Eagle Mills Research Articles

Early Mesozoic synrift Eagle Mills Formation and coeval siliciclastic sources, sinks, and sediment routing, northern Gulf of Mexico basin

AbstractThe sedimentary architecture and provenance of the early Mesozoic incipient northern Gulf of Mexico basin remains controversial due to both lack of outcrop exposure and sample scarcity across the southern United States with subcrop depths approaching 6 km. The Eagle Mills Formation and coeval deposition across the northern Gulf of Mexico provides both a stratigraphic foundation for some ∼15-km-thick overlying Mesozoic and Cenozoic deposits, and a coeval pre-salt equivalent for southern synrift deposits, in one of the most economically significant hydrocarbon basins in the world. This study presents more than 3200 new detrital zircon U-Pb analyses from sixteen Late Triassic pre-salt, siliciclastic, subcrop well samples, and combines over 14,000 linear kilometers of 2-D multi-channel seismic reflection data, 1511 geophysical well logs, and biostratigraphic data from 2478 wells to construct basin-scale pre-salt isochore and structure maps spanning the northern Gulf of Mexico margin from Florida to the USA-Mexican border.The data show that incipient Gulf of Mexico paleodrainage pathways held individual distinctions between basement sources and tectonic controls in three primary regions across the northern Gulf of Mexico: (1) The western Gulf of Mexico paleodrainage extended from the Central Texas uplift highlands to the submarine Potosi Fan on the western margin of Laurentia with local tributary sources from the East Mexico Arc, Yucatán/Maya, and Marathon-Ouachita provinces as evidenced by inverse Monte Carlo unmixing of peri-Gondwanan (ca. 700–500 Ma), Appalachian/Ouachita (500–280 Ma), Grenville (1250–950 Ma), and Mid-Continent/Granite-Rhyolite Province (1500–1300 Ma) detrital zircon ages. Isochore and associated geophysical well and seismic data suggest that by Early Jurassic time this depocenter had shifted into the present-day western Gulf of Mexico as East Mexico Arc development continued. (2) Southerly drainage in the north-central Gulf of Mexico region bifurcated around the Sabine and Monroe uplifted terranes with southwestern flow characterized by peri-Gondwanan detrital zircon ages from late Paleozoic accreted basement or discrete flexural successor basins, and southeastern fluvial networks distinguished by traditional North American basement province sources including Grenville, Mid-Continent, and Yavapai-Mazatzal. (3) Eastern Gulf of Mexico regional paleodrainage, with regional southern flow dictated by the brittle extensional tectonics of the South Georgia Rift as well as the regional southern flexure of the South Florida Basin, resulted in almost all pre-salt detrital zircon siliciclastic ages from this region to be dominated by local Gondwanan/peri-Gondwanan aged sources including the proximal Suwannee terrane and Osceola Granite complex. These regional, synrift sediment provenance models provide the first critical allochthonous evidence of Late Triassic–Early Jurassic paleodrainage stemming from the Appalachian-Ouachita hinterlands into the incipient northern Gulf of Mexico basin with critical implications for pre-salt hydrocarbon exploration and carbon sequestration reservoir potential.

Origin and Early Development of the Gulf of Mexico Basin Based on 15-Second Record Seismic in the Louisiana Shelf and Upper Slope: ABSTRACT

A 4x4 mile grid of new 15-second record seismic data covering the entire Louisiana shelf and upper slope reveals new insights into the deep structure of the Gulf of Mexico Basin. Noteworthy is the absence of evidence for major horst and graben basement faulting predicted in some models for the early Gulf under the shelf. Instead, the basement appears generally smooth, with a reasonably conformable early section 1 to 1.5 seconds thick immediately overlying it. This suggests either that the basement was formed as lower plate continental crust (in a simple shear detachment fault scenario, below the brittle-ductile crustal transition) or as oceanic crust similar to that observed under autochthonous salt in the Perdido fold belt. If the latter is correct, the edge of oceanic crust needed to reconstruct the sea-floor spreading history may nearly coincide with the current northern limit of the sub-salt hydrocarbon play, possibly because the play location depends on 3-4 km thick salt originally deposited in the area under the southern shelf. The southern limit of thick LouAnn salt is related to depositional factors (evaporation, oceanic circulation, etc.) rather than to underlying crustal type. An unresolved question is the nature of the section overlying the smoothmore » basement event. Both deepwater Lower Cretaceous (chalks, marts ?) and Eagle Mills deepwater equivalents (shales) are plausible interpretations.« less

