Triassic Rift Basins Research Articles

New insights into lithology and hydrogeology of the northern Newark Rift Basin

AbstractThe marginal facies of the Triassic rift basins in the eastern United States are poorly documented, particularly on the hinge or hanging wall margins. This study presents a lithological description and multiscale petrophysical analysis of basement rocks, overlying marginal facies of the early synrift strata, and the basal contact of the Palisade Sill that were drilled and cored in the northeastern part of the Newark Basin, near its terminus. The expression of the Stockton Formation differs from that in the central basin in having thinner layers, with uncertain temporal relationship to the type area. The bottom 50 m is lithologically distinct with brick‐red to dark‐purple mudstones and sandstones, abundant gypsum‐filled fractures, and a thin zone with anomalously high uranium concentration, not associated with organic‐rich mudstones as other occurrences in the basin. The crystalline basement is apparently Fordham gneiss, overlain by a thin sandstone layer and a dark‐purple hydrophilic mudstone. Despite the abundance of coarse‐grained strata and multiple sets of tectonic fractures, hydraulically transmissive zones are sparse, and do not uniquely correlate to fracture and/or matrix characteristics. Enhanced transmissivity may exist along intrusion boundaries due to enhanced thermal fracturing, but more hydraulic data are needed to verify it. Comparison of petrophysical data in two boreholes ∼210 m apart shows no direct correlation of individual lithological units and their hydraulic properties, although the overall formation characteristics are similar. The results highlight challenges for outcrop correlation at the marginal edges of the rift basins and estimating reservoir properties of these heterogeneous formations.

Preserved extent of Jurassic flood basalt in the South Georgia Rift: A new interpretation of the J horizon

At the end of the Triassic, ∼200 m.y. ago, the Central Atlantic Magmatic Province (CAMP), one of the largest igneous provinces in the world, was emplaced within a very short period of time. The flows, sills, and dikes that mark the event are predominantly preserved in Triassic rift basins along the Atlantic margins. Conventional wisdom implies that the areally largest of the CAMP flows is preserved in the South Georgia Rift, a Triassic rift basin buried beneath the Atlantic Coastal Plain. The extent of this flow has been mapped on the basis of a prominent seismic reflection referred to as the J horizon. This seismic horizon has been used as a time marker for estimating the end of rifting in the southern United States and the beginning of seafloor spreading. Reanalysis of existing well and seismic data, however, shows that the extent of the flood basalt is limited to a few areas, and that the J horizon coincides with the base of the Coastal Plain. This reopens the question of how the CAMP relates to the rift-drift transition of eastern North America.

Controls on the stratigraphic development of the Triassic Fundy Basin, Nova Scotia: implications for the tectonostratigraphic evolution of Triassic Atlantic rift basins

Abstract: The Late Triassic synrift succession of the Minas sub-basin (Bay of Fundy, Nova Scotia) includes a fluvio-alluvial fan-dominated unit (lower and middle Wolfville Formation), a transitional fluvio-playa unit (upper Wolfville Formation) and an upper playa-dominated package (Blomidon Formation). We describe the facies development in the 240 m thick upper Wolfville Formation, examine vertical facies stacking patterns and develop a depositional model for this succession. The unit comprises channelized and unconfined fluvial deposits interbedded with aeolian and playa sediments that grade upward into playa deposits. The succession comprises progradational and retrogradational fluvial packages and thin aggradational playa claystones. Comparison with 11 other Atlantic margin continental Triassic rift basins allows an assessment of the controls that govern rift basin development. The transition from fluvial to playa or lacustrine conditions occurs in all basins at different times. The lack of synchronicity indicates that global climate change was not a controlling factor. We suggest that the fluvial to lacustrine or playa transition was due to a decrease in source area relief related to a decline in regional tectonic activity, and that the basin was hydrologically closed throughout sedimentation. This tectonostratigraphic model may be appropriate for other Triassic basins developed along the Atlantic margin.

