Wave Reworking Research Articles

A SEQUENCE STRATIGRAPHIC STUDY OF THE CALLOVIAN FLUVIO-DELTAIC TO MARINE SUCCESSION WITHIN THE ZOCA REGION

This paper presents the results and conclusions of an integrated sequence stratigraphic study of the Callovian marine succession across area 'A' of the Zone of Cooperation (ZOCA). This study utilised wireline log and conventional core data from within ZOCA 91–1 and ZOCA 91–12, and incorporated trade data from adjacent permits, to generate a rational depositional model for the succession. Three distinct third-order sequences have been recognised from the detailed correlation of regional flooding surfaces recognised on wireline log motifs calibrated against conventional core and biostratigraphy. The base of the oldest third-order sequence includes section previously referred to as Plover Formation, and roughly corresponds to the W.digitata/W.indotata zone boundary. The Callovian Unconformity within the ZOCA region is thus relegated to a third-order sequence boundary or disconformity. The term Elang Formation is proposed for this Callovian succession which comprises three third-order sequences mappable across ZOCA. The well-type section for the Elang Formation is Elang-1, and an additional well reference section would be Elang-2, as both these wells contain significant and complementary cored section.Detailed sedimentological studies on conventional core reveal that the Elang Formation comprises a succession of coastal plain to nearshore marine sediments, ranging from low sinuosity fluvial channel, fluvial-dominated deltaic, proximal low sinuosity estuarine channel and distal outer bay sediments. Only minor wave-dominated, open marine shoref ace intervals were interpreted, most of the cored intervals indicating a fluvially-domi-nated shoreline with minimal wave reworking. Isopach and per cent sand maps generated for each third-order sequence comprising the Elang Formation illustrate the successive sediment distribution patterns across ZOCA during the progressive marine transgression from the top of the fluvio-deltaic Plover Formation to the base of the offshore marine Lower Flamingo Group. The sand-trend maps for the three sequences which comprise the Elang Formation indicate a fluvial/estuarine-dominated delta system, sourced from the region of the Laminaria Field, AC/P8, building east and southeast out across the ZOCA region. A modern analogue of this delta system in both size and sedimentation style may be the Brahmaputra/Ganges Delta of East Bengal.

Morphology and sedimentology of two contemporary fan deltas on the southeastern Baja California Peninsula, Mexico

The San Juan de Los Planes drainage basin is located on the southeastern margin of the Baja California Peninsula, approximately 20 km southeast of the city of La Paz. Pleistocene alluvial fans have been built into the basin from the surrounding areas of high relief and are presently being eroded. At the fan apex, the dissection depth ranges from 20 to 40 m, and is 6 to 15 m at the coast.The sediments have been redeposited in several active modern fan deltas built out into the narrow shelf of La Ventana Bay. Shelf width ranges from 0 to 5 km (average 3 km). Wave energy is delivered from southeasterly storms and from the local sea breeze winds; the respective wave heights average 2 and 1 m.The Agua Caliente and Las Canoas fan deltas, the subjects of this paper, are located on the active El Sargento Fault Zone and are fed by channels that deeply dissect the older fan surfaces. The Agua Caliente fan delta is a classic Gilbert-type delta with a steeply inclined delta front. Wave reworking and redistribution of the sediments at the delta face are important and a wave-cut terrace has developed at about 6 m depth. The Las Canoas fan delta is a shoal-water-profile, Hjulstrom type with a gently inclined delta front. Wave energy is less important. The alignments of the coasts of both fans are controlled by the fault line. Sediment supply is largest for the Las Canoas fan delta. The fan deltas are examples of this depositional form as it has developed in an arid, tectonically active, high-relief and narrow neo-trailing edge margin.

Sector collapse, sedimentation and clast population evolution at an active island-arc volcano: Stromboli, Italy