Diagenesis of Deeply Buried Eagle Mills Sandstones: Implications for Paleo-Fluid Migration and Porosity Development: ABSTRACT

ABSTRACT Eagle Mills strata (Triassic-Jurassic) unconformably overlie the Paleozoic basement complex and thus, form the basal sedimentary unit within the Gulf of Mexico Basin. Jurassic-aged basaltic dikes and sills have intruded Eagle Mills strata. Werner-Louann evaporates disconformably overlie the Eagle Mills Formation. These sedimentary and igneous rocks preserve the earliest record of Gulf of Mexico rifting. Deeply buried (15,000 to 18,000 ft/4,573 to 5,488 m) Eagle Mills sandstones have subarkosic and sublithic modal compositions. These sandstones exhibit evidence of a complex and prolonged diagenetic history, including: early chlorite cementation; mechanical compaction; quartz and feldspar overgrowths; nonferroan calcite and dolomite cementation; dissolution of framework grains; kaolinite precipitation; ferroan calcite cementation; albitization; saddle dolomite and anhydrite cementation; and pyritization. Pyrobitumens coat early cements indicating that most diagenesis post-dated hydrocarbon migration. Marked shifts in paleo-water chemistry are recorded by diagenetic phases in Eagle Mills sandstones. The Eagle Mills paragenetic sequence is indicative of progressive burial into a high-temperature (>150° C) regime where thermochemical sulfate reduction was the dominant diagenetic process. Pervasive late burial dissolution of detrital feldspars in some Eagle Mills sandstones provides direct evidence for deep-seated sourcing of metal cations which promoted diagenesis of overlying Mesozoic strata. These petrographic data support interpretations of the Mesozoic Gulf Coast Basin which imply vertical mass-transfer of diagenetic fluids along faults.

Petrology, Stratigraphy, and Depositional History of the Upper Triassic-Lower Jurassic Eagle Mills Formation, Choctaw County, Alabama: ABSTRACT

ABSTRACT In southwestern Choctaw County, Alabama, the Eagle Mills Formation consists of alluvial-fan red beds deposited in a Mesozoic graben. The Exxon No. 1 Smith Lumber Co. well penetrated the upper 2,210 ft of the Eagle Mills, and it is divided into eight lithostratigraphic units designated A through H from bottom to top. Unit A consists of numerous upward-fining and -coarsening sequences of sandstones, conglomeratic sandstones, and conglomerates of variable thickness separated by red mudstones. They were deposited on the distal fan by ephemeral streams and sheetfloods and are composed of low-grade metamorphic rock fragments probably derived from the Talladega slate belt. Unit B consists of red mudstone with interbedded upward-fining and -coarsening sandstones and conglomerates with abundant calcite concretions and paleocaliche. Thin-section analyses of three sandstones of Unit B show that two are phyllarenites and one is a subphyllarenite. They vary from very fine to coarse-grained and are moderately sorted and submature, with angular to subangular framework grains. Quartz (subequal monocrystalline and polycrystalline) is the dominant framework grain with lesser amounts of schist and phyllite. In the two shallower sandstones, quartz content decreases as chert and dolomite marble rock fragments increase. Scanning electron microscope examination of one sandstone shows the presence of large well-developed pores lined with illite and authigenic quartz. Examination of drill cuttings from other Unit B sandstones, however, shows them to be poorly sorted, highly indurated, and apparently of low porosity. Unit B was deposited on the distal fan by sheetfloods and ephemeral streams. Faulting and igneous activity related to rifting occurred during the latter stage of deposition of Unit B. Units C through G are dominantly conglomerates; Units C, D, and F contain abundant igneous rock fragments. Unit C contains plutonic (syenite) and volcanic (rhyolite and dacite) rock fragments, and the igneous clasts of Units D and F are volcanic. Rare poorly defined graded beds occur in Unit D cores. Units C and D exhibit a general upward coarsening of sediments, indicating progradation of the fan. Unit E coarsens upward to a sedimentary (chert and limestone) and metamorphic (dolomite marble and lesser amounts of phyllite and schist) rock-fragment-bearing cobbly pebble conglomerate. Unit F is a mixed volcanic and sedimentary-rock-fragment-bearing conglomerate. Unit G is a massive sedimentary and metamorphic rock-fragment-bearing cobbly pebble conglomerate. Cores taken from the upper half of this unit show this portion to be a poorly sorted clast- to matrix-supported conglomerate. Although Unit G is in general massive, rare poorly defined graded beds and rare thin-bedded sandstones are also present. Units D through G were deposited in the mid-fan subenvironment. In Unit H, high log porosities and well-rounded sand grains in drill cuttings indicate the presence of eolian deposits. Log responses and the presence of anhydrite in cuttings show that the Eagle Mills is overlain by the Werner Formation. The provenance for Eagle Mills sediments was the southern Appalachian system, with sedimentary rock fragments being derived from folded Paleozoic rocks, whereas metamorphic rock fragments came primarily from metasedimentary rocks of the Talladega slate belt. Igneous rock fragments were derived from rocks emplaced in the upper-fan area as a result of igneous activity associated with rifting. End_Page 117------------------------ FIGURE 1. [Grey Scale] Unit G cobbly pebble conglomerate. Clasts are chert and dolomite marble. FIGURE 2. [Grey Scale] Upward-fining conglomerate channel deposit. Light-colored clasts are quartzite; dark elongate clasts are hematite-stained phyllite and schist. FIGURE 3. [Grey Scale] Unit D porphyritic rhyolite clast. White phenocryst is alkali feldspar altered to illite. Matrix is hematite-stained mud. FIGURE 4. [Grey Scale] Thin-section photomicrograph of oolitic sandstone clast. Most ooid nuclei are quartz coated with hematite. Clast is from Silurian Clinton Group. End_of_Record - Last_Page 118-------