Structural style and evolution of a Late Triassic rift basin in the Central High Atlas, Morocco: controls on sediment deposition

AbstractLate Triassic continental sediments deposited in an active‐rift setting are exposed in the Oukaimeden‐Ourika Valley, located in the Central High Atlas Basin of Morocco. The Oukaimeden Sandstone Formation is dominated by ephemeral and perennial braided fluvial facies, and is an outcrop analogue for Triassic sandstone hydrocarbon reservoirs found in Atlantic margin and North African basins. This paper documents detailed analysis of the outcrop data to investigate the influence of tectonics on deposition and the interplay with climatic controls. The geodynamic evolution of the basin is interpreted to be influenced by Atlantic rifting to the west and the formation of the Tethys Sea to the north, which led to the development of ENE and NNE striking normal faults. The present‐day fault and outcrop geometry reflects later inversion due to the Alpine compression that led to uplift of the High Atlas and subsequent erosion.The ENE trending rift‐basin is bound by normal faults, which are probably in part reactivated older Hercynian structures. The facies distribution was controlled by a complex interplay of tectonic and climatic controls. Evidence for syn‐sedimentary movement of both fault sets is observed, with stratigraphic thickening and associated progressive change in bedding dip. These faults controlled the basin dimension, geometry of the half‐graben and created the accommodation for sediment deposition. The presence of breccia deposits close to the main ENE bounding faults indicates footwall erosion and deposition of basin margin fans. The location and orientation of the main fluvial system was controlled by these structures, and the main channel belts ran parallel and proximal to the controlling faults, whereas away from the main syn‐depositional faults overbank deposits dominate. Smaller contemporaneous NNE oriented faults are generally shorter with less throw, and had only a limited influence on gross sedimentation patterns. These faults are interpreted to be syn‐sedimentary, displaying characteristics that suggest basal detachment within the Triassic, and as such provide evidence for the extension direction during late Triassic time. A periodic change from ephemeral to perennial systems, with associated changes in architectural style, is potentially attributed to climatic control, although a structural influence cannot be dismissed. Copyright © 2009 John Wiley & Sons, Ltd.

Arsenic in groundwaters in the Northern Appalachian Mountain belt: A review of patterns and processes

Naturally occurring arsenic in the bedrock of the Northern Appalachian Mountain belt was first recognized in the late 19th century. The knowledge of the behavior of arsenic in groundwater in this region has lagged behind nearly a century, with the popular press reporting on local studies in the early 1980s, and most peer-reviewed research articles on regional patterns conducted and written in the late 1990s and early 2000s. Research reports have shown that within this high arsenic region, between 6% and 22% of households using private drinking water wells contain arsenic in excess of 10 µg/L, the United States Environmental Protection Agency's maximum contaminant level. In nearly all reports, arsenic in drinking water was derived from naturally occurring geologic sources, typically arsenopyrite, substituted sulfides such as arsenian pyrite, and nanoscale minerals such as westerveldite. In most studies, arsenic concentrations in groundwater were controlled by pH dependent adsorption to mineral surfaces, most commonly iron oxide minerals. In some cases, reductive dissolution of iron minerals has been shown to increase arsenic concentrations in groundwater, more commonly associated with anthropogenic activities such as landfills. Evidence of nitrate reduction promoting the presence of arsenic(V) and iron(III) minerals in anoxic environments has been shown to occur in surface waters, and in this manuscript we show this process perhaps applies to groundwater. The geologic explanation for the high arsenic region in the Northern Appalachian Mountain belt is most likely the crustal recycling of arsenic as an incompatible element during tectonic activity. Accretion of multiple terranes, in particular Avalonia and the Central Maine Terrane of New England appear to be connected to the presence of high concentrations of arsenic. Continued tectonic activity and recycling of these older terranes may also be responsible for the high arsenic observed in the Triassic rift basins, e.g. the Newark Basin. There are only two well-known cases of anthropogenic contamination of the environment in the northern Appalachian Mountain belt, both of which are industrial sites with surface contamination at that infiltrated the local groundwater.

Sequence stratigraphy of continental Triassic strata of Southernmost Brazil: a contribution to Southwestern Gondwana palaeogeography and palaeoclimate

