Ancestral Gulf Research Articles

Evolution of the Cape Fear Terrace: A complex interaction between the Gulf Stream and a paleo-shelf edge delta

A high-resolution seismic stratigraphic study of the Cape Fear Terrace (outer continental shelf off Cape Fear, North Carolina) combined with bio-lithostratigraphic data have yielded a complex chronostratigraphic framework of the Quaternary sequences that comprise this portion of the North American continental margin. The Cape Fear Terrace is one of six terraces/protuberances that form significant topographic irregularities along the South Carolina and North Carolina shelf-slope break. The Cape Fear Terrace is situated upon an anomalous, point-source, prograding, shelf margin feature which has protruded out onto the upper slope throughout much of the Quaternary. This upbuilding/outbuilding followed a period of active submarine erosion in which the ancestral Gulf Stream and/or associated meanders cut an erosional path beneath the present shelf margin during the early Pliocene. The protuberance was originally constructed during a relative lowstand of sea level as a shelf-edge delta. Severe modification of this deltaic lobe occurred during a relative highstand of sea level as Gulf Stream activities began to more effectively modify the margin. The anomalous thick accumulation of shelf-edge sediments acted as a barrier to flow inducing complex flow patterns of the Gulf Stream causing preferential erosion on its downstream side. Subsequent deposition may have resulted during more elevated highstands of sea level, as evidenced by the presence of active sand waves in the terrace protuberance region today. Once this shelf-edge bathymetric irregularity had been established, the Gulf Stream acted as a dynamic force inducing cellular flow structures within the shelf environment and enabling sediments to be transported seaward along the paleo-shoals complex.

Regional Stratigraphy of Smackover Limestone (Jurassic) in South Arkansas and North Louisiana, and Geology of Chalybeat Springs Oil Field: ABSTRACT

ABSTRACT This stratigraphic and structural study of oil fields in South Arkansas and North Louisiana demonstrates the existence of at least four progradational calcarenite rock bodies or banks within the Smackover Formation of Jurassic Age. Each bank was deposited approximately parallel to the ancestral Gulf shoreline and basinward of the next older bank. All of these banks are major reservoirs for oil and gas pools. Prior to 1960, the Reynolds Oolite of the Smackover Formation of South Arkansas was correlated with the Smackover B Oolite of North Louisiana. The absence of anticlines with structural closure and the paucity of untested fault closures along the Arkansas-Louisiana state boundary produced little exploratory interest in the area. Discovery of oil at Lick Creek Field in 1960 and at Walker Creek in 1968 revealed a trend of traps and led to the geological investigation that resulted in the discovery in 1972 of Chalybeat Springs Field in Columbia County, Arkansas. Chalybeat Springs Field is a combination stratigraphic-structural trap in oolitic calcarenite at a depth of 10,250 feet in the Smackover B Limestone. The production is limited by the combination of a tilted anticline with porosity pinchout on its flank. The field originally had 28 producing wells and lO dryholes and covered approximately 4,500 productive acres. Original oil in place is estimated to have been 37 million barrels and cumulative production through 1985 was approximately 12 million barrels. The geologic relationships and principles observed in the Smackover Limestone of South Arkansas and North Louisiana can and have been successfully applied to other regions and in different aged carbonate strata in the geologic search for oil and gas production. Practical geologic studies of carbonate rocks aided by examination of drill cuttings, cores and thin sections combined with mechanical log evaluations result in lower finding costs for oil and gas reserves.

High-Resolution Seismic Stratigraphy of North Carolina Continental Margin, Cape Fear Terrace: Sea Level Cyclicity, Paleobathymetry, and Gulf Stream Dynamics: ABSTRACT

A high-resolution seismic stratigraphic study of the Cape Fear Terrace (outer continental shelf off North Carolina) combined with biolithostratigraphic data has yielded a chronostratigraphic framework of the Quaternary sequences that comprise this portion of the North American continental margin. The Cape Fear Terrace is an anomalous, point-source, prograding, shelf-margin feature that has experienced positive relief through much of the Quaternary. This upbuilding or outbuilding followed a period of active, early Pliocene, submarine erosion in which the ancestral Gulf Stream cut an erosional path beneath the present shelf margin. The terrace was originally built up during a relative lowstand of sea level with the construction of a shelf-edge deltaic feature. Severe modification of this delta front occurred during a relative highstand of sea level as the Gulf Stream began to impinge upon the margin. The anomalously thick accumulation of shelf-edge sediments acted as a barrier to flow, inducing complex flow patterns of the Gulf Stream. Excavation of these sediments yielded a ter ace feature with preferential erosion on the upstream side. Subsequent deposition in the terrace region may have resulted during fairly highstands of sea level, as evidenced by the presence of active seaward-prograding sand waves in the terrace region today. Once this shelf-edge bathymetric irregularity (the terrace) had been established, the Gulf Stream acted as a dynamic force inducing cellular flow structures within the shelf environment, which enabled sediments to be transported seaward along the paleo-shoals complex. End_of_Article - Last_Page 284------------