Sequence Stratigraphy of Middle and Upper Jurassic Strata of Southwestern Alabama

Middle and Upper Jurassic systems tracts of southwestern Alabama differ from those of the western Gulf rim, showing: (1) profound influence of antecedent topography; (2) low early subsidence rates; and (3) greater clastic influx from adjacent uplands. Werner Anhydrite and Louann Salt represent the earliest marine incursion onto the Gulf rim following initial rifting; they onlap upper Paleozoic basement and garben-filling Eagle Mills red beds. Because basin-wide evaporative drawdowns overprint even higher order eustatic sea level changes, transgressive systems tracts (TST) and highstand systems tracts (HST) are indistinguishable. Anhydrite and shale caps accumulated via interstratal halite dissolution. Oxfordian Norphlet siliciclastics form a continental lowstand systems tract as illustrated by abrupt contact with underlying marine evaporites without intervening progradational marginal marine facies. Marine-reworked uppermost Norphlet sandstone marks the base of a subsequent TST, which includes overstepping lower Smackover lithofacies (laminated mudstone, algal-laminated mudstone, and pellet wackestone). The upper Smackover HST is characterized by formation of rimmed shelves upon which algal mounds and aggrading ooid grainstone parasequences accumulated. Shallow lagoonal carbonate and evaporite saltern deposition occurred behind ooid shoals; fine-grained siliciclastics accumulated in updip areas. Equivalents of Smackover A, Smackover B, Bossier, and Gilmer sequences are largely masked by influx of Haynesvillemore » and Cotton Valley continental clastics. Lack of biostratigraphic data, a consequence of restricted fauna, precludes useful age assignments for these sequences in Alabama. Middle and Upper Jurassic systems tracts of southwestern Alabama are regionally atypical and cannot serve as a model for Gulf-wide sequences.« less

