Paralic Systems Research Articles

Sedimentology and stratigraphic architecture of Barremian synrift barrier island–estuarine depositional systems from blended field and drone‐derived data

ABSTRACTThe Lower Cretaceous (Barremian) Camarillas Formation in the Galve Sub‐basin of eastern Spain is an exceptionally muddy, synrift, aggradational then retrogradational paralic succession. Deposition within these arid, equatorial paralic systems was strongly controlled by crustal rifting of the Iberia plate linked to the geodynamic evolution of the Atlantic Ocean, the Bay of Biscay and the Tethys Ocean. Although synsedimentary extensional tectonics controlled thickness and facies distributions, the parasequence stacking patterns point to a superimposed high‐order allogenic control on the paralic succession. Field data and drone imagery are combined to document changes in sedimentology and three‐dimensional stratigraphic architecture of these deposits to interpret changes in depositional environments as this basin filled. Three evolutionary stages are identified: (i) tide‐dominated estuary; (ii) mixed‐energy estuary, with a well‐developed wave‐dominated barrier island system; and (iii) barrier island–tidal inlet suite. An exceptional record of back‐barrier‐island depositional interactions is preserved in this high‐subsidence, extensional‐basin setting, including deposition of washover fans, flood‐tidal deltas and ebb‐tidal deltas. Drone‐derived imagery facilitates three‐dimensional architectural characterization of these complex paralic deposits, including multi‐episodic tidal inlets, and correlation of basin scale stratigraphic markers. Spatio‐temporal interactions between climate change, sea‐level variations and rift‐related subsidence generated complex estuarine and barrier island geobodies. The general transgressive trend recorded in the Camarillas Formation correlates well with global eustatic sea‐level rise during the Barremian.

Tidal amplification and along‐strike process variability in a mixed‐energy paralic system prograding onto a low accommodation shelf, Edgeøya, Svalbard

AbstractThe study describes the depositional development and sediment partitioning in a prograding paralic Triassic succession. The deposits are associated with the advance of large prism‐scale clinoforms across a shallower platform area. Approaching the platform, the limited accommodation and associated relative higher rates of deposition generated straighter clinoforms with lower foreset angles. The vertical restriction across the platform is interpreted to have amplified the tidal signature. Sediment was redistributed from the coast into increasingly sandy delta‐front deposits, compared to offshore equivalents. The deposits comprise extensive compound dune fields of amalgamated and increasingly clean sandbodies up‐section. Rapid deposition of significant amounts of sand led to differential subsidence and growth‐faulting in the delta front, with downthrown fault blocks further amplifying the tidal energy through funnelling. A mixed‐energy environment created along‐strike variability along the delta front with sedimentation governing process‐regime. Areas of lower sedimentation were reworked by wave and storm‐action, whereas high sedimentation rates preserved fluvially dominated mouth bars. A major transgression, however, favoured tidally dominated deposits also in these areas, attributed to increasing rugosity of the coastline. Formation of an extensive subaqueous platform between the coast and delta front dampened incoming wave energy, and tidally dominated deposits dominate the near‐shore successions. Meanwhile formation of wave‐built sand‐bars atop the platform attest to continued wave influence. The strong tidal regime led to the development of a heterolithic near‐shore tidally dominated channel system, and sandier fluvial channels up‐river. The highly meandering tidal channels incising the subaqueous platform form kilometre wide successions of inclined heterolithic stratification. The fluvially dominated channels which govern deposition on the delta plain are narrower and slightly less deep, straighter, generally symmetric and filled with cleaner sands. This study provides important insight into tidal amplification and sand redistribution during shallowing on a wide shelf, along with along‐strike process‐regime variability resulting from variations in sediment influx.

“Grain-size bookkeeping,” a new aid for siliciclastic systems with examples from paralic environments

