian – Upper Albian Bazyab Formation. The lithofacies changes at both, the base and top of the Shah Kuh Formation are gradational, showing that the formation is part of an overall transgressive sedimentary megacycle, and that the formational boundaries are potentially diachronous on larger distances. Analyses of facies and stratal geometries suggest that the Shah Kuh carbonate system started as a narrow, high-energy shelf that developed into a large-scale, flat-topped rudist platform without marginal rim or steep slope. The Shah Kuh Platform is part of a large depositional system of epeiric shallow-water carbonates that characterized large parts of present-day Iran during Late Barremian – Aptian times (“Orbitolina limestones” of NW and Central Iran, the Alborz and the Koppeh Dagh). Their biofacies is very similar to contemporaneous deposits from the western Tethys and eastern Arabia, and they form an important, hitherto poorly known component of the Tethyan warm-water carbonate platform belt.

Platform to shelf edge transect, Triassic of northern Israel, North Arabian Plate

A transect of the Middle to Late Triassic from the epicontinental belt to the shelf edge of the north Arabian margin is presented, based on new data from two boreholes in northern Israel. Two sedimentary systems are encountered along this traverse: the inland Meged-2 borehole represents the seaward extension of the epicontinental belt, with shallow- to deep-water carbonates, salina evaporites, and basinal shales, while Ga'ash-2, on the Levant margin, was positioned on an isolated platform separating the internal shelf-basin system from the open Tethys, displaying a shallower range of facies that is free of evaporites. Proximal-distal facies shifts are bundled into five low-order (LO) T–R cycles, ranging from Anisian (Pelsonian) to Norian, each composed of numerous high-order (HO) shallowing-upwards cycles. Meged-2 paleoenvironments imply an interior basin trapping sediments of the epicontinental belt, with numerous HO facies shifts indicating pulses of subsidence. Ga'ash-2 contains Lofer-like peritidal cycles and a late Anisian — early Carnian hiatus in place of the second LO cycle. The two orders of cycling are controlled by extrabasinal regional or eustatic change, and synsedimentary vertical movements. The divergent sedimentary patterns of the late Anisian–late Carnian can be attributed to NW–SE trans-tensile movements impacting on earlier Neotethyan extensional trends. Synsedimentary movements, represented locally by uplift at Ga'ash-2 and subsidence at Meged-2, controlled the influx of sea water into the epicontinental zone, and consequently, controlled the alternation of evaporite vs. carbonate systems. In the latest Carnian–early Norian, these facies belts become locked together forming a shallow, high energy shelf. This change reflects broad uplift and shifting of the locus of active differential movement into the Levant Basin.

Sediment recycling and adjustment of deposition during deglacial drowning of a low-accumulation shelf (NW Iberia)

Based on a well-established stratigraphic framework and 47 AMS- 14C dated sediment cores, the distribution of facies types on the NW Iberian margin is analysed in response to the last deglacial sea-level rise, thus providing a case study on the sedimentary evolution of a high-energy, low-accumulation shelf system. Altogether, four main types of sedimentary facies are defined. (1) A gravel-dominated facies occurs mostly as time-transgressive ravinement beds, which initially developed as shoreface and storm deposits in shallow waters on the outer shelf during the last sea-level lowstand; (2) A widespread, time-transgressive mixed siliceous/biogenic-carbonaceous sand facies indicates areas of moderate hydrodynamic regimes, high contribution of reworked shelf material, and fluvial supply to the shelf; (3) A glaucony-containing sand facies in a stationary position on the outer shelf formed mostly during the last-glacial sea-level rise by reworking of older deposits as well as authigenic mineral formation; and (4) A mud facies is mostly restricted to confined Holocene fine-grained depocentres, which are located in mid-shelf position. The observed spatial and temporal distribution of these facies types on the high-energy, low-accumulation NW Iberian shelf was essentially controlled by the local interplay of sediment supply, shelf morphology, and strength of the hydrodynamic system. These patterns are in contrast to high-accumulation systems where extensive sediment supply is the dominant factor on the facies distribution. This study emphasises the importance of large-scale erosion and material recycling on the sedimentary buildup during the deglacial drowning of the shelf. The presence of a homogenous and up to 15-m thick transgressive cover above a lag horizon contradicts the common assumption of sparse and laterally confined sediment accumulation on high-energy shelf systems during deglacial sea-level rise. In contrast to this extensive sand cover, laterally very confined and maximal 4-m thin mud depocentres developed during the Holocene sea-level highstand. This restricted formation of fine-grained depocentres was related to the combination of: (1) frequently occurring high-energy hydrodynamic conditions; (2) low overall terrigenous input by the adjacent rivers; and (3) the large distance of the Galicia Mud Belt to its main sediment supplier.

