Berry H. Tew

Recognition of maximum flooding events in mixed siliciclastic-carbonate systems: Key to global chronostratigraphic correlation

The maximum flooding event within a depositional sequence is an important datum for correlation because it represents a virtually synchronous horizon. This event is typically recognized by a distinctive physical surface and/or a significant change in microfossil assemblages (relative fossil abundance peaks) in siliciclastic deposits from shoreline to continental slope environments in a passive margin setting. Recognition of maximum flooding events in mixed siliciclastic-carbonate sediments is more complicated because the entire section usually represents deposition in continental shelf environments with varying rates of biologic and carbonate productivity versus siliciclastic influx. Hence, this event cannot be consistently identified simply by relative fossil abundance peaks. Factors such as siliciclastic input, carbonate productivity, sediment accumulation rates, and paleoenvironmental conditions dramatically affect the relative abundances of microfossils. Failure to recognize these complications can lead to a sequence stratigraphic interpretation that substantially overestimates the number of depositional sequences of 1 to 10 m.y. duration.

Eustasy versus subsidence: Lower Paleocene depositional sequences from southern Alabama, eastern Gulf Coastal Plain

Four distinct sequences of rise and fall in relative sea level may be inferred from the lower Paleocene strata in southern Alabama. Southern Alabama is part of the eastern Gulf Coastal Plain, a passive continental margin. These third-order sequences are traceable from the western part of the study area, where a thick section of deltaic-influenced siliciclastic sediments was deposited in a subsiding salt withdrawal basin, to the eastern part of the study area, where a somewhat thinner sequence of carbonate and siliciclastic deposits accumulated on a stable platform formed over Paleozoic basement rocks. The fact that these sequences and their component systems tracts can be recognized and traced throughout southern Alabama, considering the significant differences in subsidence and sedimentation rates from the western part to the eastern part of the study area, indicates that some factor other than subsidence rates or sediment supply controls the cyclicity exhibited by these sequences. Eustasy is interpreted as that controlling factor. This interpretation is consistent with the observations of previous writers that the conditions of differential subsidence, climate, and sediment supply may overprint the effects of global sea-level change, but that these factors usually do not mask the impacts of eustasy.

Geologic Framework of Norphlet and Pre-norphlet Strata of the Onshore and Offshore Eastern Gulf of Mexico Area

ABSTRACT Hydrocarbon accumulations have been discovered in the Jurassic Norphlet Formation in the onshore and offshore eastern Gulf of Mexico area. An understanding of the regional geologic framework of the Norphlet and pre-Norphlet stratigraphic succession in the study area is crucial to successful exploration for and development of Norphlet hydrocarbon reservoirs in the eastern Gulf region. These strata include Jurassic sedimentary rocks and pre-Jurassic sedimentary and crystalline rocks. Of these strata, only the Norphlet Formation has produced economic quantities of hydrocarbons from reservoirs which include eolian dune, interdune, wadi, and marine sandstones. However, the pre-Norphlet geology and paleotopography of the area controlled, to a large degree, Norphlet depositional patterns and subsequent structural development. Especially important were the locations of pre-Jurassic paleohighs and the distribution and thickness patterns of the underlying Jurassic Louann Salt. Norphlet hydrocarbon traps are generally associated with structures that have resulted from halokinesis of the Louann. Traps include salt anticlines, faulted salt anticlines, and extensional faults associated with salt movement. In addition to these salt-related structural traps, traps resulting from pre-salt basement paleotopography may exist in the study area. These traps might include Norphlet structural highs associated with preexisting basement highs and Norphlet stratigraphic terminations along the flanks of basement highs. Where Norphlet reservoir facies are present, the key factor controlling hydrocarbon accumulation potential is structural setting. In the Mississippi-Alabama-Florida (MAFLA) region that is underlain by the Norphlet Formation, four structural settings have been identified. Types A and B structural settings are characterized by Norphlet sediments overlying basement or thin salt; no Norphlet fields have been established in these settings. Types C and D structural settings are characterized by structures related to movement of thick Louann Salt; all Norphlet fields established in the MAFLA region occur in these settings. Four geographic regions have been defined in the study area for the purpose of characterizing Norphlet petroleum potential. Region 1 has excellent potential for future Norphlet discoveries, Region 2 has low to moderate potential, and Regions 3 and 4 have very little to no potential.