ABSTRACTSedimentary logs form the foundation of many studies of ancient and modern sedimentary successions. In siliciclastic settings in particular, vertical grain-size trends are important records of past depositional processes and environments, so that they are recorded with care and are often central to final interpretations. It is rare for the actual grain size to play a similar role. Yet there is significant value in (i) digitizing sedimentary logs to produce statistical grain-size data and (ii) the deliberate study of grain size, a process described here as “grain-size bookkeeping.” To illustrate this, over 5.9 km of sedimentary logs have been digitized from the Miocene to Pliocene paleo–Orinoco delta, the Cretaceous Ferron Last Chance and Notom deltas, and the Jurassic Ravenscar Group. The digital data reveal how grain size partitions into distinct sedimentary facies, proximal-to-distal changes, changes related to stratigraphy and base level, and the overall grain size of paralic systems. It emerges that fluvio-distributary channels are the coarsest-grained sediment bodies in each of the studied systems. The coarsest material does not reach the shoreline, though the grain sizes of fluvio-distributary channels and shoreline sand bodies overlap, in accordance with the concept that the former feeds the later. By contrast, overbank sediments are relatively fine-grained, suggesting that, with the exception of channel belts, coarse sediment can largely bypass the delta plain. Grain-size changes occur across some key stratigraphic surfaces, but not consistently so. Channels in valleys are, on average, coarser than similar channels in unconfined systems, but, in the presented datasets, valleys do not contain the coarsest channels. The data have also allowed the analysis of down-system fining rates in ancient, sandy fluvio-distributary systems, with grain size being measured to decrease at rates ranging from 0.7 to 7.7 μm/km—values that compare favorably with modern rivers. Such large-scale trends are ornamented by, and link to, smaller-scale spatial changes associated with, for example, channel bars, crevasses, and mouth bars, and an initial dataset of associated fining rates has been collected. In general, very large systems (rivers) have low fining rates because of their great size, whereas the converse is true for small systems, especially if the grain size range is large. Consideration of downstream fining rates has led to the insight that avulsion initiates an unequal race to the shoreline. Suspended very fine sand and silt is likely to reach the shoreline with the avulsion flood waters, but bedload will advance far more slowly, perhaps too slowly to reach the shoreline before the river avulses again. Some avulsions may lead to notable temporal variations in the caliber of sediment supplied to shorelines. As expected, the largest system, the paleo-Orinoco, is the finest grained. The Ferron deltas have catchment areas 12 and 22 times smaller than the Orinoco, and are the coarsest grained. Remarkably, though their catchment areas differ by a factor of two, they have almost identical sand grain-size distributions. The data have also proved powerful in refining paleogeographic reconstructions, in particular suggesting “missing” depositional elements needed to complete local sediment routing systems. Careful tracking of grain size is also beneficial in that it is a key control on permeability, the description of which is crucial to the prediction of subsurface fluid flow.

Anatomy of a mixed-influence shelf edge delta, Karoo Basin, South Africa

Abstract The position and process regime of paralic systems relative to the shelf edge rollover is a major control on sediment transfer into deep water. The depositional strike and dip variability of an exhumed Permian shelf edge succession has been studied in the Paardeberg Ridge, Karoo Basin. Siltstone-rich slope turbidites are overlain by 25–75 m-thick prodelta parasequences. These are truncated by a 30 m-thick sandstone-prone unit of tabular or convex-topped sandstones, interpreted as wave-modified mouth bars, cut by multiple irregular concave-upwards erosive surfaces overlain by sandstones, interpreted as distributary channels. The stratigraphic context, lithofacies and architecture are consistent with a mixed-influence shelf edge delta; the erosional base to the unit marks a basinwards shift in facies, consistent with a sequence boundary. Channels become thicker, wider, more erosive and incise into deeper-water facies downdip and correlate with sandstone-rich upper slope turbidites, all of which support the bypass of sand across the rollover. The overall progradational stacking pattern results in a stratigraphic decrease in channel dimensions. The results of this study suggest a predictable relationship between channel geometry, facies and position on the shelf-to-slope profile under a mixed wave and fluvial process regime.

Environmental significance of Ophiomorpha in a transgressive–regressive sequence of the Spitsbergen Paleocene

The study focuses on occurrences of Ophiomorpha burrows in a transgressive–regressive succession composing the Early Paleocene Firkanten Formation deposited in paralic, prodelta to delta front conditions in the Central Basin of Spitsbergen. The burrows colonize sandstones of the Todalen Member at four sites and belong to two ichnospecies: Ophiomorpha cf. nodosa , forming dominantly vertical shafts; and O. cf. irregulaire , consisting of horizontal sinuous tunnels ending in subconical shafts. Both species are observed in shoreface sandstones deposited as a barrier bar. Lithological features and stratigraphic positions suggest that the trace-makers preferentially colonized high-energy sand environments. Foraminiferal faunas occurring below and above the barrier sandstones indicate brackish water conditions for the Ophiomorpha levels, which accordingly are of restricted, monospecific nature. It is inferred that the trace-makers had a dominantly suspension feeding habit as a modern analogue Calianassa major . Portrayed in a sequence stratigraphic framework, the Ophiomorpha -bearing sandstones in middle reaches of the Central Basin were deposited in the final stage of the transgressive systems tract, which drowned the underlying coal-bearing paralic facies. Moreover, in the northern, coal-rich parts of the basin, occurrence of Ophiomorpha signals marine ingression into the paralic system. Keywords: Trace fossils; Paleocene; shoreface sandstones; brackish water; transgression signal. (Published: 6 April 2016) Citation: Polar Research 2016, 35 , 24192, http://dx.doi.org/10.3402/polar.v35.24192