Clastic sedimentation in a late Archaean accretionary terrain, Midlands greenstone belt, Zimbabwe

Abstract Late Archaean (∼2.68 Ga) clastic sedimentary rocks of the Shamvaian Group form a shear zone-bounded lithotectonic unit in the Midlands greenstone belt (MGB) of Zimbabwe. The sedimentary rocks are wedged in between mid-Archaean granitoid gneisses and late Archaean mafic and bimodal volcanic rocks. The ≥2000-m thick sedimentary sequence consists of marine turbidite deposits overlain by high-energy shelf sandstone, fluvial braid-plain pebbly sandstone and alluvial fan conglomerate. Deposition took place in a northward-deepening basin with fluvial systems draining a source dominated by granitoid gneiss and banded chert. Basin formation was associated with a west-directed thrusting event, and the basin fill was incorporated into the thrust stack soon after deposition. The Shamvaian Group is similar to modern foreland and forearc successions and possibly accumulated during collision of a continental fragment with an island arc-like complex.

Lowstand carbonates, highstand sandstones?

The sedimentary facies, sediment dynamics and sequence architecture of modern high-energy shelves in the mid and high latitudes are largely governed by wave abrasion processes. Cool-water carbonates may form there, if the influx and/or net accretion of siliciclastics is kept at a minimum. Little dilution of the carbonate produced in situ is generally promoted by a wide “epicontinental” shelf, subdued topography of the adjacent mainland, the predominance of limestone outcrops, and an arid climate. The aforementioned requirements are rarely met, and thus will automatically lead to the formation of mixed siliciclastic–cool-water carbonates. Such an example is found in the Early to Mid-Miocene Lagos-Portimão Formation (Algarve, S-Portugal), which formed on a narrow high-energy shelf of the Atlantic Ocean that was bounded by a mountain range. The sediments of the formation consist of fossiliferous sandstone (FS), shell beds, and rhodolith blankets. Along strike, the stratification of the formation is monotonous for tens of kilometres and well exposed in coastal cliffs, whereas no outcrops of dip sections exist. The bulk skeletal composition of the sediments is typical for the warm-temperate climatic zone: various endo- and epibenthic bivalves, bryozoans, coralline algae, echinoderms, gastropods, and large foraminifers ( Heterostegina). In some very rare beds, a few isolated, not framework-forming specimens of zooxanthellate corals ( Porites, Tarbellastrea) indicate temporally elevated surface water temperatures close to the lower threshold of the coral reef ecosystem. In sandstones, the fauna is well preserved and burrowing bivalves are commonly found in life position. In limestone beds, the state of preservation of the grains ranges from intact to disintegrated and abraded specimens. We infer an accumulation of the shell beds through winnowing of fine materials (siliciclastic sand and carbonate mud) at wave abrasion depth and concentration of calcareous skeletons associated with the subsequent attraction of new epibiota in a complex shell bed. The vertical alternation of fossiliferous sandstone and shell beds, and in-phase variations of the “Photo Index” (photic biota vs. bryozoans) and “Bryozoan Index” (bivalves vs. bryozoans) is envisaged to document variations of water depth (and sea level). Sandstone units built up when wave abrasion depth (WAD) rose above the sea floor during TST (and early HST), whereas the shell beds formed during LST when the WAD for sand intersected with the sea floor. Clastic sediments were probably brought on the outer shelf during early transgression, and by longshore currents. Sea-level signatures inferred in the mixed siliciclastic–cool-water carbonate shelf setting of S-Portugal therefore significantly deviate from conventional concepts of carbonate sequence stratigraphy, which were developed for flat-topped platforms. Successful interpretations of ancient mixed sequences must therefore take into consideration the processes of production, concentration and accretion of the carbonate sediments.