Jurassic Geology and Hydrocarbon Potential of Southwestern Alabama: ABSTRACT

The Jurassic Haynesville-Buckner-Smackover-Norphlet sequence in southwestern Alabama includes excellent deep reservoirs and laterally equivalent source rocks. The complex interaction of stratigraphy, syndepositional salt tectonics and faulting, and draping due to differential compaction over basement structures provides mechanisms for early entrapment of potentially significant hydrocarbon reserves in this area. Major Smackover-Norphlet production to the northwest and northeast, at Hatters Pond, Chunchula, and Jay fields, is from facies similar to those found in southwestern Baldwin County, Alabama. Detailed petrographic analyses of cores, chips, and cuttings samples from seven wells in southern Alabama and western Florida provide data for conceptualizing Upper Jurassic s ratigraphy (Norphlet-Lower Haynesville) and paleofacies relations in this area. The development of a regional Triassic Eagle Mills paleotopography in this area, including the interior salt basin, was influenced by existing Paleozoic basement structural trends and tensional tectonism related to incipient opening of the ancestral Gulf of Mexico. In the easternmost part of the salt basin, deposition of the Werner Anhydrite and Louann Salt was succeeded by the southward progradation of fluvial lower Norphlet facies. Upper Norphlet facies include littoral and possibly eolian sandstones, which are among the oldest Jurassic hydrocarbon reservoirs in the region. Rapid inundation of the Norphlet surface in lower Smackover time resulted in the deposition of argillaceous carbonates (brown dense facies) in the area. This event was followed by the establishment and rogressive southward progradation of a shoal-water limestone facies mosaic, including high-energy grainstones. Many of these subsequently dolomitized grainstones have porosities of as much as 24% and good permeabilities despite their burial below 19,000 ft (5,791 m) in southern Baldwin County. However, commercial hydrocarbon production from such reservoirs has not yet been extended south into this area. Evaporitic sabkha facies of the uppermost Smackover and Buckner-Lower Haynesville subsequently prograded over shelf carbonates throughout most of the study area. Whereas the area south of the Baldwin County graben generally appears to have remained basinal in character throughout all of Smackover time, Buckner-Lower Haynesville deposits in the Amoco 2 Amos well in Sec. 32, T7S, R4E, include thick grainstone shoal deposits which indicate the local existence of an emerging positive area south of the graben. This facies could provide for a significant stratigraphic-structural hydrocarbon accumulations in this heretofore unexplored region. End_of_Article - Last_Page 1561------------

Subsurface Wilcox Lignite in West-Central Louisiana: ABSTRACT

ABSTRACT The Wilcox Group in west-central Louisiana occurs as a wedge of terriginous clastic sediment of Paleocene age which prograded into the northern margin of the ancestral Gulf of Mexico. In this study, subsurface correlations provide the basis for dividing the Wilcox Group into three primary intervals. Proceeding upsection, these intervals are the lower Wilcox, upper Wilcox, and Carrizo Formation. The lower Wilcox is further subdivided into four regional lithologic units, informally referred to as Intervals 1, 2, 3, and 4. Lignite occurs as component facies of alluvial valley, deltaic, and lagoonal rocks. Lignite is identified from electric-log response on the basis of an operational definition, and environmental interpretations of associated facies are derived from log responses characteristic of deltaic environments. Expected properties of lignites are predicted by analogy with modern peat deposition and comparison with other ancient lignite accumulations. The Carrizo Formation was deposited as meanderbelt facies within an alluvial plain, but is devoid of any significant lignite accumulation because of destruction of overbank deposits. The upper Wilcox is a lagoon barrier-bar complex characterized by deposition of fine-grained sediment updip and strike-oriented accumulation of coarse-grained sediment downdip. The interval is basically not lignite bearing. A maximum of 8 seams occur updip of major strike-oriented sandstone. These lignites are expected to be of lagoonal origin and of poor quality. The lower Wilcox is a typical progradational deltaic complex to the east and marginal delta plain to the west. The lower Wilcox is the major lignite-bearing interval of the Wilcox Group and contains up to 35 lignite seams. The lignites are associated with interdistributary flood-basin deposits and should be of good quality, based on similarity to analogous deposits elsewhere.