Global distribution of modern shallow marine shorelines. Implications for exploration and reservoir analogue studies

Deposits of marginal marine depositional systems make up significant hydrocarbon reservoirs in the rock record. These systems are deposited by a complex interaction between competing depositional processes which can result in heterogeneous and compartmentalized reservoirs. Shallow marine systems are described using a ternary classification describing the relative importance of wave, tide and fluvial processes at the coastline. With the advent of freely available remote sensing data, modern systems are being increasingly used as analogues for the ancient, however to date, there has been no systematic quantification of global modern paralic systems. The aim of the present study has been to map and classify all the world's shorelines by ternary process and to consider the distribution and controls on different shoreline types.The semi-automated classification of marginal marine clastic shorelines has been achieved by combining data from a series of proxies for the ternary processes. Combined with coastline morphology, an algorithm predicts shoreline classification with an 85% success rate when compared to manual interpretation. Using this algorithm, the global shoreline has been subdivided into 246,777, 5 km segments and the distribution and proportions of these analyzed.The first order classification subdivides 28% of the world's coastlines as depositional. Within the depositional coastlines 62% are Wave-dominated, 35% Tide-dominated and 3% Fluvial-dominated. Analysis of shoreline type distribution suggests a complex network of inter-related controlling factors. Of these, climate and tectonic setting are reasonably well constrained in the ancient and can be used to predict the probability of a specific shoreline type. In addition to shedding insight into the controls on the distribution of different shoreline types, the results of this study can also be used to identify suitable modern analogues for ancient systems, which in turn can be used to extract data for better reservoir characterization.

Micromorphological, geochemical, and diagenetic characterization of sirenian ribs preserved in the Late Miocene paleontological site of Cessaniti (southern Calabria, Italy)

The site of Cessaniti (Vibo Valentia, Italy) has been well known since the 19th century for the richness and good preservation of its Miocene fauna and flora. The sedimentary succession of the site represents a paralic system that evolved toward an open-marine environment recording the Tortonian transgression. The fossil assemblage contains rich invertebrate (corals, bivalves, gastropods, brachiopods, echinoids, benthic and planktonic foraminifers) and vertebrate faunas (proboscideans, rhinoceroses, giraffids, bovids, sirenids, marine turtles, and fish remains). The fossils recovered at the Cessaniti site have a relevant role in phylogenetic studies and paleogeographic reconstructions of Late Miocene environments of the southern Italy. This research is focused on the microstructure and preservation state of the fossil bones. Samples of Metaxytherium sp. bones have been analyzed to understand the diagenetic profile of the bone assemblages that characterizes the taphonomic history of the Cessaniti site. The analyses provided a comprehensive account of how bone mineral (bioapatite) has been altered and demonstrated that the post-burial processes did not significantly affect the micromorphological and biogeochemical features of the bones. The excellent preservation state of the bones strengthens the importance of the Cessaniti site for studies of the Mediterranean Miocene vertebrate fauna.

Preliminary Report on a Late Cretaceous Vertebrate Fossil Assemblage in Northwestern Coahuila, Mexico

A new vertebrate fossil assemblage of Late Cretaceous age is reported here. This discovery represents the results of the 2007 and 2008 field seasons in the locality known as �Las Jicoteas� situated in the municipality of Ocampo, in northwestern Coahuila, Mexico. The stratigraphic sequence where this fossil fauna occurs can be correlated to the Lower Member of the continental Aguja Formation that outcrops in the neighbouring Big Bend region of Texas. This new fauna bears the first record of a nodosaur for Mexico, and, also additional dinosaur remains attributed to the families Tyrannosauridae and Hadrosauridae are recorded. Associated fossil taxa include lepisosteid fishes, turtles and crocodile remains. The palaeocological conditions inferred by this new faunal assemblage suggest a paralic system with prodelta, marshes, lagoons, and nearshore marine deposits.