Mixed carbonate-siliciclastic sedimentation on a tectonically active margin: Example from the Pliocene of Baja California Sur, Mexico

Bioclast-rich, coarse-grained deposits in the Pliocene Loreto basin provide a record of mixed carbonate and siliciclastic sedimentation at the steep hanging-wall margin of this small, fault-controlled basin. Sedimentary facies consist of sand- to gravel-sized carbonate debris mixed with volcaniclastic sand and gravel in a proximal to distal facies tract that includes matrix-rich and matrix-poor shelly conglomerate, impure calcirudite and calcarenite, mixed-composition turbidites, and bioturbated calcarenitic sandstone. Carbonate material was produced by mollusks and other benthic organisms on a narrow, high-energy shelf and mixed with volcaniclastic sand and gravel in cross-shelf channels. These mixtures were transported down a steep subaqueous slope by debris flows, grain flows, and turbidity currents, forming foresets and bottomsets of marine Gilbert-type deltas. This style of mixed carbonate-siliciclastic sedimentation has not been documented in detail elsewhere but should be locally abundant in the stratigraphic record of fault-bounded basins, particularly those with cool or nutrient-rich waters that support relatively few binding and framework-building faunas. Recognition of similar facies in other settings can provide useful insights into ancient conditions of carbonate production, oceanography, climate, and tectonics.

Discussion: Upper Lower Cambrian depositional sequence in Avalonian New Brunswick

The authors (E. Landing and S.R. Westrop) wish to abandon most of the local stratigraphic nomenclature of the Saint John Group in southern New Brunswick, in order to emphasize the broader tectonic significance of these lower Paleozoic platformal rocks throughout the Avalon terrane. Their approach, based in part on biostratigraphic data, is to use a somewhat arbitrary mixture of stratigraphic terms taken from widely separate type sections in New Brunswick and Newfoundland. Thus, the Ratcliffe Brook Group (a formation defined in New Brunswick), would include the Rencontre and Chapel Island formations (both defined in Newfoundland), whereas the Chamberlain's Brook Formation (defined in Newfoundland) would include the Fossil Brook Member (defined in New Brunswick). The Saint John Group, which clearly has historical precedence (Williams et al. 1985), is left out of their stratigraphic scheme. Since the establishment of a formal lithostratigraphic unit requires that its lower and upper boundaries be explicitly defined, it is difficult to see how this can be accomplished when the supposedly adjacent units are from type sections or reference sections located about 1000 krn apart. Even if separate sections contain similar successions, it is unlikely that lithological boundaries would be identical over large areas in such an episodically high-energy shelf environment. Difficulties in adequately describing formational boundaries are compounded in the case of the Hanford Brook Formation since, as the authors point out, it does not even occur in the Newfoundland type section where the underlying and overlying strata are defined. The two type sections are, therefore, not truly homotaxial, so extrapolation of Newfoundland terminology to New Brunswick may not be justified. The authors insist on placing the upper part of the Glen Falls Formation into the overlying Hanford Brook Formation since they claim a major unconformity exists within the former. Although such a revision is not unreasonable, it is not required. Stratigraphic nomenclature is supposed to be based on empirical lithological changes rather than interpretive depositional history, so the traditional lithological components of the Glen Falls Formation could be retained even if the claimed unconformity were proven to exist. In conclusion, the authors have made a significant contribution to the understanding of tectonic controls on the development of Paleozoic shelf facies covering the Avalon terrane. However, this could have been accomplished without resorting to wholesale name changes, the sole purpose of which is to create a general stratigraphic nomenclature to encompass what is surely a complex depositional setting containing many depocentres. A standard correlation chart retaining local terminology would be a more practical way to convey the reasoning behind the authors' tectono-stratigraphic synthesis. I believe that the composite stratigraphic column proposed by Landing and Westrop creates more problems than it solves and is unlikely to be adopted by others working in this part of the northern Appalachians.