The continental Triassic succession of southernmost Brazil comprises two second-order depositional sequences—the Sanga do Cabral (Early Triassic) and the Santa Maria (Middle to Late Triassic) supersequences. The first one includes ephemeral, low-sinuosity fluvial deposits developed on a low gradient plain. Based on fossil tetrapods, especially procolophonids, an Upper Induan age is estimated for this sequence. Facies association of the Santa Maria Supersequence indicates low-sinuosity fluvial rivers, deltas and lakes. This supersequence can be further subdivided into three third-order sequences (age provided by palaeovertebrate biostratigraphic data) as follows: Santa Maria 1 (Ladinian), Santa Maria 2 (Carnian to Early Norian) and Santa Maria 3 (probably Raethian or Early Jurassic) sequences. The Gondwanides paroxysms I and II in the Sierra de la Ventana–Cape Fold Belt are directly related to the development of both supersequences. The source area of the Sanga do Cabral Supersequence was located to the south. It consisted of an uplifted peripheral bulge situated landward of the retro-foreland system, from where older sedimentary rocks were eroded. The source area of the Santa Maria Supersequence was also positioned southwards and related to the uplifted Sul-Rio-Grandense and Uruguayan shields. The Santa Maria Supersequence stratigraphic architecture is comparable to the Triassic rift basins of Western Argentina. Diagenesis, facies and palaeontology of the studied succession suggest a dominantly semiarid climate during the Triassic.

Post-collisional crustal extension setting and VHMS mineralization in the Jinshajiang orogenic belt, southwestern China

The Jinshajiang orogenic belt (JOB) of southwestern China, located along the eastern margin of the Himalayan–Tibetan orogen, includes a collage of continental blocks joined by Paleozoic ophiolitic sutures and Permian volcanic arcs. Three major tectonic stages are recognized based on the volcanic–sedimentary sequence and geochemistry of volcanic rocks in the belt. Westward subduction of the Paleozoic Jinshajiang oceanic plate at the end of Permian resulted in the formation of the Chubarong–Dongzhulin intra-oceanic arc and Jamda–Weixi volcanic arc on the eastern margin of the Changdu continental block. Collision between the volcanic arcs and the Yangtze continent block during Early–Middle Triassic caused the closing of the Jinshajiang oceanic basin and the eruption of high-Si and -Al potassic rhyolitic rocks along the Permian volcanic arc. Slab breakoff or mountain-root delamination under this orogenic belt led to post-collisional crustal extension at the end of the Triassic, forming a series of rift basins on this continental margin arc. Significant potential for VHMS deposits occurs in the submarine volcanic districts of the JOB. Mesozoic VHMS deposits occur in the post-collisional extension environment and cluster in the Late Triassic rift basins.

Sediment-hosted micro-disseminated gold mineralization constrained by basin paleo-topographic highs in the Youjiang basin, South China

The Youjiang basin is a Devonian–Triassic rift basin on the southern margin of the Yangtze Craton in South China. Strong syndepositional faulting defined the basin-and-range style paleo-topography that further developed into isolated carbonate platforms surrounded by siliciclastic filled depressions. Finally, thick Triassic siliciclastic deposits covered the platforms completely. In the Youjiang basin, numerous sediment-hosted, micro-disseminated gold (SMG) deposits occur mainly in Permian–Triassic chert and siliciclastic rocks. SMG ores are often auriferous sedimentary rocks with relatively low sulfide contents and moderate to weak alteration. Similar to Carlin-type gold ores in North America, SMG ores in the Youjiang basin are characterized by low-temperature mineral assemblages of pyrite, arsenopyrite, realgar, stibnite, cinnabar, marcasite, chalcedony and carbonate. Most of the SMG deposits are remarkably distributed around the carbonate platforms. Accordingly, there are platform-proximal and platform-distal SMG deposits. Platform-proximal SMG deposits often occur in the facies transition zone between the underlying platform carbonate rocks and the overlying siliciclastic rocks with an unconformity (often a paleo-karst surface) in between. In the ores and hostrocks there are abundant synsedimentary–syndiagenetic fabrics such as lamination, convolute bedding, slump texture, soft-sediment deformation etc. indicating submarine hydrothermal deposition and syndepositional faulting. Numerous fluid-escape and liquefaction fabrics imply strong fluid migration during sediment basin evolution. Such large-scale geological and fabric evidence implies that SMG ores were formed during basin evolution, probably in connection with basinal fluids. It is well known that basinal fluids (especially sediment-sourced fluids) will migrate generally (1) upwards, (2) towards basin margins or basin topographic highs, (3) and from thicker towards thinner deposits during basin evolution. The isolated carbonate platform (as a basin paleo-high) and related syndepositional fault system, together with the unconformity-related facies succession, may have controlled the migration pathway of ore-forming basinal fluids and subsequently determined the location of SMG deposits in the Youjiang basin. Unlike Carlin-type gold deposits, SMG mineralization in the Youjiang basin may represent an integral aspect of the dynamic evolution of extensional basins along divergent continental margins.