THECAMOEBIANS (TESTATE AMOEBAE) AND FORAMINIFERA FROM THREE ANCHIALINE CENOTES IN MEXICO: LOW SALINITY (1.5-4.5 psu) FAUNAL TRANSITIONS

Research Article| October 01, 2008 THECAMOEBIANS (TESTATE AMOEBAE) AND FORAMINIFERA FROM THREE ANCHIALINE CENOTES IN MEXICO: LOW SALINITY (1.5–4.5 psu) FAUNAL TRANSITIONS P. J. Van Hengstum; P. J. Van Hengstum 1 School of Geography and Earth Sciences, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada, L8S 4K1 1Correspondence author. Current Address: Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, B3H 4J1, Canada. E-mail: vanhengstum@dal.ca Search for other works by this author on: GSW Google Scholar E. G. Reinhardt; E. G. Reinhardt School of Geography and Earth Sciences, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada, L8S 4K1 Search for other works by this author on: GSW Google Scholar P. A. Beddows; P. A. Beddows School of Geography and Earth Sciences, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada, L8S 4K1 Search for other works by this author on: GSW Google Scholar R. J. Huang; R. J. Huang School of Geography and Earth Sciences, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada, L8S 4K1 Search for other works by this author on: GSW Google Scholar J. J. Gabriel J. J. Gabriel School of Geography and Earth Sciences, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada, L8S 4K1 Search for other works by this author on: GSW Google Scholar Journal of Foraminiferal Research (2008) 38 (4): 305–317. https://doi.org/10.2113/gsjfr.38.4.305 Article history received: 24 Mar 2008 accepted: 18 Jun 2008 first online: 03 Oct 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation P. J. Van Hengstum, E. G. Reinhardt, P. A. Beddows, R. J. Huang, J. J. Gabriel; THECAMOEBIANS (TESTATE AMOEBAE) AND FORAMINIFERA FROM THREE ANCHIALINE CENOTES IN MEXICO: LOW SALINITY (1.5–4.5 psu) FAUNAL TRANSITIONS. Journal of Foraminiferal Research 2008;; 38 (4): 305–317. doi: https://doi.org/10.2113/gsjfr.38.4.305 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyJournal of Foraminiferal Research Search Advanced Search Abstract This study presents the first systematic documentation of thecamoebians and foraminifera in anchialine cenotes (sinkholes) from Quintana Roo, Mexico. Thirty-three surface sediment samples (upper 5 cm) were collected from cenotes Carwash (1.5 psu), Maya Blue (2.9 psu) and El Eden (>3.3 psu). Q-mode cluster analysis of the faunal distributions isolated four low-diversity (Shannon diversity index 1.0–1.5) and salinity-controlled assemblages. Assemblage 1 (1.5 psu) is dominated by the thecamoebians Centropyxis aculeata “aculeata” (53%) and Arcella vulgaris (21%). Assemblage 2 (2.9 ± 0.2 psu) is dominated by Centropyxis aculeata “discoides” (41%) and Centropyxis aculeata “aculeata” (27%). Dwarfed (~50 μm) Centropyxis constricta “aerophila” (20%) with an autogenous test and Jadammina macrescens (29%) dominate Assemblage 3 (3.4 ± 0.2 psu). Finally, Ammonia tepida (51%), Tritaxis sp. (29%) and Elphidium sp. (11%) dominate Assemblage 4 (3.7 ± 0.4 psu). Thecamoebian and foraminiferal populations in the subtropical cenotes are distributed according to salinity variations as found in other temperate paralic systems. The centropyxid taxa trended towards ecophenotypes without spines with increasing salinity, and dwarfed and autogenous-shelled Centropyxis constricta “aerophila” were determined as the most euryhaline thecamoebian, persisting at the ecological boundary of the group (~3.3 psu). Importantly, the transition from a thecamoebian-dominated assemblage to a foraminiferan-dominated assemblage occurs at a salinity of approximately 3.5 psu. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geology and geochemistry of paleoproterozoic magnesite deposits (∼1.8Ga), State of Ceará, Northeastern Brazil