An Integrated Lithostratigraphic, Biostratigraphic, and Sequence Stratigraphic Approach to Paleogeographic Reconstruction: Examples from the Upper Eocene and Lower Oligocene of Alabama and Mississippi

ABSTRACT Paleogeographic reconstruction involves determining the spatial and temporal relationships of strata that can represent diverse ancient environments of deposition. The production of a paleogeographic map at a given temporal horizon generally requires that a time datum be drawn between spatially separated stratigraphic intervals that often have different lithofacies which represent the same time interval. This method of reconstruction is often hampered by the fact that chronostratigraphic resolution provided by the most often used tool (biostratigraphy) is commonly inadequate when used in paleogeographic reconstruction across lithologically diverse strata that represent different environments of deposition at a specific temporal horizon. Unconformity-bounded depositional sequences determined from sequence stratigraphic analysis can be used as a basis for paleogeographic reconstruction. Major surfaces associated with depositional sequences present a physical stratigraphic framework that provides relative chronostratigraphic datums which may be used to construct paleogeographic maps. Surfaces associated with a depositional sequence that have chronostratigraphic significance include upper and lower sequence bounding unconformities, transgressive surface, and surface of maximum sediment starvation/maximum transgression. These bracketing surfaces are used as constraints to interpret relative time lines at various points within the sequence. Interpreted time correlative points are then used in conjunction with lithostratigraphic and biostratigraphic relationships to produce paleogeographic maps illustrating the regional distribution of paleoenvironments and rock types at various time horizons within relative sea level cycles. The Tejas A Gulf Coast (TAGC)-4.3 and TAGC-4.4 depositional sequences of Alabama and Mississippi provide excellent examples of the sequence stratigraphic method of paleogeographic reconstruction. The TAGC-4.3 sequence is a type 2 sequence that includes the Cocoa Sand (shelf margin systems tract), Pachuta Marl (lower transgressive systems tract), and Shubuta (upper transgressive systems tract/lower condensed section) Members of the Yazoo Clay the Red Bluff Clay/Bumpnose Limestone interval (lower highstand systems tract/upper condensed section), and the Forest Hill Sand (upper highstand systems tract). The TAGC-4.4 sequence includes the Mint Spring Marl Member of the Marianna Limestone (lower transgressive systems tract), the Marianna Limestone (upper transgressive systems tract/lower condensed section), and the Glendon Limestone Member of the Byram Formation (upper condensed section/highstand systems tract). Application of the methodology outlined above to these sequences indicates that sequence stratigraphy can be a useful tool in paleogeographic reconstruction and can greatly enhance the understanding and interpretation of chronostratigraphic relationships within a depositional basin that might not be resolved using other methods.

Paleocene-Eocene Lignite Beds of Southwest Alabama: Parasequence Beds in Highstand Systems Tracts

ABSTRACT In southwest Alabama, lignite or lignitic beds are present in at least five stratigraphic intervals that span approximately 8 million years of geologic time. Lignite is found in the Paleocene Oak Hill and Coal Bluff members of the Naheola Formation, the Paleocene Grampian Hills Member of the Nanafalia Formation, the Paleocene Tuscahoma Sand, and the Eocene Hatchetigbee Formation. Lignite beds range in thickness from 0.5 to 11 feet and consist of 30 to 53 percent moisture, 13 to 39 percent volatile matter, 4 to 36 percent fixed carbon, and 5 to 51 percent ash. The organic matter includes structured plant material characteristic of brackish water coastal marsh and freshwater swamp environments. Pollen of herbaceous angiosperms, bisaccate gymnosperms, and fungal elements dominate marsh-derived lignite assemblages, whereas pollen of arborescent angiosperms and fern and moss spores dominate swamp-derived lignite assemblages. These Paleocene and Eocene lignite beds occur as parasequence deposits in highstand systems tracts of five distinct third-order, unconformity-bounded depositional sequences. These sequences consist of lowstand systems tract sand or shelf margin systems tract sand, transgressive systems tract glauconitic sand and marl, and highstand systems tract fluvial-deltaic sand, silt, carbonaceous clay, and lignite. Generally, lowstand systems tract incised valley fill sand deposits overlie the lignite beds; however, where these sand beds are absent, a distinct transgressive surface (merged with sequence boundary) marked by clasts and marine fossils is developed. The lignite beds are interpreted as strata within highstand systems tract parasequences that occur in mud-dominated regressive intervals. As many as six individual lignite beds have been observed within a single highstand systems tract. Lignite beds were deposited in interdistributary coastal marsh and swamp environments as part of the delta plain of fluvial-deltaic systems, which prograded into southwest Alabama from the northwest. As sediment was progressively delivered into the basin from these deltas, the effects of relative sea level rise during an individual cycle were overwhelmed, producing a net loss of accommodation and concomitant overall basinward progradation of the shoreline (regression). Small-scale fluctuations in water depth, resulting from the interaction of eustasy, subsidence, and sediment influx and distribution, led to cyclical flooding of the coastal marshes and swamps, followed by periods of progradation and regression, as recorded by the superimposition of upward-shallowing, lignite-capped, mud-dominated parasequences on the overall regressive stratigraphic succession. Highstand systems tract deposition within a particular depositional sequence culminated with a relative sea level fall that resulted in a lowering of base level and an abrupt basinward shift in coastal onlap. During this lowstand, fluvial incision and erosion characterized landward areas. Following sea level fall, significant relative sea level rise resulted in marine inundation of the area previously occupied by coastal marshes and swamps.