Sciara del Fuoco is the subaerial part of a partially filled sector-collapse scar that extends to 700 m below sea level on Stromboli volcano. The collapse occurred 2200 m and >10 km from the shore to the NNW, a fan-shaped mounded feature comprises debris avalanche deposits (>4 km3) from two or more sector collapses. Volcaniclastic density currents originating from Sciara del Fuoco follow the topographic margin of the debris avalanche deposits, although overbank currents and other unconfined currents widely cover the mounded feature with turbidites. Historical (recorded) eruptive activity in Sciara del Fuoco is considerably less than that which occurred earlier, and much of the partial fill may have formed from eruptions soon after the sector collapse. It is possible that a mass of eruptive products similar to that in the collapse scar is dispersed as volcanogenic sediment in deep water of the Tyrhennian basin. Evidence that the early post-collapse eruptive discharge was greater than the apparent recent flux (≈2kg/s) counters suggestions that a substantial part of Stromboli's growth has been endogenous. The partial fill of Sciaria del Fuoco is dominated by lava and spatter layers, rather than by the scoria and ash layers classically regarded as main constituents of Strombolian (‘cinder’) cones. Much of the volcanic slope beneath the vents is steeper than the angle of repose of loose tephra, which is therefore rapidly transported to the sea. Delicate pyroclasts that record the magmatic explosivity are selectively destroyed and diluted during sedimentary transport, mainly in avalanches and by shoreline wave reworking, and thus the submarine deposits do not record well the extent and diversity of explosive activity and associated clast-forming processes. Considerable amounts of dense (non-vesicular) fine sand and silt grains are produced by breakage and rounding of fragments of lava and agglutinate. The submarine extension of the collapse scar, and the continuing topographic depression to >2200 m below sea level, are zones of considerable by-passing of fine sand and silt, which are transported in turbidity currents. Evidently, volcanogenic sediments dispersed around island volcanoes by density currents are unlikely to record well the true spectrum and relative importance of clast-forming processes that occurred during an eruption. Marine sedimentary evidence of magmatic explosivity is particularly susceptible to partial or complete obliteration, unless there is a high rate of discharge of pyroclastic material into the sea.

Post glacial lacustrine event sedimentation in an ancient mountain setting: Carboniferous Lake Malanz�n (Western Argentina)

Lacustrine deposits of the Malanzan Formation record sedimentation in a small and narrow mountain paleovalley. Lake Malanzan was one of several water bodies formed in the Paganzo Basin during the Late Carboniferous deglaciation. Five sedimentary facies have been recognized. Facies A (Dropstones-bearing laminated mudstones) records deposition from suspension fall-out and probably underflow currents coupled with ice-rafting processes in a basin lake setting. Facies B (Ripple cross-laminated sandstones and siltstones) was deposited from low density turbidity currents in a lobe fringe environment. Facies C (Massive or graded sandstones) is thought to represent sedimentation from high and low density turbidity currents in sand lobes. Facies D (Folded sandstones and siltstones) was formed from slumping in proximal lobe environments. Facies E (Wave-rippled sandstones) records wave reworking of sands supplied by turbidity currents above wave base level.

Sedimentary dynamics and evolutionary history of a Late Carboniferous Gondwanic lake in north‐western Argentina

ABSTRACTLacustrine deposits are well represented in the lower part of the Late Carboniferous Agua Colorada Formation in the north‐west Sierra de Narváez, Catamarca Province, Argentina. Lake Narváez was one of the several water bodies formed in the region immediately after the Gondwana glaciation.The lacustrine transport system has been divided into three distinct zones: delta, shallow lake and deep lake. Delta progradation proceeded from the ESE. Coarse‐grained delta plain and turbidite delta front deposits suggest that the delta was formed close to the headwaters (‘short‐headed stream delta’type). During periods of high discharge, river mouths acted as bypass zones and fine and very fine sands were transported further into the lake by underflow currents. The clastic material supplied by the deltaic system was partially reworked by wave action. Sands accumulated in unstable conditions at the upper delta front as a consequence of delta progradation. As a result of the addition of clastics in the steep delta front, turbidity currents were formed, spreading their load along the lower delta slope. Deep lacustrine deposits are typically stacked, forming two different kinds of progradational turbidite lobe sequences. Type I lobes were formed in a basinal setting and were probably detached from their feeder systems as a result of sediment‐bypassing in a shallow lake during periods of low lake level. These turbidite lobes are replaced upwards by type II lobes, which were formed on the delta slope during periods of lake level rise that allowed the onset of delta progradation. The presence of highly deformed sandstone bodies suggests rapid depositional rates in a high slope setting, whereas the occurrence of hummocky cross‐stratified sandstones indicates wave reworking of the sands initially emplaced by turbidity currents. Therefore, the inner part of type II lobes was formed above storm wave base. The depositional history of Lake Narváez can be traced through four evolutionary stages: lake transgression, formation of type I lobes, formation of type II lobes and delta progradation. Tectonic activity was probably important at the early stage of lake evolution, but the subsequent depositional history was mainly controlled by fluctuations of lake level.