Five important magnesite mines and small occurrences in the state of Ceará form a sequence of lenses that extend, discontinuously, for over 140km. The magnesite rocks are hosted by metadolomites with lutecite, sulphate nodules pseudomorphosed by fiber-radiated quartz, scapolite and dissolution breccias. This metacarbonate sequence is more voluminous and more calcic in the southwestern extremity of the belt, but less voluminous and more magnesian towards the northeastern portion of the belt, the highest Mg contents being observed in the Alencar-Orós region. Magnesite deposits pass gradually into metadolomites and then to almost pure calcitic marbles westward the belt. This group is hosted in a greenschists to amphibolite-facies metavolcanic-sedimentary sequence crosscut by basic sills and granitic intrusions of variable size, form, composition and age (middle to late Proterozoic ∼550Ma). Two types of magnesite marble may be distinguished: (1) medium-grained (1 to 9mm) and (2) sparry-magnesite marble (1 to 15cm). The latter displays wide textural variation (porphyric, rosette, layered and palisadic types) and exhibits remnants of the original sedimentary features. The sparry crystals, in spite of their deformation, are hypidiomorphic and pinolitic. The colors vary from white to light or dark gray, or even red. The dark banded term presents traces of microfossils and stromatolites structures. The medium-grained magnesite marble, on the other hand, shows more homogeneous texture and color and anhedral crystals are more abundant. The sparry-magnesite marble is less enriched in SiO2, Fe2O3, Al2O3 and CaO and richer in MgO than the medium-grained ones. The magnesitic marbles display positive Ce, and negative Eu anomalies. This suggests a shallow marine platform environment with continental, reduced influence (lagunar). These magnesitic marbles also show positive Ce and Eu anomalies in relation to the present seawater, suggesting compositional differences, materialized mainly by an enrichement of these elements in the past seawater. Fluid inclusions in the medium-grained magnesites of the Riacho Fundo deposit are aqueous inclusions, while in the sparry magnesite deposits, particularly in Cabeça de Negro, aquo-carbonic inclusions dominate. Hydrocarbons, however, are found in both cases. The calcitic marble, deposited on open-marine environment, displays positive13C (+0.5 to 2.1 ‰ PDB), while dolomitic to dolostone marbles, deposited on less open marine environment, display negative shift (0.7 to −3.2‰ PDB). This13C fluctuation is within the range for carbonates deposited around 1.8Ga. The depositional environment of these carbonates is close to a paralic system, lagoon, with strong evaporitic conditions. The depressions were of variable depth and lengths and could be isolated, in particular by stromatolitic barriers, dried and flooded by seawater with continental inflows. The largest and deepest ones correspond to the sparry magnesite deposits. These magnesites are located in a lateral way to almost pure calcitic marbles, through dolomitic marbles. This points to a process of chemical differentiation within a paralic system, from open sea to landwards, more calcic in the first to more magnesian in the confined environment, in which lagoon waters are progressively purified from Ca through preferential precipitation of calcic carbonates and secondly of gypsum/anhydrite, which increases the Mg/Ca ratio, and lead carbonate deposits towards the magnesite pole. These metacarbonates also reveal a fractionation of the light REE, starting from the calcitic, through the dolomitic to magnesitic marbles. In the calcitic marbles, REE are in association with the clay fraction, while in magnesitic marbles, they seem to be in association with organic and/or soluble complexes. The magnesitic marbles are of sedimentary origin and underwent important diagenetic evolution before being metamorphosed during the Brasiliano orogenic cycle.

A glacial sequence stratigraphic model for temperate, glaciated continental shelves

Abstract Temperate, glaciated continental shelves are by nature complex basins in that they not only have typical low-latitude siliciclastic processes acting to produce a sedimentary record and depositional architecture, but they also have the consequences of glacial action superimposed. For these settings glacial systems tracts are defined and related to glacial advance and retreat signatures, which can then be evaluated relative to changes in other external forces. We define glacial maximum (GMaST), retreat (GRST), minimum (GMiST) and advance (GAST) systems tracts separated by bounding discon-formities, which are respectively, the grounding-line retreat surface (GRS), the maximum (glacial) retreat surface (MRS) and the glacial advance surface (GAS). Each glacial advance and retreat sequence is bounded by a regionally significant unconformity, a glacial erosion surface (GES), or its equivalent conformity. Parasequence motifs vary across the shelf but include facies dominated by the movement of grounding lines. Facies motifs may have subglacial till above a GES and this is the main facies of the GMaST; however, tills are commonly absent. Although diamictic debrites are often associated with groundingline deposystems, the GRST succession is dominated by sorted deposits of gravel, rubble and poorly sorted sands due to the dominance of glacial meltwater. These deposits commonly have the geometry of banks or wedges/fans. They form the offlap-break at the outer continental shelf and also form a retrogradational stacking of bank systems on the shelf. Banks are capped by glacimarine rhythmites including thin debrites, turbidites, cyclopsams and cyclopels and perhaps iceberg-rafted varvites in fan to sheet-like geometries. Above these are draped sheets of bergstone muds that grade into paraglacial muds. The GMiST occurs with glacial retreat onto land or into fjords. The GMiST is represented in nearshore areas by progradational deposits of paralic systems dominated by deltaic and siliciclastic shelf systems, and in offshore areas by condensed sections. The GAST is represented by the inverse of the GRST facies succession, but is also the most likely interval to be eroded during readvance.