An Ultramafic Inclusion from the Weeks Island Salt Dome, South Louisiana

Mines in Gulf Coast salt domes commonly encounter entrained clastic sedimentary material. The clastic sediments include red, brown, green and yellow sands, silts and clays in brecciated seams which may represent shear zones within or between salt stocks. In January of 1992, miners encountered a hard, dark-gray block of distinctive lithology within a clastic inclusion zone at the 1,000 foot level of the Morton Salt Company's mine at Weeks Island, Iberia Parish, Louisiana. The square block measured approximately 18 inches on a side and had a depth into the freshly exposed salt face of a few inches. Thin-section petrography established that the dark gray is an ultramafic porphyry. Phenocrysts of serpentinitized olivine, pyroxene and amphibole occur in a groundmass dominated by serpentine and opaque materials. This new occurrence is significant because it extends the known range of mafic and ultramafic igneous activity within the Gulf area. Olivine nephelinites and their differentiation products are widely distributed in the Austin-San Antonio-Brackettville areas of central Texas, both as intrusions and extrusions and are probably related to the Balcones fault zone. Potassium-Argon dates for these rocks published by Adams and Baldwin (1971) range between 85 and 63 million years. Other igneous p lugs are known or inferred from subsurface data. In 1976, Braunstein and McMichael briefly described an intrusive body of altered porphyritic basic rock from offshore St. Bernard Parish, Louisiana (Chandeleur Sound), at a depth of 7,200 feet. The was dated at 82 M.Y., ± 8 m.y., although the published cross section implies intrusion through (or possibly onlap of) early Tertiary Midway strata. There is no published record of foraminifera or other paleontological materials to enable the sediments to be dated, although late Triassic or possibly Jurassic sporomorphs from the salt have been reported by Jux (1961). It is normally concluded that the entrained clastic sediments are younger than the salt and were picked up during the salt's upward diapirism, although salt domes in the Arabian Gulf contain blocks of material that are clearly from the older underlying basement rocks, apparently sucked from below the rising salt mass. The age of the new block of porphyritic peridotite is problematical. If a real consanguinity exists between the Weeks Island sample and these other examples, a late Cretaceous age similar to the ages discussed above may be realistic; the sample itself is not a promising subject for dating. Another possible interpretation exists. The serpentinitized peridotite (and the associated red elastics) may have been derived from beneath the salt, in a manner comparable with the Arabian Gulf examples. The red sands may therefore correlate with the Triassic Eagle Mills Formation of North Louisiana and the peridotite may thus represent a sample derived ultimately from the upper mantle beneath the Gulf, brought near to the late Triassic surface by tectonics or volcanism early in the Mesozoic opening phase. This scenario is highly speculative but raises some interesting possibilities for future consideration.

Diagenetic And Sedimentologic Aspects Of Eagle Mills-Werner Conglomerate Sandstones (Triassic-Jurassic), Northeast Texas

ABSTRACT The Eagle Mills-Werner sequence occurs in deep wells (12,000 to 18,000 ft.) in northeast Texas and southwest Arkansas. Eagle Mills sandstones unconformably overlie the Paleozoic complex and disconformably underlie the Werner Conglomerate. Werner-Louann evaporites overlie Eagle Mills-Werner siliciclastics. Basaltic sills and dikes transect these sandstones and conglomerates. Eagle Mills-Werner strata consist of greenish-gray conglomerates and green, red, and pink, coarse-grained, lithic-feldspathic arenites which are interbedded with red, gray, and green shales and siltstones. Lithic arenites and conglomerates contain volcanic and sedimentary rock fragments. These strata represent nonmarine paleoenvironments developed within grabens and are the earliest record of Gulf of Mexico rifting. Eagle Mills-Werner sandstones have undergone a complex diagenetic history, including: chlorite cementation, compaction, quartz and feldspar overgrowths, early carbonate cementation, dissolution of framework grains, chloritization and albitization of feldspars, late cementation by ferroan calcite and ferroan dolomite and late replacement by anhydrite and pyrite. Pyrobitumens coat early chlorite cements indicating that most diagenesis post-dated hydrocarbon migration. This suite of authigenic phases records progressive burial of Eagle Mills-Werner strata into a high-temperature diagenetic regime where thermochemical sulfate reduction was the dominant process.