Upper Jurassic Smackover Oil Plays in Alabama, Mississippi and the Florida Panhandle

ABSTRACT Five Smackover (Upper Jurassic, Oxfordian) oil plays can be delineated in the eastern Gulf Coastal Plain. These include the basement ridge play, the regional peripheral fault trend play, the Mississippi Interior Salt basin play, the Mobile graben fault system play, and the Wiggins arch complex play. Plays are recognized by basinal position, relationships to regional structural features, and characteristic petroleum traps. Within two plays, subplays can be distinguished based on oil gravities and reservoir characteristics. Reservoirs are distinguished primarily by depositional setting and diagenetic overprint. The basement ridge play is updip of the regional peripheral fault trend where the Jurassic Louann Salt is thin or absent; structures in this trend formed on pre-Jurassic basement rocks. The basement ridge play is characterized by structural and combination traps. Reservoirs in the Choctaw ridge complex subplay are peritidal, partially to completely dolomitized, oolitic, peloidal, and oncoidal grainstone. Reservoirs of the Conecuh and Pensacola-Decatur ridge complexes subplay are subtidal to supratidal oolitic, oncoidal, intraclastic, and peloidal dolograinstone and dolopackstone, fenestral dolostone, quartz sandstone, and algal doloboundstone. The regional peripheral fault trend play is basinward of the updip limit of the Louann Salt and is typified by salt related structural features. These structural features occur in association with the Pickens, Gilbertown, West Bend, Pollard, and Foshee fault systems and are generally parallel to the basin margin. The regional peripheral fault trend play is exemplified by salt-related structural and combination traps. Reservoirs of the Pickens, Gilbertown, and West Bend fault systems subplay are peritidal, nondolomitic to completely dolomitized, oolitic, oncoidal, and peloidal grainstone. Reservoirs of the Pollard and Foshee fault systems subplay are subtidal to supratidal, partially to completely dolomitized, peloidal grainstone to wackestone, and dolomitized algal boundstone. The Mississippi interior salt basin play is downdip from the Pickens and Gilbertown fault systems and is characterized by structural and combination traps associated with salt tectonism in this basin. Reservoirs are peritidal, nondolomitic to completely dolomitized, oolitic and peloidal grainstone and packstone. The Mobile graben fault system play is located along the eastern limit of the Mississippi interior salt basin and is typified by salt-induced structural and combination traps and Smackover peritidal peloidal and oolitic dolograinstone to dolowackestone and dolostone reservoirs. The Wiggins arch complex play is in a downdip basinal position and is characterized by structural and combination-petroleum traps associated with stratigraphic thinning and salt flow. The traps occur along the flanks of pre-Mesozoic paleohighs associated with this complex. Reservoirs are subtidal to supratidal peloidal, oolitic and oncoidal dolograinstone and dolopackstone, thrombolitic dolostone, and crystalline dolostone.