Rhythmic Sedimentation in a Mixed Tide and Wave Deposit, Hazel Patch Sandstone (Pennsylvanian), Eastern Kentucky Coal Field

ABSTRACT The Hazel Patch sandstone is an informal unit of the lower Middle Pennsylvanian Breathitt Formation, which is locally dominated by facies containing alternating sand and shale layers, or rhythmic bedding. Three types of rhythmites are defined. Cyclic rhythmites show a sinusoidal vertical stacking trend of thickening and thinning laminae, amalgamated rhythmites show partially or possibly cyclic trends that have been reworked or erosively truncated by overlying formsets, and noncyclic rhythmites show only random thickness distribution and no evidence of original cyclicity. Lower in the sandstone, noncyclic rhythmites are associated with swaly to quasi-planar bedding, even bedding, and ripples with rounded crests, and are inferred to have formed in response to storm waves and combined flows. Higher in the sandstone, noncyclic, amalgamated, and cyclic rhythmites are present in association with herringbone stratification and pinstripe laminae, and are inferred to have formed under the influence of tidal currents. Cyclic tidal rhythmites are present near the base of a shallow channel deposit. These rhythmites contain sandstone-shale couplets indicative of presumed diurnal tides, 11-12 laminae cycles indicative of neap-spring tidal sedimentation, and 25-27 laminae cycles indicative of lunar monthly sedimentation. Upward and laterally within the channel trunc tion of neap-cycle laminae by spring-cycle ripples, and a loss of continuous clay drapes in amalgamated rhythmites, led to preservation of only monthly or greater depositional cycles. Vertical facies trends in the sandstone are interpreted to indicate upward shallowing from a wave-influenced outer estuarine or shallow shelf environment to an intertidal sand flat to an exposed low-lying coast upon which peats accumulated. The fact that cyclic rhythmites were preserved only in a shallow channel, stratigraphically near the middle of the mail may indicate that accommodation space and a medial estuarine position with little wave reworking and bioturbation controlled the preservation of the various orders of cycles.

Multiyear Fast Ice along the Taymyr Peninsula, Siberia

A 20 km wide zone of fast ice, reaching offshore to the 20 m isobath and sealing off coastal embayments along the Taymyr Peninsula, survived until at least 23 November 1993, becoming multiyear land-fast sea ice (MLSI). Barrier islands show characteristic recurved spits indicating dominant southeastward direction of longshore transport in intermittent years. At the time of observation, the fast ice was undisturbed, only heaving vertically with tides, protecting the coastal zone from wave reworking. The 1 to 2 m thick ice cover affects light and oxygen, creating profound effects on marine life in the zone. A herd of walrus, in other summers reported to haul out on local beaches, was displaced by the fast ice. We saw no evidence for sediment loading of the ice by rivers or eolian deposition on ice. MLSI is considered an incipient ice shelf, which could grow in thickness and strength in successive winters. Salinity and delta 18O profiles in ice cores from some smaller areas of MLSI provide a record of environmental conditions during the first year of ice growth, such as local hydrography and contribution of congealed snow to the solid fast ice. On ice charts we found no record of waxing and waning MLSI in the area, and do not know whether the observed ice survived the next summer. Since arctic ice shelves have developed from MLSI in the past, the observed phenomenon permits a view of how transitions from interglacial to full glacial conditions can begin.Key words: multiyear land-fast sea ice, incipient ice shelves, sediment entrainment, interglacial/glacial transition, Siberia, Laptev Sea

Ostracod biostratinomy at lagoonal shorelines: examples from the Great Estuarine Group, Middle Jurassic, Scotland

The Duntulm and Kilmaluag formations of the Middle Jurassic (Bathonian) Great Estuarine Group represent deposition in brackish-marine and fresh-brackish lagoons respectively. Deposition of ostracod valves at or near to the shoreline of these lagoons is described. Populations of Darwinula and Paracypris , composed almost entirely of carapaces of adults and older juveniles, are ascribed to storm deposition above the normal water level. Desiccation cracks and ‘cup-in-cup’ preservation, the latter indicative of wave reworking, are used to document fluctuations between subaqueous and subaerial conditions during deposition of the Kilmaluag Formation.