GEOLOGIA E GEQQUÍMICA DOS ELEMENTOS MAIORES DOS DEPÓSITOS DE MAGNESITA PRÉ-CAMBRIANA (~1.8Ga) DA FAIXA MÓVEL ORÓS ( CEARÁ)

The five most important magnesite mines and some small occurrences of the Ceara State form an array of discontinuous lenses extending for about 140 km. The magnesite-rich rocks are hosted in metadolomites with lutecite, fibroradiated quartz pseudomorph after sulfate nodules, scapolite and dissolution breccias. The marbles are hosted by a metavolcanosedimentary sequence of the greenschist to amphibolite facies, and were intruded by basic sills and granites of size, form, composition and age that vary from the Meso to Neoproterozoic. Two types of magnesite marbles may be distinguished: (1) magnesite marble of medium grain-size (1 to 9mm) and (2) sparry magnesite marble (1 to 15cm). The sparry magnesite marble has porphyric, rosette, layered, and palisade textures, the latter having remains of the original sedimentary features. In spite of deformation, the sparry crystals are hipidiomorphic and pinolitic. The rocks vary from white to light gray or dark gray or even red. The dark banded terms have traces of microfossils and stromatolitic structures. The texture and color of the medium-grained magnesite marbles are more homogeneous and the crystals are more anhedral. The sparry magnesite marble is in general poorer in SiO 2 , Fe 2 O 3 , Al 2 O 3 and CaO and richer in MgO as compared to the medium-grained types. The paleogeographic interpretation suggests that these rocks formed near a paralic system (lagoon), with strong evaporitic conditions. The depressions were of variable depth and length and probably isolated by stromatolite barriers, dried and flooded by seawater with continental inflows. The largest and deepest deposits correspond to the sparry magnesite. Regionally, the magnesite deposits give place to dolomitic marbles, which grade into almost pure calcitic marbles, indicating a chemical differentiation within the paralic system, by which the lagoon waters turned progressively poorer in Ca by the precipitation of calcium carbonate, thus increasing the Mg/Ca ratio and leading to the precipitation of magnesite. The magnesite marble is than interpreted as of sedimentary origin, and underwent strong diagenesis before metamorphism and Neoproterozoic deformation.

The Vicksburg Formation of Texas: Depositional Systems Distribution, Sequence Stratigraphy, and Petroleum Geology

The lower Oligocene Vicksburg Formation of the Gulf Coastal plain contains major petroleum reservoirs in the Rio Grande embayment and is an economically viable target in other areas of Texas. Knowledge of the distribution of Vicksburg depositional systems is essential to understanding sandstone concentrations and, therefore, is fundamental to effective exploration and production of the Vicksburg section. The depositional setting of the Vicksburg reservoirs, their position in a sequence stratigraphic framework, and the influence these factors have on the petroleum geology of the Vicksburg are the focus of this paper. Surface and subsurface geological and geophysical data provided the framework for an analysis of the depositional systems and the petroleum geology of the Vic sburg. The two primary Texas Vicksburg depocenters, the Rio Grande embayment and the Houston embayment, are separated by the San Marcos arch, a deep-rooted structural nose. Within the embayments, sand-rich deltaic complexes merged along strike with barrier/strand plains. Contemporaneous growth faulting controlled depositional patterns of shelf-edge deltas in the Rio Grande embayment, but had only a minor effect on the configuration of the shelfal deltas in the Houston embayment. Smaller wave-dominated shelf delta complexes interspersed with barrier/strand plains extended across the San Marcos arch. Updip of these sandy paralic depocenters, fluvial systems traversed mud-rich coastal plain units. Seaward of the paralic systems, sand and mud deposits prograded across and built up over the r lict Jackson shelf and shelf-margin shales. These depositional complexes are contained in the systems tracts of one eustatic (Exxon) sequence. Vicksburg production from each of the three structural regions of Texas is characterized by reservoirs from different systems tracts and of distinct, different depositional origins.