A Geotraverse Across Northwestern Turkey: Tectonic Units of the Central Sakarya Region and their Tectonic Evolution

Abstract In the Central Sakarya area of Turkey there are two main Alpine continental units, separated by a south verging ophiolitic complex which represents the root zone of the İzmir-Ankara Suture Belt. The Central Sakarya Terrane in the north includes two ‘Variscan’ tectonic units in its basement. The Söğüt Metamorphic rocks represent a Variscan ensimatic arc complex and the Tepeköy Metamorphic rocks are characteristically a forearc-trench complex. The unconformably overlying Triassic Soğukkuyu Metamorphic rocks correspond to a part of the Karakaya Formation and are interpreted as a Triassic rift basin assemblage. These units are unconformably overlain by a transgressive sequence of Liassic-Late Cretaceous age that represents the northeastward deepening carbonate platform of the Sakarya Composite Terrane. The middle tectonic unit (the Central Sakarya Ophiolitic Complex) comprises blocks and slices of dismembered ophiolites, blueschists and basic volcanic rocks with uppermost Jurassic-Lower Cretaceous radiolarite-limestone interlayers. Geochemical data from basalt blocks suggest mid-ocean ridge basalt (MORB)- and suprasubduction-type tectonic settings within the Neotethyan İzmir-Ankara Ocean. The southern tectonic unit includes basal polyphase metamorphosed clastic rocks (Sömdiken Metamorphics), intruded by felsic and basic dykes and overlain by thick-bedded marbles. This assemblage is unconformably overlain by continental clastic rocks gradually giving way to thick-bedded recrystallized limestones, cherty limestones and pelagic limestones intercalated with radiolarites, and finally by a thick high pressure-low temperature (HP-LT) metamorphic synorogenic flysch sequence. This succession is identical to the passive continental margin sequences of the Tauride Platform. It is suggested that this passive margin was subducted during the Late Cretaceous in an intraoceanic subduction zone and affected by HP-LT metamorphism. The emplacement of the allochthonous oceanic assemblages and the collision with the Central Sakarya Terrane was complete by the end of the Cretaceous.

Paleofluid-flow circulation within a Triassic rift basin: Evidence from oil inclusions and thermal histories

The migration of subsurface fluid flow within continental rift basins has been increasingly recognized to significantly affect the thermal history of sediments and petroleum information. To gain insight into these paleofluid flow effects, the thermal history of the Taylorsville basin in Virginia was reconstructed from fluid-inclusion studies, apatite fission-track data, and vitrinite reflectance data. Models of thermal history indicate that the basin was buried to the thermal maximum at 200 Ma; a cooling event followed during which the eastern side of the basin cooled earlier and faster than the western side, suggesting that there was a differential uplift and topographically driven fluid flow. This hypothesis is supported by analyses of secondary oil and aqueous inclusions trapped in calcite and quartz veins during the uplift stage. Gas chromatograms of inclusion oils exhibit variable but extensive depletion of light molecular-weight hydrocarbons. The relative abundance of n-alkanes, petrographic observations, and the geological data indicate that the alteration process on these inclusion oils was probably neither phase separation nor biodegradation, but water washing. Water:oil ratios necessary to produce the observed alteration are much greater than 10,000:1. These exceedingly high ratios are consistent with the migration of inclusion oils along with fluid flow during themore » early stages of basin evolution. The results provide significant evidence about the role of a subsurface flow system in modifying the temperature structure of the basin and the composition of petroleum generated within the basin.« less

Seismotectonic structures along the Savannah River Corridor, South Carolina, U.S.A.