Tectonic Event Stratigraphy in a Fluvio-Lacustrine, Strike-Slip Setting: The Boss Point Formation (Westphalian A), Cumberland Basin, Maritime Canada

ABSTRACT The Boss Point Formation (Westphalian A) was deposited in a rapidly subsiding strike-slip basin. The formation, about 800 m thick, comprises alternate braidplain and lacustrine associations. Sixteen megacycles, defined by sharp but nonerosive lacustrine flooding surfaces, each comprise a lower lacustrine and an upper braidplain package. Braidplain deposits invariably scour deeply into lacustrine strata. The lacustrine association consists of very fine sandstone, mudstone, claystone, coal, and limestone deposited in shallow, hydrologically open freshwater lakes. Lakes were filled by small, elongate, river-dominated deltas, and by suspended and biogenic sediment. Sand-rich deltaic represent mouth bar, subaqueous levee, crevasse channel and splay, and straight and sinuous distributary-channel subenvironments. Evidence of wave reworking is very scarce. Interdeltaic areas accumulated platy gray mudstone. As interdistributary bays aggraded to water level they were subject to oxidation and pedogenic modification, forming bright colors, prismatic (desiccation) fabrics, pseudo-anticlines, and calcretes. Thin but laterally extensive (possibly up to 40 km) abandonment facies comprising black, organic-rich mudstones, coals, and ostracod limestones suggest periods of clastic starvation. Dunes, current ripples, and rarely adhesion ripples are common on the top surfaces of braidplain sandstone packages, preserved beneath a blanket of lacustrine mudstone. Rare oscillation ripples attest to localized reworking of the fluvial sand during or soon after flooding. Flooding surfaces can be traced for 12 km, and possibly up to 40 km, and are attributed to instantaneous subsidence along the northwest margin of the basin during major earthquakes. One locality preserves evidence of abrupt, 180° paleoflow reversals above and below a lacustrine unit. This is interpreted to record abrupt basin tilting, first to the northwest, forming a lake that filled from the southeast, followed by tilting to the southeast. We interpret Boss Point megacycles to reflect the interaction of episo ic tectonic subsidence with longer-term changes in sediment supply controlled by cyclical climatic variation in the Milankovitch band. During periods of pronounced wet-dry seasonality when clastic supply was high, sandy and gravelly braidplain deposits prevailed, and any lakes that formed by earthquake-related subsidence were rapidly filled. During more humid periods, higher vegetative cover greatly reduced clastic supply, and lakes persisted for many thousands of years. Hydrologic changes accompanying the transition from relatively humid to strongly seasonal wet-dry periods might have resulted in the deep fluvial erosion observed at the base of each braidplain package.

The role of wave reworking on the architecture of storm sandstone facies, Bell Island Group (Lower Ordovician), eastern Newfoundland

ABSTRACTStacked shallow marine cycles in the Lower Ordovician, Bell Island Group, of Bell Island, Newfoundland, show upward thickening and upward coarsening sequences which were deposited on a storm‐affected shelf. In the Beach Formation each cycle has a facies sequence comprised, from base to top, of dark grey mudstones, light grey mudstones, tabular sandstones and mudstones, lenticular sandstones and mudstones, and thick bedded lenticular sandstones, reflecting a progressive increase of wave orbital velocities at the sediment surface. The mudstones and tabular sandstones reflect an environment in which the sea floor lay in the lower part of the wave orbital velocity field and in which tempestites were deposited as widespread sheets from weak combined flow currents. The lenticular sandstones in the succeeding facies are wave reworked sands, commonly lying in erosional hollows and having erosional tops and internal hummocky cross‐stratification. Planar lamination is relatively uncommon and sole marks are mainly absent. In this facies oscillatory currents were dominant and accumulated sand in patches generally 10–30 m in diameter. The facies formed on the inner shelf where the oscillatory currents generated by storm waves had powerful erosional effects and also determined the depositional bedforms. Mud partings and second‐order set boundaries within sandstone beds are believed to separate the products of individual storms so that many lenticular sandstone beds represent the amalgamation of several event beds. This interpretation has important implications for attempts to estimate event frequency by counting sandstone beds within a sequence and for estimates of sand budgets during storm events.The thick bedded lenticular facies appears to have been formed by erosion of the mud beds between the lenticular sands, leading to nearly complete amalgamation of several lenticular sand bodies except for residual mud partings.In the overlying Redmans Formation the process of amalgamation progressed even further so that nearly all the mud partings were removed, resulting in the formation of thick bedded tabular sandstones.Sequence stratigraphic analysis of the cyclical sequence suggests that the cycles were eustatically controlled. The rising limb of the sea level curve produced only the dark grey mudstone part of the cycle while the remainder of the cycle was deposited on the falling limb. There is a gradational but rapid facies transition from the tabular to the lenticular sandstone facies which is interpreted as occurring at the inflexion point on the falling limb. The thick bedded facies of the Beach Formation and the thick bedded tabular facies of the Redmans Formation represent periods of maximum sea level fall.The stacked cycles in the Beach Formation are interpreted as an aggradational, high frequency sequence or parasequence set bounded at the top by a sequence boundary and succeeded by the three aggradational parasequences of the Redmans Formation.The recognition of storm facies with sandstone beds of very different bed length has important implications for the reservoir modelling of such facies.