Depositional, Diagenetic, and Structural Controls on Reservoir Properties of Low-Permeability Sandstone, Travis Peak Formation, East Texas

ABSTRACT The Lower Cretaceous Travis Peak Formation in East Texas is a low-permeability gas-bearing sandstone that commonly requires hydraulic fracture treatment to produce at economic rates. Stratigraphic, petrographic, and structural characterization of the Travis Peak has resulted in information that can be used by operators to evaluate and complete these reservoirs more efficiently. Depositional systems of the Travis Peak Formation in this region include braided to meandering fluvial, deltaic, and paralic systems. Reservoirs in the upper Travis Peak are tidal-channel, tidal-flat, and meandering fluvial channel sandstones, whereas reservoirs in the lower Travis Peak are braided fluvial deposits in wide channelbelts. Mudstones through out the Travis Peak are highly silty and thus do not stop vertical growth of hydraulic fractures created during well stimulation. As a result, hydraulic fractures are circular and may connect several stacked sandstones. Petrographic studies indicate that extensive quartz cement is the major cause of low permeability in Travis Peak sandstones. Because the volume of quartz cement increases with depth, matrix porosity and permeability decrease. However, natural fractures are more abundant in structurally deeper, more intensely quartz-cemented sandstones than in shallower sandstones. Mean azimuth of natural fractures is N80°E. Drilling-induced fractures and borehole breakouts indicate that maximum horizontal stress (SHmax) is oriented east-northeast, which is the direction in which hydraulic fractures will propagate. Because the hydraulic fractures propagate in a direction subparallel to the natural fractures, hydraulically induced fractures may not intersect as many natural fractures as they might if SHmax had some other orientation relative to fracture strike.

Tertiary Depositional Systems, Gulf of Mexico Basin: ABSTRACT

Thick, offlapping, terrigenous clastic wedges make up the principal fill of the Gulf basin. Proximal parts of these wedges consist of paralic deposits formed either as large-scale, high-constructive delta systems (with related strike systems) or as a series of smaller, high-destructive delta systems. Distal parts accumulated as continental slope deposits associated with salt diapir fields at the terminus of prograded paralic systems. High-constructive delta systems (e.g., lower Wilcox, Yegua, and Jackson) are comparable in scale and facies to Holocene Mississippi deltas. They were supplied by rivers with large-volume sediment discharge; fluvial facies are concentrated locally along the basin margin. These deltas consist dominantly of fluvial and fluvially influenced deposits, with extensive coal-bearing delta-plain facies, thick progradational delta-front sandstone facies, and very thick, organic-rich, prodelta mud facies. Progradational sandstone facies show either lobate or elongate patterns in plan. Delta systems of this type supported extensive strike-fed systems comparable to strandplain and barrier bar systems of the Holocene northwestern Gulf Coast. High-destructive delta systems (e.g., upper Wilcox and Frio) are analogous to the Rhone and other Holocene deltas with significant marine modification (chiefly wave action) of fluvially introduced sediments. These deltas were supplied by numerous, relatively small rivers with moderately high sand load; updip fluvial facies persist along the entire basin margin. High-destructive deltas are composed of a series of sandstone bodies with thickness axes roughly parallel with End_Page 152------------------------------ regional strike. Each of these deltas consists of local progradational sandstone facies (channel and channel-mouth bars) flanked marginally by extensive sandstone units reworked from channel-mouth bars. Associated prodelta mud facies are moderately thick to thin. High-destructive deltas supported local rather than areally extensive strike-fed systems. Principal oil and gas reservoirs in high-constructive deltas occur in the progradational delta front sandstones with trends controlled by geometry and distribution of these lobate or elongate sandstone bodies. Vertical stacking of sandstone bodies is common, resulting in multipay fields. Trends within these delta systems are discontinuous along strike, as facies between main prograded lobes consist mostly of muds and tight sands. Attendant growth faulting, salt doming, and mud intrusion cause structural traps. In related barrier bar and strandplain systems trends are regionally persistent with stratigraphic traps. Oil and gas trends in high-destructive deltas are defined by local cuspate-trending coastal barrier sands and downdip progradational channel-mouth bars; principal traps are tratigraphic. Continental slope systems, making up the distal parts of Gulf basin terrigenous wedges, have been penetrated only in younger units of the basin or in very deep wells. Systems are comparable in scale, composition, and structural association to modern continental slope deposits of the northwestern Gulf. Thick and rapidly deposited delta systems of the Gulf basin mobilized underlying deep-seated salt. Principal flowage was toward the distal front of the prograding systems resulting in distinct diapir fields coextensive with continental slope systems; minor flowage was toward thinner interdeltaic areas. Salt mobilization was a significant control in determination of facies fabric and growth faulting. Younger offlapping units inherited and perpetuated the tectonic grain established by unde lying systems. End_of_Article - Last_Page 153------------