The Savannah River Corridor is defined herein to extend from the Brevard zone in South Carolina to just northwest of Charleston, South Carolina. The length of the Corridor is 400 km, the width 300 km, and the strike is S 40°E. The Corridor includes (1) the Savannah River Site, which is the locus of the most detailed seismic reflection data set in the eastern United States, and is the keystone for the extrapolation of regional reflection seismic data away from the Site, and (2) the South Carolina–Georgia seismic zone. The principal data used for the conclusions of this study are reflection seismic data acquired by the Consortium for Continental Reflection Profiling (COCORP), the Appalachian Ultra-Deep Core Hole (ADCOH) project, and Conoco, Inc. (SRS). Relatively higher seismicity appears to be associated with a crustal scale antiform that has been imaged in the Savannah River Corridor as well as to the northeast along strike in Virginia. An increase in earthquake activity occurs in that part of the crystalline allochthon that overlies and conceals a foreland-dipping imbricate stack that has developed in early Paleozoic rocks beneath the overthrust Piedmont crystalline allochthon. To the southeast, hypocenters of two SRS earthquakes (1985 and 1988), projected along the strike of the Pen Branch northwest-bounding fault of the Dunbarton Triassic rift basin plot on a highly reflective southeastward-dipping shear zone that underlies the basin. Here, the proximity of the hypocenters to the border fault of the Dunbarton basin suggests that syntectonic structures associated with the Pen Branch border fault might be involved. Some of the seismicity is probably associated with reactivation of older Paleozoic and Mesozoic structures.

Carboniferous through Jurassic paleomagnetic data and their bearing on rotation of the Colorado plateau

Small, yet systematic, differences between paleomagnetic poles derived from strata on the Colorado plateau and paleopoles determined from rocks on the North America craton have been interpreted to support the hypothesis of modest post‐Late Cretaceous clockwise rotation of the plateau, as a quasi‐rigid body, with respect to the craton. Using an iterative search for the best fit Euler pole and rotation angle, comparison of the best quality Late Carboniferous through Late Jurassic paleomagnetic poles from the Colorado plateau and the North America craton gives a cumulative rotation estimate (based on a rotation pole at 34°N, 105°W) of 7.4°±3.8° (95% confidence limits). A similar comparison using subsets of the cratonic database from localities in (1) northeast North America and (2) the craton platform interior give larger (8.8°±3.6°) and smaller (5.1°±3.8°) estimates, respectively, reflecting the fact that poles from localities in northeast North America, in particular those from Triassic rift basins, indicate a larger rotation (as concluded in direct pole to pole comparisons). The Euler pole, as determined by the paleomagnetic data only, can lie anywhere within a relatively large area that encompasses locations in the western United States previously proposed from geological observations. Paleomagnetic data and geologic observations, together or independently, do not support the hypothesis of a large Colorado plateau rotation (of 11° to 15°). If geologically reasonable, previous estimates of significant (>∼20 km) dextral slip along the eastern margin of the plateau require a position for the Euler pole east of the 105°W meridian.

Evolution of late Triassic rift basin evaporites (Passaic Formation): Newark Basin, Eastern North America

ABSTRACTThe Passaic Formation of the late Triassic Newark Supergroup is 2700 m thick and was deposited in series of wide, deep to shallow lacustrine environments in the Newark rift basin (eastern North America). The Passaic Formation can be divided into lower, middle, and upper sections based on depositional structures, composition and the distribution and morphology of its evaporites. Evaporites formed as a result of syndiagenetic cementation and/or displacive processes. Evaporitive minerals now include gypsum and anhydrite, although other mineral species, such as glauberite, may have originally existed. Most of the evaporites of the Passaic Formation occur within massive red mudstone and siltstone lithologies in the form of diffuse cements, void‐fillings, euhedral crystals, crystal clusters and nodules. These evaporites grew displacively within the fine siliciclastic matrix as a result of changes in the hydrochemical regimes of the rift basin.A well‐developed upward increase in the amount of evaporite material is present in the Passaic Formation. This resulted from: (1) long‐term, progressive increase in aridity, and (2) significant increase in evaporation surface area of the basin during its tectonic evolution. A nonmarine source for the evaporites is evident from the isotopic data. Sulphate δ34S ranges from 11%. to 3.3%. CDT, while δ18O ranges from + 15.1%. to + 20.9%. SMOW, indicating derivation from early diagenetic oxidation of organic sulphur and pyrite within the organic‐rich, lacustrine deposits. The 87Sr/86Sr ratios in sulphate are radiogenic (average 0.71211), showing the interaction of basin waters with detrital components and that the Newark Basin was isolated from the world ocean.Most of the original evaporites show evidence of diagenetic change to polycrystalline and polymineralic pseudomorphs now filled with recrystallized coarse‐grained anhydrite (1–3 mm size) and low‐temperature albite. Homogenization temperatures of fluid inclusions within the coarse‐grained anhydrite indicate crystallization temperatures for anhydrite in the range of 150° to 280°C. Such elevated temperatures resulted from circulation of hot water in the basin. Later exhumation of these rocks caused partial to total replacement of anhydrite by gypsum in the upper part of the section. The resulting increase in volume due to hydration of anhydrite at shallow depths also emplaced non‐evaporative satin‐spar veins (fibrous gypsum) along bedding planes and in fractures.While the local geology of the Newark rift basin controlled the distribution of facies, the sedimentological development of the Passaic Formation evaporites resulted from the world‐wide climatic aridity that prevailed during the late Triassic. because the Newark Basin sequence was only covered with about 3 km of sedimentary overburden that correspond to about 100°C and hence suggests that evaporites have experienced alteration by hot fluids.5 As the Triassic marks the greatest evaporite formation world‐wide and profound sense of parched continentality throughout the world existed before the final break‐up of the Pangea, the Passaic Formation evaporites are an example of the influence of these palaeoclimatic conditions at the eastern margin of North America.