Riverine—tidal interactions in the monsoonal Gilbert River fandelta, northern Australia

The Gilbert River fandelta drains into the Gulf of Carpentaria, northern Queensland, providing a marginal deltaic facies in a stable intracratonic epeiric sea. The Holocene deltaic facies unconformably overlies an irregular Pleistocene surface and forms a lensoidal body up to 8 m thick and 25 km wide. Progradation of this facies has resulted from a high rate of sedimentation in an area with a low regional gradient. Channel avulsion in the mid to upper fluvial reaches of the fandelta has controlled sediment supply and the rate of progradation along the delta front. A succession of strandline sand ridges marks prior positions of the prograding delta but the rate of progradation has been enhanced by a slight decrease in sea level over the last 6 ka. The sand ridges, in turn, restrict the position in which distributary channels can reach the coast. Sediment movement within the delta region is controlled by a combination of tidal and monsoonal river hydrodynamics. Sand only reaches the delta front during monsoonal river flow conditions and as the latter decrease sand plugs are deposited within the river channel due to tidal retardation of river flow. During the remainder of the year channels in the tidal portion of the delta redistribute mud derived from wave reworking of the delta front deposits. This alternating bedload and suspended load depositional system is recorded by distinctive annual sand and silt couplets deposited on point bars and vertical accretion on the interchannel supratidal mudflats. Channel morphology in the tidally influenced portion of the Gilbert River fandelta can be divided into a lower sinuous tide-dominated segment and a landward river-dominated tidal channel section. These morphological features are similar to channel planforms shown in other northern Australian deltas with differences being attributed to variation in tidal range and annual river discharge.

Origin, evolution, and distribution of shoreface sand ridges, Atlantic inner shelf, U.S.A.

A computer mapping system was employed to document the location of 259 shoreface-attached and detached sand ridges in water <20 m deep and the temporal and spatial distribution of 309 historical and active tidal inlets along the U.S. Atlantic coast (Montauk Point, New York, to Miami Beach, Florida). This database was compiled through the analysis of over 600 historical maps, 50 bathymetric charts, and other published data. A genetic relationship between the location of certain historical and active tidal inlets and shoreface-attached sand ridges is documented. It is inferred that ebb-tidal deltas provided the initial sand source for the development of many shoreface-attached sand ridges. Although shoreface-attached sand ridges appear to have several different modes of formation, a two-step process for the development of most shoreface-attached and detached sand ridges along U.S. Atlantic barrier island and cape coastlines is proposed: (1) sand is deposited as ebb-tidal deltas or river deltas along the lower shoreface and/or inner continental shelf prior to or during transgression, followed by (2) further transgression, which reworks the deltaic sand bodies into linear sand ridges at the base of the shoreface by shelf processes. The best-developed shoreface sand ridge fields along the U.S. Atlantic shelf lie adjacent to shorelines characterized by all of the following: (1) transgression, (2) mixed energy, wave-dominated barrier islands, and (3) laterally migrating tidal inlet systems. Tidal inlet systems are natural sediment sinks that capture sand carried by longshore sediment transport. Ebb-tidal delta deposits associated with these migrating tidal inlets provide the initial sand body for the development of shoreface-attached sand ridges. The oblique orientation and linear form of shoreface-attached sand ridges appear to be a function of shoreline transgression, lateral inlet migration, and wave reworking of ebb-tidal delta deposits concentrated along an ebb-tidal delta retreat path. Shelf processes act as modifying agents in the evolution of sand ridges during and after ebb-tidal delta deposition. In general, shoreface-attached sand ridges are poorly developed or absent along eroding headlands, spits, and barrier island shorelines characterized by naturally stabilized tidal inlets. These latter inlets and shoreline types are dominated or influenced by one or all of the following: (1) antecedent topography, (2) higher tidal ranges, (3) larger tidal prisms, (4) lower wave energies, or (5) finer grain sizes. The coupling of shoreline and shallow marine sedimentary processes during a transgression is critical to the origin, evolution, and distribution of shoreface sand ridges in the study area. Modern shoreface-attached sand ridges are also known to occur in different coastal and shelf settings where large amounts of sediment were supplied to the shoreface and inner shelf during, or immediately before, transgression. Commonly observed vertical and lateral interrelationships of Holocene shoreface-attached sand ridges and tidal inlets or distributary channels have important ramifications for the development of shelf sandstone facies models. In addition, the geometric relationship documented in this study provides predictive petroleum and hard mineral exploration models of the spatial and temporal distribution of shoreface and shelf sand ridges.