The influence of inherited geological framework upon a hardbottom-dominated shoreface on a high-energy shelf: Onslow Bay, North Carolina, USA

Abstract The southeastern coast of North Carolina is a major tourist destination that has experienced rapid population growth resulting in increased revenues. Most development is sited on transgressive barriers and headland beaches located along chronic erosion zones. Beach replenishment is viewed as the only viable option for erosion mitigation. However, a detailed sand resource assessment is non-existent. Furthermore, few data exist on the inter-relationships between the underlying geological framework and the morphology, sediments and evolution of the coastal system. The shoreface of the two constraining headlands of this sand-starved system in Onslow Bay was studied utilizing cores, remotely sensed data, and diver surveys. The (coquina-dominated) subaerial headland at Fort Fisher and the (limestone) submarine headland at New River clearly have influenced the coastwise evolution of the intra-headland perched beaches. These shorefaces, as well as those of adjacent barriers, are characterized by hardbottoms of varying relief, morphology, and lithology. Prominent submarine scarps and karstic topographic highs that extend above the ravinement surface have resulted in major changes in barrier orientation and development of distinct subcompartments with respect to coastal processes. Holocene sea-level rise has produced a thin (<70 cm) variable sequence, deposited unconformably over bioeroded hardbottoms. Bioerosion represents a major source and supply of new sediment. The thin sediment veneer is a mosaic of remobilized graded palimpsest and residual shell-rich sand and gravels. Data reinforce the concept that a common equilibrium profile for all shorefaces is neither realistic nor adequate for an understanding of coastal processes in the study area.

Shelf Sand Body Distribution and Transgressive Systems Tract Development on a High Energy Shelf: An Example from the East China Sea: ABSTRACT

Shelf Sand Body Distribution and Transgressive Systems Tract Development on a High Energy Shelf: An Example from the East China Sea: ABSTRACT

Lithostratigraphy and depositional environment of the Permian Nowra Sandstone in the southwestern Sydney Basin, Australia

Lithofacies in the mid‐Permian Nowra Sandstone indicate a middle/upper shoreface to foreshore environment of deposition under the influence of storm‐generated waves and north‐northeasterly directed longshore currents. Palaeogeographic reconstruction for the Nowra Sandstone portrays a sand‐dominated high energy shelf and offshore shoal forming a sequence thickening seaward away from the western shore of the Sydney Basin. The shoal‐crest at the outer edge of the shelf trends north‐northeast. It is characterized by fine‐ to medium‐grained sandstone with upper flow regime structures and a high proportion of conglomerate, whereas coarser sandstone with lower energy bedforms occurs along the seaward side of the shoal. In the deeper water to the east, the lower Nowra Sandstone becomes rapidly thinner as it passes seaward, via bioturbated storm redeposited sandstone beds, into the shelf deposits of the Wandrawandian Siltstone. This sequence accumulated during a regressive event and the base of the formation becom...

Holocene Sediment Dynamics on a Cool-Water Carbonate Shelf: Otway, Southeastern Australia