Dynamic modeling of divergent margin formation: application to the U.S. Atlantic margin

A finite element model of extension in the continental lithosphere shows that, on the U.S. Atlantic margin, the presence of pre-existing weaknesses within the lithosphere can account for variations in subsidence and extension, hinge zone development and some aspects of magmatic activity during rifting. The lithosphere is treated as a layered viscoplastic material consisting of a quartz diorite crust and dunite mantle. The crustal weakness may results from thrust faulting during the Paleozoic Applachian orogeny, and is treated as a region within the crust with a weaker rheology. The mantle weakness results from thickening of the crust below the pre-rift Appalachian orogen. The model indicates that subsidence in the early stages of rifting is focused in the region encompassing in crustal weakness, at the location where the Triassic rift basins formed at about 225-195 Ma. Extension within the mantle during this stage of rifting is diffuse, and a shear zone develops within the lower crust connecting the crustal weakness to the mantle weakness, in a manner akin to simple shear. As rifting continues, extension within the mantle becomes focused in a narrow region under the mantle weakness. The crust above the mantle weakness is thermally weakened, and the locus of subsidence and crustal extension shifts to the region overlying the mantle weakness. The style of extension from this time onward is similar to pure shear. Seafloor spreading ultimately develops over the position of the original mantle weakness at about 175 Ma. The model accounts for a number of hitherto unexplained features of the U.S. Atlantic margin, including the shifting of the locus of extension from the Triassic basins to the deep offshore basins, hinge zone development, timing and position of magmatic activity, the location of the initiation of seafloor spreading, and the asymmetry of the conjugate margins of the rift. The predicted shift from simple shear to pure shear rifting style may be a characteristics of other continental rifts and passive margins.

Geochemical evidence for Triassic rifting in southwestern Florida

Mesozoic rhyolites and basalts from three drillholes penetrating the pre-Cretaceous basement of southwestern Florida were analyzed for their trace element and Sr and Nd isotopic compositions. Major and trace element abundances suggest the suite was erupted in an extensional environment, and isotopic data indicate derivation primarily from aged, enriched Precambrian continental lithosphere combined to varying degrees with an asthenospheric component. Nd model ages show no significant difference in the age of the lithosphere from which these rocks and volcanic rocks of Paleozoic age from North Florida were derived, arguing against major displacement across the peninsula, including along proposed extensions of the Jay fault and/or Bahamas fracture zone. We suggest that the geophysically inferred boundary across the peninsula, if a fault, may have had a strong normal component, consistent with the greater depth to basement south of the boundary. In this view, the southwestern Florida Mesozoic volcanic province (SFMV) may represent a part of a major Triassic rift basin associated with the breakup of Pangaea.