Natural Gas Plays in Jurassic Reservoirs of Southwestern Alabama and the Florida Panhandle Area

ABSTRACT Three Jurassic natural gas trends can be delineated in southwestern Alabama and the Florida panhandle area. These include a deep natural gas trend, a natural gas and condensate trend, and an oil and associated natural gas trend. Trends are recognized by hydrocarbon types, basinal positions, and relationships to regional structural features. Within these natural gas trends, eight distinct natural gas plays can be identified. Plays are recognized by characteristic hydrocarbon traps and reservoirs. The deep natural gas trend includes the Mobile Bay area play that is characterized by structural hydrocarbon traps associated with salt tectonism and Norphlet sandstone reservoirs at depths exceeding 20,000 feet. The natural gas and condensate trend includes the Mississippi interior salt basin play, the Mobile graben play, the Wiggins arch complex play, and the Pollard fault system play. The Mississippi interior salt basin play is typified by salt-related structural and combination hydrocarbon traps and Smackover dolomitized oolitic, oncolitic, and peloidal grainstone and packstone reservoirs at depths of approximately 16,000 feet. The Mobile graben play is exemplified by salt-induced structural hydrocarbon traps and Smackover dolostone and Norphlet sandstone reservoirs at depths ranging from 12,400 to 18,400 feet. The Wiggins arch complex play is characterized by structural and combination hydrocarbon traps associated with stratigraphic pinch-outs and salt flow. These traps are salt-related and occur along the flanks of paleohighs associated with the Wiggins arch complex. Smackover dolostone reservoirs at depths ranging from 16,100 to 18,400 feet are typical of this play. The Pollard fault system play is typified by salt-induced structural hydrocarbon traps and reservoirs at depths of approximately 15,000 feet. These reservoirs are Smackover dolomitized oolitic and peloidal grainstones and packstones and Norphlet sandstones. The oil and associated natural gas trend includes the Gilbertown and West Bend fault systems play, the Foshee fault system play, and the basement ridge play. The Gilbertown and West Bend fault systems play is exemplified by salt-related structural or combination traps and Smackover dolomitized oolitic, oncolitic, and peloidal grainstone and packstone reservoirs and Norphlet sandstone reservoirs at depths ranging from 11,000 to 14,000 feet. The Foshee fault system play is characterized by structural and combination hydrocarbon traps related to salt movement and Smackover dolomitized peloidal grainstone and packstone and Norphlet sandstone reservoirs at depths of approximately 15,000 feet. The basement ridge play, which is typified by structural and combination traps associated with the Conecuh and Pensacola-Decatur ridge complexes and Smackover oolitic and peloidal grainstone and packstone and algal boundstone and Haynesville sandstone reservoirs at depths ranging from 11,800 to 15,500 feet, has potential for significant undiscovered natural gas.

Comparison of Upper Cretaceous and Paleogene Depositional Sequences in the Eastern Gulf Coastal Plain

ABSTRACT Genetic depositional sequences representing durations of 0.5 to 11 million years, their component systems tracts and associated physical surfaces have been identified and mapped in Upper Cretaceous and Paleogene strata of the eastern Gulf Coastal Plain. The time duration in which these sequences were deposited appears to have little impact on sequence development in that the component systems tracts can be recognized in all of the sequences. However, the Upper Cretaceous sequences represent longer periods of time, resulted from slower sedimentation rates, experienced lower subsidence rates, and, generally, reflect more marine paleoenvironmental conditions. In the eastern Gulf Coastal Plain, the Late Cretaceous was characterized by warm climates, relatively high sea levels, and stable depositional conditions. Conversely, Paleogene sequences represent shorter durations of time and were greatly affected by differential rates of sedimentation and subsidence. In Paleogene sequences, usually only the transgressive systems tracts were characterized by marine depositional conditions. In the eastern Gulf Coastal Plain, the Paleogene was typified by widely fluctuating climates and dynamic changes in depositional conditions. Generally, Upper Cretaceous sequences span more than one biozone, whereas Paleogene sequences usually are restricted to a single biozone. The sedimentation rates and patterns, the stable depositional conditions, and high biologic productivity during the Late Cretaceous results in a stratigraphic section that is useful for high resolution sequence analysis. Thus, the diachroneity of sequence boundaries and first transgressive surfaces and the synchroneity of maximum flooding surfaces are more easily discerned by mapping Upper Cretaceous system tracts and associated physical surfaces in the eastern Gulf Coastal Plain. The study of Upper Cretaceous depositional sequences, therefore, is potentially more helpful in identifying the factors affecting global sea level change.