Petrography and Diagenesis of Eagle Mills Sandstones, Subsurface--Northeast Texas and Southwest Arkansas

The Eagle Mills Formation (Triassic-Jurassic) has been penetrated by several deep wells (12,000 to 18,000 ft) in northeast Texas and southwest Arkansas. It consists of green, red, and pink conglomeratic lithic arenites and fine- to coarse-grained feldspathic arenites, interbedded with red and greenish gray shales and siltstones. Lithic arenites contain basalt, chert, quartzite, and dolomite rock fragments; plagioclase is the predominant feldspar. All Eagle Mills sandstones have low textural and mineralogic maturities. Eagle Mills red beds and associated intrusive igneous rocks (diabase and basalt dikes and sills) represent the fillings of grabens or rift basins that actively subsided during deposition (in alluvial, fluvial-deltaic, and lacustrine paleoenvironments). Eagle Mills lithic and feldspathic sandstones have undergone a complex diagenetic history, including chlorite cementation (pore linings and pore fillings), compaction, quartz and feldspar overgrowths, dolomite cementation, chloritization and albitization of detrital feldspars, local dissolution of framework grains (igneous lithics and feldspars), precipitation of kaolinite, late Fe-calcite cementation, and saddle dolomite formation. Cement mineralogies are strongly correlative with lithofacies. Lithic sandstones contain the highest frequency of chlorite cements, whereas feldspathic sandstones are preferentially cemented with carbonates and anhydrite; quartz and feldspar overgrowths are ubiquitous. The suite of authigenic minerals in Eagle Mills sandstones recordsmore » progressive burial into a deep, high-temperature (120-150C), semiclosed, diagenetic regime.« less

Jurassic Faults of the Southwest Alabama and Offshore Areas

Four fault groups affecting Jurassic strata occur in the southwest and offshore Alabama areas. They include the regional basement rift trend, the regional peripheral fault trend, the Mobile graben fault system, and the Lower Mobile Bay fault system. The regional basement system rift and regional peripheral fault trends are distinct and rim the inner margin of the eastern Gulf Coastal Plain. The regional basement rift trend is genetically related to the breakup of Pangea and the opening of the Gulf of Mexico in the Late Triassic-Early Jurassic. This fault trend is thought to have formed contemporaneously with deposition of Late Triassic-Early Jurassic Eagle Mills Formation and to displace pre-Mesozoic rocks. The regional peripheral fault trend consists of a group of en echelon extensional faults that are parallel or subparallel to regional strike of Gulf Coastal Plain strata and correspond to the approximate updip limit of thick Louann Salt. Nondiapiric salt features are associated with the trend and maximum structural development is exhibited in the Haynesville-Smackover section. No hydrocarbon accumulations have been documented in the pre-Jurassic strata of southwest and offshore Alabama. Productive hydrocarbon reservoirs occur in Jurassic strata along the trends of the fault groups, suggesting a significant relationship betweenmore » structural development in the Jurassic and hydrocarbon accumulation. Hydrocarbon traps are generally structural or contain a major structural component and include salt anticlines, faulted salt anticlines, and extensional fault traps. All of the major hydrocarbon accumulations are associated with movement of the Louann Salt along the regional peripheral fault trend, the Mobile graben fault system, or the Lower Mobile Bay fault system.« less

Pollen and Spores Date Origin of Rift Basins from Texas to Nova Scotia as Early Late Triassic

Palynological studies of the nonmarine Newark Supergroup of eastern North America and of rift basins in the northern Gulf of Mexico facilitate correlation with well-dated marine sections of Europe. New information emphasizes the chronological link between the Newark basins and a Gulf of Mexico basin and their common history in the rifting of North America from Pangea. Shales from the subsurface South Georgia Basin are shown to be of late Karnian age (early Late Triassic). The known time of earliest sedimentation in the Culpeper Basin is extended from Norian (late Late Triassic) to mid-Karnian, and the date of earliest sedimentation in the Richmond and Deep River basins is moved to at least earliest Karnian, perhaps Ladinian. The subsurface Eagle Mills Formation in Texas and Arkansas has been dated palynologically as mid- to late Karnian. The oldest parts of the Newark Supergroup, and the Eagle Mills Formation, mostly began deposition in precursor rift basins that formed in Ladinian to early Karnian time. In the southern Newark basins, sedimentation apparently ceased in late Karnian but continued in the northern basins well into the Jurassic, until genesis of the Atlantic ended basin sedimentation.

Palynostratigraphy of the subsurface eagle mills formation (Triassic) from a well in east‐central texas, U.S.A.