The Otway Shelf is covered by cool ocean waters and veneered by bryozoan-dominated carbonate sediments. Radiocarbon dating and stratigraphy of shelf vibracores and slope gravity cores document late Pleistocene/Holocene deposition. Shelf sediments of the late Pleistocene highstand (60-26 ky) are rare, either never having been deposited or having been removed during the following sea-level fall (26-17 ky). During the subsequent lowstand the shelf was exposed, facies shifted basinward, and beach/dune complexes were constructed near the shelf edge. The deep shelf was characterized by nondeposition and hardground formation, and the shelf margin became locally erosional. Upper-slope bryozoan/sponge assemblages continued to grow actively, and lower-slope foraminifera and nannofossil ooze was increasingly enriched in hemipelagic terrigenous mud swept off the wide shelf. Coarse shelf debris and lowstand dune sands were erosively reworked and transported onto the upper slope and redistributed to deep-slope aprons during early transgression (17-10 ky). This event locally terminated shelf-edge bryozoan and sponge growth. Continued erosive shoreface retreat formed a bored and cemented ravinement surface, mantled by bivalves and lithoclasts, across the open shelf. Base-level rise reduced the amount of terrigenous material reaching the continental slope. Slower transgressive flooding from 10 to 6.5 ky allowed infilling of shallow embayments on the shelf, and outer-shelf sediment accumulation began by 7 ky. Sea level stabilized near its present level by 6.5 ky and the area is now partitioned into three zones by an energetic wave and swell regimen: (1) inboard, protected embayments and/or beach/dune complexes with high sediment accumulation rates (about 100 cm/ky), (2) an open shelf where, although sediment production is active, accumulation is patchy (3-5 cm/ky) to nonexistent within the zone of wave abrasion, and minimal (about 23 cm/ky) between maximum abrasion depth and swell wave base, and (3) a deep shelf edge and upper slope where sediment is accumulating at rates of 2-50 cm/ky. Interpreted deep-water (> 100 m) bryozoan/sponge mounds are growing at rates of 105 cm/ky. Accumulation decreases progressively down slope with deep-slope, pelagic carbonate sedimentation at rates of 5 cm/ky. The late Quaternary shelf record resembles that of flat-topped, warm-water platforms with Holocene sediment overlying Pleistocene/Tertiary limestone, but for different reasons. Much of this high-energy shelf is still within lag depth, more than 10 ky after flooding. The steady-state deep-water, shelf-margin bryozoan factory has been largely unaffected by 50-100 m eustatic changes in sea level, and has contributed to slow but continuous platform progradation. The slow growth potential, uniform profile of sediment production and distribution, and inability of constituent organisms to construct rigid frameworks favor maintenance of a shallow ramp profile and makes the cool-water carbonate system an excellent modern analog for interpretation of many ancient ramp successions.

Perriwinkle and Perriwinkle North Fields, Martin County, Texas: A Cisco-Canyon Lowstand Reef Complex

During the middle-early through early-middle deposition of sediments in Canyon field, a series of glacio-eustatically controlled sea level lowstands resulted in carbonate buildups seaward of the Horseshoe Atoll in Martin County, Texas. The resulting reef tracts consist of algal-bryozoan boundstones, clean forereef talus conglomerates, and other high-energy shelf carbonates. The reef complex was chemically eroded into a tower karst terrain during subsequent sea level lowstands and associated subaerial exposure. The highly sculptured paleotopography was mechanically eroded during the onset of a sea level highstand, filling lows with locally derived conglomerates. In addition, highstand basinal foreshelf conglomerates from the atoll were deposited. These conglomerates contain at least one reservoir-size upper Strawn high-energy shelf allochthon. A transgressive-regressive shelf margin reef tract was deposited seaward of the now exhumed Canyon paleokarst surface during a lowstand that occurred early in Cisco deposition. The tract consists of an algal-bryozoan boundstone, associated forereef talus conglomerates, immediate backreef grainstones and packstones, and erosional foreshelf detritus. Although subaerially exposed, the Cisco reef tract is not as highly solutioned as the Canyon tract, resulting in a lower relief paleotopographic surface. During subsequent sea level highstands, both the Cisco and Canyon reef tracts were buried under lower Cisco basinal foreshelfmore » conglomeraters, masking the stratigraphic intricacies of the paleobathymetric surface and obscuring the relationship of time correlative reservoirs within the shelf-edge complex.« less

B. C. Canyon Field, Howard County, Texas: An Ancient Analogy to Modern Tropical Tower Karst Terrains

Late in the early deposition of sediments in Canyon field, a series of glacio-eustatically controlled sea level lowstands resulted in a carbonate buildup seaward of the Horseshoe Atoll in Howard County, Texas. The resulting satellite reef tract consists of fringing boundstone; high-energy shelf grainstones; lower energy shelf packstones and wackestones; and thin, highstand, black shales and mudstones. The original extent and thickness of deposits were extensively modified during karstification coincident with successive sea level lowstands. The resulting paleotopographic landforms appear to be similar to tower karst features of Puerto Rico. During the beginning of each sea level highstand, the paleoterrain was modified by erosion. The basinal foreshelf conglomerates resulting from initial highstand erosion contain dipping strata that commonly can be detected by the dipmeter tool. The mechanism for the formation of these strata may be depositional or the result of diagenetic alteration of the rock fabric in the burial environment. Using dipmeter data, an uneconomic producer has been offset by one of the better producing wells in the field.