Petroleum Geochemistry and Geology of Southeast Georgia Embayment and Florida-Hatteras Slope: ABSTRACT

End_Page 1583------------------------------Petroleum geochemical and geologic studies were carried out on the COST GE-1 well (Southeast Georgia Embayment), and on the Atlantic Margin Coring Project (AMCOR) core 6004 on the Florida-Hatteras slope. Mud additives contaminated some of the COST GE-1 samples, but by analyzing several duplicate handpicked sample suites the effect, although not totally removed, was minimized. In the COST well, the Tertiary shale-chalk-limestone section to a depth of approximately 1,088 m contains very small quantities of indigenous, biogenic hydrocarbons that are not believed to have had a thermal-chemical history. Upper Cretaceous rocks (Maestrichtian, Campanian, Santonian, Coniacian, and Turonian) from 1,088 to 1,814 m consist of gray, calcareous deep-water shales that contain the most organic-rich intervals in the well which are composed of thermally immature, amorphous, hydrogen-rich algal marine kerogens. If these Upper Cretaceous rocks were buried more deeply or found in a region of higher thermal gradient, they could be significant potential oil and gas source rocks. The Lower Cretaceous sedimentary rocks from 1,814 to 2,700 m are dominantly of continental origin wi h intercalated marine carbonate and sand units that contain very small amounts of terrestrial organic matter (less than 0.1% organic carbon). Although the kerogen in these rocks is of marginal thermal maturity, it promises little as a potential petroleum or natural-gas source. Sediments in the AMCOR hole 6004 range from Holocene to Upper Cretaceous and are of a predominantly outer shelf-upper slope depositional character. The hydrocarbon and fatty-acid distributions and molecular compositions are typical of marine biogenic sources that are thermally immature with regard to petroleum generation. The organic geochemistry of Paleocene sediments taken from the AMCOR core 6004 may reflect the influence of erosion by the ancestral Gulf Stream. End_of_Article - Last_Page 1584------------

On the hydrology of the Edwards Limestone, south-central Texas

The Edwards Limestone (Albian) is a mosaic of shallow water, back-reef, carbonate lithofacies averaging about 150 m thick, that have been dolomitized, chertified, and calcitized. Intermittent subaerial exposure during and shortly after deposition of the Edwards resulted in solution-enlargement of some primary voids. Slow upwarping of the northwestern margin of the subsiding ancestral Gulf of Mexico basin elevated the Edwards Group above sea level late in the Cretaceous. Down-to-the-coast, en echelon, normal faulting along the Balcones system during the Early Miocene accentuated the topographic position of the Edwards above sea level. Rejuvenated Gulf-ward flowing streams cut into the upthrown fault block and exposed the uppermost Edwards Limestone in the bottoms of deep canyons, which created the discharge sites that initiated a continuously circulating groundwater system within the Edwards Limestone in the Miocene. The early porosity systems have increased in volume through the self-ramifying cavern solution process that occurs in carbonate rocks. The down-dip boundary of the aquifer, or “bad-water line”, represents an early bypass boundary of the aquifer that has become deeply ingrained with time. Cavern development is largely sub-horizontal and reflects the control of bedding partings and beds of differing lithology. The Edwards Limestone aquifer has only a few widely separated natural discharge sites which, when viewed in scale, make the confined aquifer appear like a regional master conduit. This cavernous system is known as the Edwards underground reservoir and it supplies the water for most of south-central Texas.

Deposition and Diagenesis of James Limestone (Early Cretaceous) in East Texas Basin: ABSTRACT