Subtidal carbonate cycles: Implications for allocyclic vs. autocyclic controls

Thick successions of repetitive metre-scale subtidal carbonate cycles require that short-term sedimentation rates approximate long-term accumulation rates in order to maintain submergence below peritidal depths. Intrinsic processes such as storm and wave reworking and redistribution may act to inhibit aggradation into the zone of optimal carbonate production. The presence of subtidal vs. peritidal carbonate cycles may be a function of energy regime that in turn may relate to platform morphology. The ultimate control on stacked subtidal cycles appears to be eustasy acting in concert with intrinsic processes such as storm and wave reworking.

Geologic Reservoir Model for the Triassic Doig Formation, Northeast British Columbia, Canada

A subsurface investigation of the mid-Triassic Doig formation in northeastern British Columbia documented two main reservoir facies. Both are a product of mass movement and sediment gravity flow processes on a progradational, tectonically active continental shelf margin. Substrate instability was likely a product of sediment loading, perhaps in concert with seismic activity. Sedimentary facies and reservoir parameters were determined from analysis of approximately 150 cores and 900 well logs. Laterally discontinuous Doig sandstones are up to 60 m thick and trend northeasterly within the study area. The main reservoir facies are incised density flow deposits and laterally extensive slump deposits. Reservoir quality within these sands is extremely variable with porosity ranging from less than 5% to 15%. In core, these deposits consist of moderately well sorted, very fine grained sandstones with no vertical grain size variation. The best production to date is in the Buick Creek field with initial flows of 346 BOPD. The slump deposits are thinner and tend to be more elongate parallel to paleoshoreline. These sands were subject to some wave or current reworking. Modern analogs where similar processes and products of deposition are known to occur include the Gulf of Alaska continental shelf and the Frasermore » River Delta slope. Doig sandstones usually are enclosed in fine-grained shelf deposits that provide a good stratigraphic trapping mechanism. Successful development of Doig reservoirs must incorporate geologic modes that assist in understanding the complex and highly variable reservoir quality of sandstones units.« less

Significance of styles of epicontinental shale sedimentation in the Belt basin, Mid-Proterozoic of Montana, U.S.A.

Within the strongly shale dominated sediment fill of the Mid-Proterozoic Belt basin a large variety of shale types can be distinguished. Sedimentological investigations of several formations have yielded valuable data about lateral associations of shale types as well as about the principal mechanisms of shale deposition. Depositional environments of shales in the Belt basin range from red bed mudstones of ancient flood plains to deep water mudstones in a turbidite setting. Graded silt/mud couplets are ubiquitous in most shales of the Belt basin. However, they differ in detail and can be related to a variety of depositional processes, such as low-density turbidity currents, sheet floods, storms, and wave reworking. In several formations there is evidence that microbial mats colonized the sediment surface and probably protected the sediment surface from erosion. Shales from comparable Phanerozoic settings generally lack the silt/mud couplets that are so commonly observed in shales from the Belt basin and other Proterozoic epicontinental basins. This difference probably reflects the proliferation of bioturbating organisms in the Phanerozoic. The absence of benthic microbial mats in Phanerozoic shales is probably due to the evolution of metazoan grazers towards the end of the Proterozoic.

Facies and sequence stratigraphic analysis in an intracratonic, thermal‐relaxation basin: the Early Proterozoic, Lower Quilalar Formation and Ballara Quartzite, Mount Isa Inlier, Australia