Ancient Delta Systems of Gulf of Mexico Basin: ABSTRACT

Thick, offlapping, terrigenous clastic wedges make up the principal fill of the Gulf basin. Proximal parts of these wedges consist of paralic deposits formed either as large-scale high-constructive delta systems (with related strike systems) or as a series of smaller high-destructive delta systems. Distal parts accumulated as continental slope deposits associated with salt-diapir fields at the terminus of prograded paralic systems. High-constructive delta systems (e.g., lower Wilcox, Yegua, and Jackson) are comparable in scale and facies to Holocene Mississippi deltas. They were supplied by rivers with large-volume sediment discharge; fluvial facies are concentrated locally along the basin margin. These deltas consist dominantly of fluvial and fluvially influenced deposits, with extensive coal-bearing delta-plain facies, thick progradational delta-front sand facies, and very thick, organic-rich prodelta mud facies. Progradational sand facies show either lobate or elongate patterns in plan. Delta systems of this type supported extensive strike-fed systems comparable to strandplain and barrier-bar systems of the Holocene northwestern Gulf Coast. High-destructive delta systems (e.g., upper Wilcox and Frio) are analogous to the Rhone and other Holocene deltas with significant marine modification (chiefly wave action) of fluvially introduced sediments. These deltas were supplied by numerous, relatively small rivers with moderately high sand load; updip fluvial facies persist along the entire basin margin. High-destructive deltas are composed of a series of sand bodies with thickness axes roughly parallel with regional strike. Each delta consists of local progradational sand facies (channel and channel-mouth bars) flanked marginally by extensive sand units reworked from channel-mouth bars. Associated prodelta mud facies is moderately thick to thin. High-destructive deltas supported local rather than an extensive strike-fed system Principal oil and gas reservoirs in high-constructive deltas occur in the progradational delta front sands with trends controlled by geometry and distribution of these lobate or elongate sand bodies. Vertical stacking of sand bodies is common, resulting in multipay fields. Trends within these delta systems are discontinuous along strike as facies between main prograded lobes consist mostly of mud and tight sand. Attendant growth faulting, salt doming, and mud intrusion cause structural traps. In related barrier-bar and strandplain systems trends are regionally persistent and stratigraphic traps are common. Oil and gas trends in high-destructive deltas are defined by local cuspate-trending End_Page 1968------------------------------ coastal barrier sands and downdip progradational channel mouth bars; principal traps are stratigraphic. Continental-slope systems that make up the distal parts of Gulf basin terrigenous wedges have been penetrated only in younger units of the basin or in very deep wells. Systems are comparable in scale, composition, and structural association to modern continental-slope deposits of the northwestern Gulf. Thick and rapidly deposited delta systems of the Gulf basin mobilized underlying deep-seated salt. Principal flowage was toward the distal front of the prograding systems resulting in distinct diapir fields coextensive with continental-slope systems; minor flowage was toward thinner interdeltaic areas. Salt mobilization was a significant control in determination of facies fabric and growth faulting. Younger offlapping units inherited and perpetuated the tectonic grain established by und rlying systems. End_of_Article - Last_Page 1969------------

THE AMERICAN ASSOCIATION OF PETROLEUM GEOLOGISTS

Thick, offlapping, terrigenous clastic wedges make up the principal fill of the Gulf Basin. Proximal parts of these wedges consist of paralic deposits formed either as large-scale high-constructive d e I t a systems (with related strike systems) or as a series of smaller high-destructive delta systems. Distal parts acc u mu I a ted as continental slope deposits associated with salt diapir fields at the terminus of prograded paralic systems.