Northeast-trending facies of the James Limestone of the East Texas basin reflect three depositional settings. From northwest to southeast they include (1) an area of quartz-sandstone deposition, most of which can be inferred to be of continental origin, (2) a reef and skeletal-oolitic grainstone belt that formed under high-energy conditions in nearshore shallow water, and (3) a broad area of open-marine shelf on which argillaceous lime mudstone and calcareous shales formed under relatively quiet-water conditions. Three depositional trends and their associated carbonate bodies can be differentiated in the reef and carbonate grainstone belt. One is an east-northeast-trending oolitic grainstone ridge of dune which can be mapped for at least 30 mi. A second is marked by northwest trending rudistid-reef complexes, one of which forms a conspicuous salient projecting southeastward from the main grainstone belt (Fairway reef) and another within the main grainstone belt (Quitman reef). The third depositional trend is also in the northwest and is seen as very large skeletal-oolitic grainstone bars separated from one another by channels. The northwest-trending channels, reefs, and oolitic grainstone bars strongly suggest that tidal currents from the ancestral Gulf in the southeast played an important r le in the deposition of the James reef and grainstone belt. Porosity and permeability distribution within the reef and grainstone belt were controlled profoundly by a sequence of three diagenetic events. In the first event, subaerial exposure of the reef and grainstone belt shortly after deposition resulted in the development of solution or moldic porosity and the precipitation of an iron-free calcite cement. Maximum development of solution or moldic porosity is in the thickest reef and oolitic buildups indicating that (a) areas of higher relief played an important role in the gathering and movement of meteoric water, and (b) areas of higher relief contained enough unstable carbonate (probably aragonite) so that the solution by meteoric water was maximized. Although a calcite cement was precipitated during this first diagenetic event, it did n t seriously occlude void space and, therefore, did not significantly lower porosity and permeability. In contrast to the first diagenetic event, a second event of ferroan calcite cementation and a third event of ferroan dolomite cementation resulted in widespread elimination of porosity and permeability, especially in areas between the thickest carbonate buildups and in limited zones within the carbonate buildups. The textural relations of these second and third cements clearly indicate that they were precipitated some time following subaerial exposure and prior to migration of hydrocarbons into the producing reservoirs. However, fundamental problems remain, the foremost of which is the determination of the source, or perhaps sources, of the fluids from which the cements were precipitated. End_of_Article - Last_Page 2205------------

Raham Conglomerate – new evidence for Neogene tectonism in the southern part of the Dead Sea Rift

SummaryA newly discovered, mainly continental, deformed, Neogene formation – the Raham Conglomerate – records the early existence of the southern part of the Dead Sea Rift. The field relations, the predominance of clasts derived from formations which have been virtually completely eroded, and the presence of pebbles of Precambrian rocks, all attest that a considerable structural relief – possibly exceeding 2 km – already existed in the Middle (?) Miocene. Brackish marine fossils indicate that an ancestral Gulf of Elat (Aqaba) also already existed.

Deltaic and Associated Facies of Difunta Group (Late Cretaceous to Paleocene), Parras and La Popa Basins, Coahuila and Nuevo Leon, Mexico: ABSTRACT

ABSTRACT The Difunta Group is gray calcareous mudstone, siltstone, and sandstone that interfingers with wedge-shaped redbed units. These complexly interfingered units are divided into 13 formations and numerous members, tongues, and lentils; the group has a thickness of 10,000 feet. Low sinuosity, high-bedload streams transported volcanic and sedimentary rock debris from the Sierra Madre Oriental eastward to the ancestral Gulf of Mexico. Separate rivers fed the Parras and La Popa basins. Sediment accumulated in arcuate, wave-dominated deltas that prograded slowly into water several hundred feet deep. Some sediment was transported by marine processes to shallow delta-flank environments. Delta-plain lake and interfluve deposits are bioturbated mudstone containing sparse leaves, charophyte oogonia, dinosaur bones, and oyster debris. Many beds were reddened by postdepositional oxidation of iron-bearing minerals. Delta-front sheet sands are 20 to 60 feet thick, well sorted, flat bedded, and have sparse Ophiomorpha. Delta-platform deposits are characterized by fantastic sequences of ball-and-pillow structure and a sparse molluscan fauna. Prodelta deposits are interbedded graded sandstone and bioturbated mudstone; the sandstone beds were deposited by turbidity currents generated at the delta front by hyperpycnal inflow. Carbonate banks up to 1000 feet thick developed in distal prodelta (shelf) environments on submarine highs that were sea-floor expressions of rising evaporite diapirs. At least four major regressive-transgressive cycles developed in response to episodic basin subsidence. In contrast to the Rocky Mountain Cretaceous transgressions, the Difunta accumulated sheet sandstone units 20 to 60 feet thick. Parts of the transgressive units are delta-destructional deposits, but many are coalesced progradational sheet sands.

Structure and Development of the Continental Margin of British Honduras

The continental margin of British Honduras is characterized by elongate ridges that are sub-parallel with the coast and that diverge slightly northward. Each ridge apparently is formed by aligned fault blocks composed of continental basement rock that rotated and subsided during rifting of the margin. The western part of the Cayman Ridge may be underlain by similar rock that extends southwestward into the margin. Little sediment has been trapped in the basins between the ridges, presumably as a result of long-term, low rates of sedimentation. A canyon system that is controlled by both fault-block ridges and cross faults transects the margin and opens into the Yucatan Basin. Deep-sea sediments are characterized by a succession of pelagic deposits above acoustic basement rocks; these deposits are covered by turbidites that have formed deep-sea fan deposits in some places. The change from pelagic to turbidite deposition may have occurred at the end of Cretaceous time when regional uplift provided a source for clastic materials. The principal channel for transport of sediment to the deep sea is believed to be a graben that extends northeastward from the southwest corner of the Gulf of Honduras. Development of the margin may have begun in Late Jurassic time when sphenochasmic rifting created the ancestral Gulf of Honduras. Near the end of Cretaceous time, a new set of motions began, with the Caribbean plate moving relatively eastward and slightly away from the North American plate. This new motion resulted in continued movement of basement blocks through Tertiary and Quaternary time. Although this margin is not young, it has a geomorphically youthful aspect in that the sedimentary sequences do not obscure the rift structures.