Basement Rocks and Structure, Northeast Gulf of Mexico

ABSTRACT Multifold seismic data tied to wells were used to study basement rocks in the northeast Gulf of Mexico region and to determine the influence of basement structure on the deposition of overlying Jurassic salt and sedimentary rocks. Basement is defined here as all rocks older than Middle Jurassic Louann Salt. Two large rift basins filled with Middle(?) Triassic-Lower Jurassic redbeds and volcanics occur northeast of the inferred extension of the Bahamas Fracture Zone (BFZ). These features appear to be the offshore extension of the South Georgia rift basin. South of the BFZ are wide areas of dipping intra-basement reflections (monoclines and synclines) truncated by a prominent erosional unconformity marking the top of basement. These rocks may represent Paleozoic sedimentary rocks (encountered in nearby wells) and/or further extensions of the Triassic rift basins. The basic architecture of the Gulf of Mexico basin formed during the Middle Jurassic when attenuation produced areas of thick and thin transitional crust as South America and Africa began moving away from North America. Thick transition al crust in the northeast Gulf of Mexico is characterized by broad highs (Middle Ground arch, Southern platform, Pensacola arch, and Sarasota arch) and lows (Apalachicola basin and Tampa embayment). Basement structure controlled the distribution and thickness of the Middle Jurassic Louann Salt. Salt was deposited in the Apalachicola basin and lower Tampa embayment but is absent over the basement highs. The updip limit of thick salt and the updip limit of salt tectonics coincide with a basement hingeline in the northern Apalachicola basin. Distribution, thickness, and paleogeography of the three over-lying Upper Jurassic sequences (Norphlet-Smackover, Haynesville, and Cotton Valley) also reflect the configuration of the underlying basement.

Depositional and Structural Histories of Richmond and Taylorsville Triassic Rift Basins: ABSTRACT

The Richmond and Taylorsville basins of Virginia display similar structural and sedimentological histories, which differ somewhat from those of other eastern North American rift basins. Both basins formed during the middle Carnian in response to the initial opening of the North Atlantic Ocean. Both are westward tilted half grabens whose border faults share a reactivated Paleozoic basement thrust (the Hylas zone). A dipmeter log from the Bailey 1 well (Richmond basin) shows that bedding dips toward the border fault and increases with depth. These relationships are interpreted to reflect syndepositional rotation and tilting of the basin above a listric normal fault. Unlike the red beds typical of other eastern North American rift basins, sediments of the Richmond and Taylorsville basins are mostly unoxidized gray sandstones and siltstones. The sediments of both basins can be divided into three facies associations. The lowest is a poorly sorted, coarse-grained, lithic arkose rapidly deposited in alluvial fans and braided streams. This was succeeded by sand, silt, and mud of a lower End_Page 1449------------------------------ gradient, fluvial-paludal setting. Organic-rich black shales accumulated in lakes (oxbows?) and overbank deposits. An influx of cross-bedded fluvial sands, possibly triggered by renewed border faulting on the west, characterizes the final stage of basin fill. These sediments represent fan-delta systems that prograded northeastward across the lacustrine-paludal lowlands. End_of_Article - Last_Page 1450------------

COCORP seismic profiling of the Appalachian orogen beneath the Coastal Plain of Georgia

A southeastward extension onto the Coastal Plain of an earlier COCORP traverse, which confirmed large-scale, thin-skinned thrusting of crystalline rocks of the southern Appalachians, has provided some of the most spectacular reflections yet seen in crustal seismic data. Most of the reflectors can be interpreted as either fault surfaces or as metamorphosed strata of late Precambrian—early Paleozoic age. They are consistent with the hypothesis that a major detachment extends eastward beneath this part of the orogen, although other interpretations with a more complex pattern of detachments or sutures are also possible. Large-scale overthrusting provides a mechanism for incorporating sedimentary rocks into the lower crust and may help to explain many of the layered features on crustal seismic data. Reflections from deep beneath the Coastal Plain indicate that the structural configuration of the rocks is complex and that the remains of a collision zone are being observed. Several east-dipping horizons, which bear strong similarities to thrust faults in Valley and Ridge sedimentary rocks, are seen in the basement at shallow and mid-crustal levels beneath the Coastal Plain. The Augusta fault, for example, displays a reflection which extends at a low angle some 80 km or more southeast of its surface position. In conjunction with surface geologic information, these new data demonstrate that late Paleozoic compressive deformation was pervasive and resulted in lateral movements in the upper crust extending from the Valley and Ridge to the crystalline rocks beneath the Coastal Plain — a distance of 400 km or more. A large antiform, cresting at about 2.3 sec, or about 6 km below the surface, and other structures beneath the Coastal Plain of Georgia deserve further consideration for petroleum exploration, although metamorphism may have eliminated petroleum from these rocks. Refracted arrivals and fault geometries indicate two Triassic rift basins beneath Coastal Plain sedimentary rocks, one of which has apparently not been recognized previously.