ABSTRACTThe Quilalar Formation and correlative Mary Kathleen Group in the Mount Isa Inlier, Australia, conformably overlie rift‐related volcanics and sediments and non‐conformably overlie basement rocks. They represent a thermal‐relaxation phase of sedimentation between 1780 and 1740 Ma. Facies analysis of the lower siliciclastic member of the Quilalar Formation and the coeval Ballara Quartzite permits discrimination of depositional systems that were restricted areally to either N‐S‐trending marginal platform or central trough palaeogeographic settings.Four depositional systems, each consisting of several facies, are represented in the lower Quilalar Formation‐Ballara Quartzite; these are categorized broadly as storm‐dominated shelf (SDS), continental (C), tide‐dominated shelf (TDS) and wave‐dominated shoreline (WDS). SDS facies consist either of black pyritic mudstone intervals up to 10 m thick, or mudstone and sandstone associated in 6–12‐m‐thick, coarsening‐upward parasequences. Black mudstones are interpreted as condensed sections that developed as a result of slow sedimentation in an outer‐shelf setting starved of siliciclastic influx. Vertical transition of facies in parasequences reflects flooding followed by shoaling of different shelf subenvironments; the shoreface contains evidence of subaerial exposure. Continental facies consist of fining‐upward parasequences of fluvial origin and tabular, 0·4–4‐m‐thick, aeolian parasequences. TDS facies are represented by stacked, tabular parasequences between 0·5 and 5 m thick. Vertical arrangement of facies in parasequences reflects flooding and establishment of a tidal shelf followed by shoaling to intertidal conditions. WDS facies are preserved in 0·5–3‐m‐thick, stacked, tabular parasequences. Vertical transition of facies reflects initial flooding with wave reworking of underlying arenites along a ravinement surface, followed by shoaling from lower shoreface to foreshore conditions.Parasequences are stacked in retrogradational and progradational parasequence sets. Retrogradational sets consist of thin SDS parasequences in the trough, and C, TDS and probably WDS parasequences on the platforms. Thick SDS parasequences in the trough, and TDS, subordinate C and probably WDS parasequences on the platforms make up progradational parasequence sets.Depositional systems are associated in systems tracts that make up 40–140‐m‐thick sequences bounded by type‐2 sequence boundaries that are disconformities. Transgressive systems tracts consist of C, TDS and probably WDS depositional systems on the platforms and the SDS depositional system and suspension mudstone deposits in the trough. The transgressive systems tract is characterized by retrogradational parasequence sets and developed in response to accelerating rates of sea‐level rise following lowstand. Condensed‐section deposits in the trough, and the thickest TDS parasequences on the platforms reflect maximum rates of sea‐level rise and define maximum flooding surfaces. Highstand systems tract deposits are progradational. Early highstand systems tracts are represented by TDS and probably WDS depositional systems on the platforms and suspension mudstone deposits in the trough and reflect decreasing rates of sea‐level rise. Later highstand systems tracts consist of the progradational SDS depositional system in the trough and, possibly, thin continental facies on the platforms. This stage of sequence development is related to slow rates of sea‐level rise, stillstand and slow rates of fall. Lowstand deposits of shelf‐margin systems tracts are not recognized but may be represented by shoreface deposits at the top of progradational SDS parasequence sets.

Biofacies and sediments in an emergent Late Pleistocene glaciomarine sequence, Skerries, east central Ireland

Late Devensian sedimentation (ca. 18 ka B.P.) on the coastal margins of the Irish Sea Basin occurred in shallow-water glaciomarine environments around retreating tidewater ice margins. High relative sea levels were the result of glacioisostatic crustal depression around the periphery of the last British ice sheet. At Skerries, in east central Ireland, ice-proximal glaciomarine diamict, mud, sand and gravel facies are selectively preserved in a bedrock depression. The coastal exposure, about 500 m long, can be divided into three lithofacies associations. (1) A diamict/gravel association resting on bedrock was emplaced by sediment gravity flows and records the downslope reworking of glacial debris released from a retreating ice margin. (2) An overlying mud association was deposited from suspended sediment plumes, with minor ice-rafting, and records ice retreat and increasing distality of the site. Muds contain a mixed foraminiferal microfaunal assemblage typical of open-water shallow-marine shelves. Species can be grouped into reworked temperate, cosmopolitan and well-preserved boreo-arctic elements and indicate mixing of derived and retransported temperate species with an in-situ glaciomarine fauna. (3) At the top of the section a gravel association, derived from a nearshore fan delta, rests on planated muds and is attributed to wave reworking and slumping of the fan delta front in shallow water during rapid basin rebound. This and other sequences elsewhere in the Irish Sea Basin is a record of rapid evacuation of the basin by calving tidewater glaciers and the uplift and erosion of glaciomarine sedimentary sequences in response to rapid late glacial crustal rebound.