Carbonate Ramp--Alternative to Shelf Model: ABSTRACT

One of the most common depositional models for carbonate rocks is the shallow shelf. This model typically is constructed to show a nearly flat platform and a clearly defined shelf-slope break. Typically, there are detrital carbonates on the platforms, reefs or banks at the shelf-margin and basinal rocks seaward of the reefs. The usual analogs are the Florida-Bahamas (recent), the Cretaceous of Texas and northern Mexico (Edwards-El Abra), and the Permian of West Texas-New Mexico (Capitan model). Less commonly presented, but very important, is the ramp depositional model. The ramp model is an inclined platform that extends basinward without a pronounced break in slope. Carbonate facies, therefore, are not protected necessarily by a shelf-margin barrier. Reefs and facies patterns of the detrital carbonates tend to be distributed in bands which parallel the coastline and reflect the greater wave and current activity near the mainland shore. A modern example of the ramp model is the Campeche Bank. The detrital carbonates of the Campeche Bank are in concentric bands which range from grainstones and boundstones in shallow, agitated water to mudstones and wackestones on the seaward reaches of the ramp. Coral-algal reefs are common, but they do not occupy a position at the margin of the ramp. The ramp model is appropriate to explain the Jurassic carbonates of the Smackover and Cotton Valley around the ancestral Gulf of Mexico. The Jurassic ramp is modified by, and facies patterns are complicated by, salt tectonics. End_of_Article - Last_Page 1826------------

Smackover's Significant Walker Creek (Arkansas): ABSTRACT

Thirty years ago, the oil industry knew all there was to know about the Smackover southern Arkansas. All production from that formation came from large anticlines that were found by seismograph--then came Creek! Now, stratigraphic traps at 11,000 ft, oolite bars trending across low relief structural noses, and porosity pinchouts bring a new frontier for the wildcatter. Leave your old ideas and your old tools behind if you join this search. Gravity and shooting are not much help. The in combination is an imaginative subsurface geologist working with an operator who is interested trend plays. When you are tracing the pattern of offshore bars along an ancestral Gulf of Mexico shoreline, it is a big country. Wherever oolites are formed, beach or bar deposits can be present, and these are the ingredients for lucrative but elusive oil fields. How many Walker Creeks will be found from Mexico to Florida? That is an interesting challenge facing the industry today. End_of_Article - Last_Page 1689------------

Deltaic Origin of Difunta Group (Late Cretaceous to Paleocene), Parras Basin, Coahuila and Nuevo Leon, Mexico: ABSTRACT

The Difunta Group is predominantly gray calcareous mudstone, siltstone, and sandstone with 3 wedge-shaped redbed units. The redbeds provide a basis for subdividing the group into 7 formations with a composite thickness of 12,000 ft. Simple structure, continuous exposures, and lithologic variability provide an excellent opportunity for paleoenvironmental interpretations. The Difunta Group is largely the product of deltaic sedimentation in the Parras basin, a shallow embayment off the ancestral Gulf of Mexico. Major delta progradation was from southwest to northeast as shown by facies changes, thickness trends, and paleocurrent data. The redbed units are interpreted as delta-plain deposits that accumulated as lake and bay muds and silts and fluvial and distributary-channel sands. Redbeds are bounded by resistant gray blanket sandstones 20-60 ft thick that have sharp bases, scour-and-fill bedding, Ophiomorpha, and local mudstone, wood, and oyster clasts. These sandstones are interpreted as delta-front and delta-destructional deposits formed during progradation and retrogradation of the deltaic complex. Prodelta facies are gray sequences of either (1) burrowed mudstone or muddy sandstone, many beds of which have ball-and-pillow structure and a sparse molluscan fauna, or (2) interbedded graded sandstone and burrowed mudstone. The graded sandstone beds of the later facies were deposited by turbidity currents that were generated at the delta front probably by hyperpycnal flow. Patchlike carbonate banks up to 1,000 ft thick developed in the north during episodes of low terrigenous influx in this area. Bank deposits are chiefly micritic bioclastic rocks; boundstone is rare. End_of_Article - Last_Page 352------------