Abstract Well cuttings from the 8300–9500 ft (= 2530–2896 m) levels of the Hinton No. 1 Snider well in Falls County, east‐central Texas, were studied palynologically. The cuttings were prevailingly gray to reddish‐brown shales, and most samples were productive of palynomorphs. The undoubted indigenous palynoflora of the rock indicates late Carnian (Late Triassic) age for the Eagle Mills Formation, representing sedimentation in a Newark‐Basins‐style rift basin. The flora of most of the sampled interval was dominated by monosaccate and bisaccate conifer pollen and spores supporting late Carnian age. These forms also indicate relatively dry climatic conditions. Above 8650 ft the palynoflora is dominated by Corollina (al. Classopollis) pollen, and spores, indicating terminal Triassic to Early Jurassic age. However, the upper levels contain abundant dinoflagellates and spores and pollen of Jurassic to Late Cretaceous age, which clearly came from up‐well caving. It is therefore probable that the post‐Carnian in...

Triassic/Jurassic Faulting Patterns of Conecuh Ridge, Southwest Alabama: ABSTRACT

Two major fault systems influenced Jurassic structure and deposition on the Conecuh Ridge, southwest Alabama. Identification and dating of these fault systems are based on seismic-stratigraphic interpretation of a 7-township grid in Monroe and Conecuh Counties. Relative time of faulting is determined by fault geometry and by formation isopachs and isochrons. Smackover and Norphlet Formations, both Late Jurassic in age, are mappable seismic reflectors and are thus reliable for seismic-stratigraphic dating. The earlier of the 2 fault systems is a series of horsts and grabens that trends northeast-southwest and is Late Triassic to Early Jurassic in age. The system formed in response to tensional stress associated with the opening of the Atlantic Ocean. The resulting topography was a series of northeast-southwest-trending ridges. Upper Triassic Eagle Mills and Jurassic Werner Formations were deposited in the grabens. The later fault system is also a series of horsts and grabens trending perpendicular to the first. This system was caused by tensional stress related to a pulse in the opening of the Gulf of Mexico. Faulting began in Early Jurassic and continued into Late Jurassic, becoming progressively younger basinward. At the basin margin, faulting produced a very irregular shoreline. Submerged horst blocks became centers for shoaling or carbonate End_Page 268------------------------------ buildups. Today, these blocks are exploration targets in southwest Alabama. End_of_Article - Last_Page 269------------

Recent Jurassic Discoveries in Southeastern Cass County, Texas: ABSTRACT

Southeastern Cass County had lain virtually dormant as a prospective Jurassic oil and gas province since the mid-1960s, when East Linden field was discovered and developed. Then, in 1978, Hilliard Oil and Gas drilled the 1 Johnson and discovered Kildare field (Smackover). Subsequent development thru 1982 proved additional reserves in several Cotton Valley sandstones as well, reconfirming southeastern Cass County as territory for viable Jurassic drilling. Additional drilling occurred when Marshall Exploration redrilled and expanded the old Bloomburg field and Heflin redrilled Queen City field. All of this drilling was successful in the Smackover reservoir, finding sour gas and condensate. Wildcat activity included the two Smackover completions finding South Atlanta field, as well as two completions in formations that are highly debated as to their nomenclature. Cities Service reportedly completed their well in the Eagle Mills. This well brought national attention to southeastern Cass County, when it was reported on the CBS Saturday evening news. The well initially flowed at rates that were as high as 1,800 BOPD, 1,396 MCFGD, and 32 BWPD, with pressure of 3,250 psi. Just as the excitement was dying down, Primary Fuels, Inc. reentered and deepened the Highland Resources 1 Glass and completed that well in a zone correlative to the Cities Service 1-A Pruitt. The 1 Glass potentialed for 200 BOPD, 570 MCFGD, and 32 BWPD, at pressure of 2,900 psi. The producing zone was determined to be the Norphlet, which once again made wildcatters of all previous upper Smackover explorers. The activity in this county has not slowed. At present, 30 wells are reported in various stages of drilling. This success has brought with it the advent of higher acreage cost, royalties, and shorter lease terms. End_of_Article - Last_Page 1209------------

Stratigraphy and Environmental Significance of Continental Triassic Rock of Texas: ABSTRACT