Early Carboniferous (Tournasian-early Visean) Global Paleogeography, Paleostorm Tracts, and the Distribution of Waulsortian and Waulsortian-Like Carbonate Mud Mounds: ABSTRACT

Tournasian-early Visean mud mounds (i.e., Waulsortian and Waulsortian-like mounds) are unlike other carbonate buildups in the stratigraphic record because they lack an identifiable frame-building organism. Waulsortian mounds are comprised mainly of carbonate mud; Waulsortian-like mounds are mud-rich and contain a significant percent of skeletal grains, especially crinoids and bryozoa. This study has revealed that all of the reported Waulsortian and Waulsortian-like mounds developed in low paleolatitudes either on the southern shelf margin of the Laurussian paleocontinent or in the Laurussian interior seaway. Waulsortian and Waulsortian-like mounds are specifically not present in low-latitude regions of other paleocontinents. As Tournasian-early Visean carbonate deposition was widespread in the range of 30{degree}N to 10{degree}S, the very restricted paleogeographic distribution of Waulsortian and Waulsortian-like mound locations suggests a mechanism or set of conditions that effectively limited the distribution of mud mounds. Considering the Tournasian-early Visean distribution of paleocontinents and the principles that govern the movement of modern hurricanes, tropical storms, and winter storms, the tracts of hurricanes, tropical storms, and winter storms probably crossed all main submerged paleocontinental areas except the southern Laurussian shelf margin and the Laurussian interior seaway, the two areas where mud mounds developed. The lack of storm energy in these twomore » large areas of Laurussia provided long-term stability and thus enhanced the growth prospects of the frame-deficient Waulsortian and Waulsortian-like mud mounds. Lack of extensive periodic wave reworking and other storm-induced devastation helps to account for enigmatic features such as general mound symmetry, great size, high depositional relief (as much as 220 m), and side steepness (as steep as 50{degree}).« less

Sedimentology and Depositional Environment of the Oligocene-Miocene Gas-Bearing Succession in the Yacheng 13-1 Field, South China Sea, People's Republic of China: ABSTRACT

The giant Yacheng 13-1 gas field, discovered in 1982, is located in the South China Sea some 70 mi south of Hainan Island. The Oligocene-Miocene reservoir succession accumulated on the shallow-dipping flank of an extensional half-graben basin. The reservoir comprises an at least 800-ft (250-m) thick clastic interval that forms part of a transgressive basin fill. Basin sedimentation commenced with localized fan deltas sourced from the underlying Mesozoic granitic-metamorphic basement and fed directly into a lacustrine environment Paleosols (gleysols, planosols, and possible podzoluvisols) and palynomorphs, together with the textural characteristics of the sediments and the common occurrence of coal, suggest a prevailing humid, but seasonal, paleoclimate and abundant vegetation. Marine transgression into the basin occurred abruptly from the southwest. An elongate marine embayment was formed, the geometry of which served to amplify prevailing tidal currents. Marine processes had a pronounced effect upon the fan deltas, which continued to supply sediment to the basin. Tidal current and wave reworking winnowed and removed much of the unstable lithic components from the sediments, resulting in increased maturity. With continued transgression the basement topography was eventually drowned and the fan delta/shoreface sequences were sealed by shelfal muds and silts. Reservoir quality is controlled bymore » burial diagenesis superimposed on the textural and mineralogical characteristics of the sediments. Poor reservoir quality typifies the immature, poorly sorted, lacustrine, fan-delta deposits, whereas good reservoir quality is characteristic of the texturally more mature, tidally reworked, marine, fan-delta, and shoreface sediments.« less

Ice Interaction with the Arctic Shelf and Coast: Constraints On and Implication for Petroleum Development: ABSTRACT

Along the Arctic coast of Alaska sea ice affects structures placed on and under the sea bed and influences the erosion and dispersal patterns of sedimentary particulates and pollutants. Most directly, ice gouges the sea floor to depths of a few meters, with return periods on the inner shelf of tens of years and, in places, annually, primarily during freezeup and breakup. These sea-floor ice-gouge patterns provide information on the motion and characteristics of the local ice canopy. The design and alignment of pipelines and coast structures should consider that ice gouging extends to the coast and onto the beaches; however, ice gouging is less apparent on beaches owing to lower gouge intensities and vigorous wave reworking. In the long term, gouging contributes to high rates of coastal and shelf erosion. Protruding ice keels extending downward from the sea-ice canopy divert and focus currents causing increased sea-floor scour to depths of a few meters below the sea floor. Sea-ice freezing during the commonly turbulent fall storms causes large volumes of sediment and pollutants to be sorted, resuspended, and, ultimately, incorporated into the ice canopy. Most entrained material is re-released to the nearshore the following open-water season however, some material maymore » be transported offshore to become part of the Arctic pack. During freezeup and breakup (4-5 months/yr) when these processes are most active, trafficability in the Arctic nearshore is nil owing to the instability of the ice canopy, and so our comprehension of this processes is limited.« less