Hydrocarbon Productivity Characteristics of Upper Jurassic Smackover Carbonates, Eastern Gulf Coastal Plain

ABSTRACT The Upper Jurassic Smackover Formation is a prolific oil and gas reservoir in the eastern Gulf Coastal Plain. To date, over 838 million barrels of oil and condensate and 2.4 trillion cubic feet of natural gas have been produced from 157 Smackover fields located in the area. The tectonic setting and depositional and postdepositional history of the Smackover carbonates and associated strata have been conducive to the formation of numerous petroleum traps, the accumulation and preservation of substantial organic matter, the thermogenic generation and preservation of large quantities of hydrocarbons, the formation of extensive porous and permeable reservoirs, the subsequent enhancement of reservoir quality, and the formation of a regional impervious seal rock. In addition, the timing of trap formation and hydrocarbon generation and migration have been important to oil and gas entrapment and preservation. An understanding of the factors that contribute to the hydrocarbon productivity of the Smackover will assist with improving the recovery of oil and gas from these limestones and dolostones.

Abstract: Late Pleistocene-Holocene Sequence Stratigraphy of the Mississippi-Alabama Shelf, Northeastern Gulf of Mexico: Implications for Global Change

ABSTRACT Sequence stratigraphy is the study of rock relationships within a framework of repetitive, unconformity-bounded depositional sequences and provides a methodology by which to study and understand stratigraphic, lithofacies, and paleogeographic relationships among the strata within a depositional basin. The primary premise of sequence stratigraphy is that stratal stacking patterns and lithofacies distributions are related to cycles of transgression and regression that result from cyclical changes in relative sea level; relative sea level changes, in turn, result from the combined effects of eustatic sea level changes, basin subsidence, sediment yield, and climatic conditions. While the concepts of sequence analysis have been applied in numerous studies of the ancient rock record, research into the applicability of this approach to the modern record is generally lacking. An understanding of the sequence stratigraphic setting and history of modern depositional systems, when combined with data from ancient analogs to which sequence analysis has been applied, may have predictive value in determining possible scenarios for change in modern coastal and offshore areas and the rates of such change. A major late Pleistocene sea level fall that exposed the continental shelf in the northern Gulf of Mexico area is well documented. During this lowstand, the exposed continental shelf was subjected to erosion and stream valley entrenchment, which resulted in an incised, type 1 unconformity surface. This surface forms the base of the late Pleistocene-Holocene sequence in the study area. Sediment bypassed the exposed shelf through these valley systems and accumulated as slope and basin lowstand systems tract deposits. In the northern central Gulf, studies have shown that the Mississippi Canyon and Fan formed during this sea level phase by these processes. On the Mississippi-Alabama shelf, incised valleys formed on the type 1 surface are much smaller than the Mississippi Canyon to the west, but are numerous and readily delineated on seismic reflection records. Preliminary data show that these features are filled with generally coarse-grained siliciclastics that were deposited as incised valley fill associated with renewed transgression. Progressive marine transgression during the Holocene eventually flooded the entire continental shelf, depositing backstepping sets of transgressive systems tract parasequences. Rapid deepening led to sediment starvation on the shelf as active depositional loci were translated landward. Submarine hardbottoms consisting of surficial rock and shell rubble are common in the offshore in the northeastern Gulf of Mexico. The thickness of the Holocene section in this area and the presence of these hardbottoms indicate that the Mississippi-Alabama shelf has been sediment starved throughout most of its Holocene history. Preliminary assessment of these hardbottom features indicates that they represent, in part, condensed section associated with the transgressive phase of the late Pleistocene-Holocene sea level cycle. This condensed section is probably correlative to that reported by Boyd et al. (1989) in the north-central Gulf area. Subsequent to maximum transgression, relative sea level has been falling in the north-central Gulf, and the Mississippi delta system has been prograding into the basin as a highstand systems tract. Sediment yield from rivers in the northeastern Gulf is not as great as that of the Mississippi River. Sequence stratigraphic analysis of late Pleistocene-Holocene strata of the Mississippi-Alabama shelf can be used as a tool for establishing the anticipated naturally-controlled lithostratigraphic succession, sea level position and rate of change, and distribution of modern geomorphic features. The recognition of deviations from the anticipated sequence stratigraphic framework may allow for the determination of physical environmental changes which may be attributed to anthropogenic events. End_of_Record - Last_Page 849-------