The continental Triassic rocks of Texas are represented by four distinct but similar rock groups that exist both in outcrop and in the subsurface and include the Eagle Mills Formation (south-central and northeast Texas), Sycamore Formation (central Texas), Dockum Group (west Texas), and Bissett Formation (southwest Texas). They are clearly terrigenous in nature derived principally from older Paleozoic sedimentary rocks. The rock groups are composed in part or entirely of mudstone, siltstone, medium to coarse-grained sandstone, and pebble to boulder conglomerate (intrabasinal and extrabasinal). The sediments were deposited in alluvial fans, braided and meandering streams, lobate deltas, fan deltas, and lakes. The coarse sandstone and conglomerate are the products of high-e ergy, short-duration depositional events. Sedimentation was greatly affected by alternating climatic conditions that produced changes in base level, water depth, and lake area as well as the type of streams that flowed into the depositional basins. The character of the rock groups strongly suggests semi-arid to arid deposition typical of the low latitude desert regions of today. Thus, the rocks comprising the Eagle Mills, Sycamore, Dockum, and Bissett Formations appear to be products of continental clastic deposition during a major semi-arid to arid climatic episode, such as that of late Triassic time. End_of_Article - Last_Page 469------------

Basement Model for Panhandle of Florida: ABSTRACT

Core samples from deep boreholes in panhandle Florida form the basis of a basement model involving at least eight separate fault blocks and basins, each with a distinct depositional history. The dominant structures are a northwest-trending fault and a large, northeast-trending Triassic graben which encompasses several secondary fault blocks and forms the Southwest Georgia Embayment (Apalachicola Embayment). This graben as well as associated perpendicular (northwest-southeast) faults were formed in response to tensional forces related to the Mesozoic separation of North American and South American landmasses and the consequent formation of the Gulf of Mexico. Granitic basement blocks, perhaps early Cambrian in age, experienced differential subsidence and changing relations ips with various sedimentary source terranes. Thus, the separate basins accommodated different combinations of Triassic Eagle Mills red beds and Jurassic deposits ranging from the Louann Salt to the Cotton Valley sandstones and shales. End_of_Article - Last_Page 1472------------

Exploration Analysis of Jurassic Apalachicola Embayment of Florida: ABSTRACT

A petrophysical evaluation was made of the Apalachicola embayment Jurassic rocks from all available wells, using ores, cuttings, and electric logs. From this information, I propose that the embayment was formed by basement faulting, which resulted in a grabenlike structure. Two basins were formed on either side and within the embayment, together with a structurally high area at the entrance of the bay. These structures strongly influenced the patterns of water circulation and sediment deposition throughout the Jurassic. The Louann Salt was laid down throughout the embayment but was later eroded, with some of the Eagle Mills Formation, by the Norphlet fluvial sands. These Norphlet sands were deposited in a very arid climate by braided streams and then moved into eolian dun s. The initial marine transgression of the Smackover eroded the top of most of these sand deposits and laid down, in place, a fine-grained dense carbonate throughout the embayment in a tidal-flat environment. During the late Smackover marine regression, additional sediment was deposited, which was subsequently subaerially exposed to freshwater percolation, resulting in dolomitization. Lower Haynesville Buckner anhydrite was laid over the Smackover limestone, but much of the anhydrite was eroded by the continental sediments of the Haynesville. The Haynesville sediments show that there were marine fluctuations into the embayment throughout this time. The Cotton Valley sandstones were deposited throughout the area including the embayment. As hydrocarbon potential exists in the Norphlet and mackover, a petrologic examination was concentrated on these formations. The fluvial Norphlet sands are conglomeratic, fine grained with interbedded red shales in the updip position, but were white to red, fine grained, and quartzitic in the center and at the mouth of the embayment. The Smackover limestones and dolomites are shallow, tidal-flat marine deposits and are composed of algal-coated grains, fecal pellets, Foraminifera, echinoid spines, and mollusk shells. Porosity and permeability reduction is due to large calcite crystal growth and closely packed dolomite rhombs in the pore spaces and pore throats. From a model application of a modern analog of a carbonate tidal flat, the most favorable reservoir rock would be expected in intertidal channels and sediment buildups at the embaym nt mouth. End_of_Article - Last_Page 1434------------