Sequence Stratigraphy of the Smackover Formation in the Northern Half of the Manila Embayment, Southwestern Alabama

ABSTRACT The Smackover Formation in the northern half of the Manila Embayment, southwestern Alabama, ranges from 100 to 540 ft (30 to 165 m) thick and encompasses two genetic packages of facies. These genetic packages developed within a single cycle of relative sea-level change during the Late Jurassic (specifically late early Oxfordian to early Kimmeridgian) and comprise parts of a single depositional sequence. The base of the lower genetic package is the sharp contact between terrestrial clastic facies of the underlying Norphlet Formation and shelf carbonate facies in the Smackover. The lower Smackover genetic package (=transgressive systems tract) includes high-energy grainstones, shelf packstones and wackestones, and shelf-margin and patch-reef algal boundstones. The contact between the upper and lower genetic packages is a maximum-flooding surface delineated by a sharp facies change to a set of ramp carbonate facies. The upper genetic package (=prograding highstand systems tract) includes, in stratigraphic order, ramp packstones and wackestones, lagoon al wackestones and mudstones, and high-energy grainstone and clastic tempestite facies. The top of the upper genetic package is the sharp transition to anhydritic sabkha facies in the overlying Buckner Anhydrite Member of the Haynesville Formation.

Habitat of Oil in the Lindsborg Field, Salina Basin, North-Central Kansas

The Lindsborg field was discovered in 1938, and is now 14 mi in length and 1-2 mi in width. It has a projected ultimate recovery of 16 MMBO. Three pay zones (5-20 ft thick) produce in the field. The Simpson pay zone (Middle Ordovician) is a well-rounded, quartzitic sandstone that is interpreted to be a paralic, high-energy shelf deposit. The Viola pay (Middle Ordovician) appears to be a dolomitic, lime grainstone but no cores are available to confirm this. The uppermost pay zone, the Upper Ordovician Maquoketa, is a finely laminated, vuggy, cherry dolomite interpreted to have been deposited as a subtidal lime mudstone in a restricted lagoon. The Simpson and Viola pays are structurally trapped in culminations along the crest of the Lindsborg anticline. Although the Maquoketa pay is structurally trapped with the other pay zones in the southern half of the field, its locus of production in the north half of the fields extends 100 ft vertically down the western flank of the anticline. The trapping mechanism is unclear due to lack of core control and modern logging suites, but it may be subtle updip diagenetic change from vuggy to nonvuggy dolomite. The Simpson and Maquoketa oils aremore » geochemically distinct. Both may reflect efficient local source-to-reservoir migration from originally rich but marginally mature Ordovician and Devonian shales that contact each pay zone. If oil in the Lindsborg field is locally generated, the prospectivity of the relatively unproductive and underexplored Salina basin may be enhanced.« less

The Llandovery—Wenlock Halysitidae from New Brunswick, Canada

A large collection of Silurian halysitids, mainly from the Limestone Point, La Vieille, and Petit Rocher Formations of northern New Brunswick, includes nine species distributed among Catenipora Lamarck, Halysites Fischer von Waldheim, and Cystihalysites Tchernyshev. One species, Cystihalysites belledunensis, is new, and, of the others, half are previously unknown in the Chaleur Bay region and half have been described before from this area.All species are defined using both qualitative and quantitative data. Statistical analysis of serial sections of a corallum of Cystihalysites belledunensis suggests that any random section of a halysitid will yield quantitative data representative of that colony.Preliminary analysis of the relationships between species distributions and stratigraphy and sedimentary associations suggests strong facies-control on the distribution of the four most abundant species. Both Catenipora simplex (Lambe) and Catenipora micropora (Whitfield) occur mostly in outer shelf facies. Cystihalysites belledunensis n. sp. and Cystihalysites encrustans (Buehler) never occur together, the former being found in predominantly high-energy inner shelf facies and the latter in the outer shelf facies.