Regional Jurassic Lithostratigraphy of Alabama: ABSTRACT

Jurassic units in the subsurface of Alabama include the Werner Formation, the Louann Salt, the Norphlet Formation, the Smackover Formation, the Haynesville Formation (including the Buckner Anhydrite Member), and the Cotton Valley Group. These units range in age from early Callovian to late Tithonian, (Middle to Late Jurassic) with the Jurassic-Cretaceous boundary possibly occurring in the upper part of the Cotton Valley Group. Deposition was controlled by pre-Jurassic paleohighs, diapiric salt structures, and the peripheral fault system that rims the Gulf Coast basin. Climates during the Jurassic ranged from hot and dry during Early Jurassic to hot and humid during Late Jurassic. The Werner Formation consists of a lower sandstone and shale sequence, overlain by an evapori ic unit, and lies disconformably either on the Paleozoic basement or the Eagle Mills Formation of Late Triassic age. Deposition of the Werner represents the initial transgression of marine waters into the Gulf Coast basin. The Louann Salt is a massive halite unit that formed a southward tilting ramp on which younger Jurassic strata were deposited. The Norphlet Formation is a sandstone sequence with thin interbedded shale that lies disconformably on the Louann. The Smackover Formation consists of two units, a lower laminated mudstone and brown dense limestone, and an upper dolomitic grain-supported limestone. The Smackover lies conformably on the Norphlet, and in places is gradational with it. The Haynesville is a sequence of calcareous, anhydritic sandstone and interbedded, anhydritic, m caceous shale. The Haynesville may consist of either a sequence of interbedded, calcareous mudstone, anhydritic shale and sandstone, limestone, dolostone, and salt stringers, or a massive anhydrite (Buckner) at the base. In places the Buckner is gradational with the upper Smackover. The Cotton Valley Group is a sequence of fine to very coarse-grained to conglomeratic sandstone, interlayered with silty micaceous shale, very thin limestone beds, and in the Mississippi Interior Salt basin, thin coal and lignite beds. The Cotton Valley was deposited in a terrestrial to littoral environment and the absence of fossils, to date, makes it difficult to distinguish from the sandstone units of the overlying Hosston of Early Cretaceous age. End_of_Article - Last_Page 1438------------

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------------

Sedimentation on Trailing Plate Margin--Northern Gulf Coast: ABSTRACT

The breakup of Pangea and the splitting of South America from North America in the early Mesozoic left a rifted and attenuated trailing margin on the latter plate which became the initial depositional surface for a sedimentary sequence of Late Triassic to recent age. The Late Triassic Eagle Mills Formation and its equivalents are interpreted as being the initial deposits confined to rift grabens of the attenuated plate margin. Deposition of Jurassic evaporites resulted from sedimentation by the brine-mixing process in the restricted circulation of a young and narrow seaway similar to the Red Sea. Upper Jurassic and Cretaceous strata represent the transgressive deposits formed as open-marine conditions prevailed as the plates diverged and the North American plate margin su sided. Laramide tectonism in the continental interior provided a rejuvenated hinterland source area that supplied the voluminous sediment for the regressive and prograding Cenozoic clastic wedge. Studies of this entire sedimentary record reveal the influence of the tensional effects of continental splitting and lower crustal creep that established the initial depositional surface that slowly subsided as proposed by crustal thinning and the thermal-decay curve of cooling oceanic lithosphere. In addition, these studies also reveal the control and influence of (1) inherited structures of the rifted margin, (2) hinterland source areas, (3) the timing and amount of differential subsidence between continental and ocean crust, (4) active syndepositional faults, (5) hinge lines, and (6) postdepositional rejuvenation due to contemporary plate movement. Not only do these studies add to our understanding of the geologic history of the area, which is most important for development of successful exploration programs, but they provide a guide for the study of sedimentary infills within ancient lithospheric plates, a neglected but important task facing all who are confronted by the complex problem of interpreting the sedimentary record of ancient basins. End_of_Article - Last_Page 1602------------

Seismic Stratigraphic Identification of Eustatic Cycles in Late Triassic, Jurassic, and Early Cretaceous Rocks--Gulf of Mexico and West Africa: ABSTRACT

ABSTRACT Seismic stratigraphic techniques permit identification of Late Triassic, Jurassic, and Early Cretaceous eustatically controlled sequences in strata from the North American Gulf Coast and West Africa. Several distinct sequences are remarkably persistent from the Florida Panhandle around the perimeter of the Gulf Coast into northern Mexico, a distance of over 1,500 miles. Their identification requires the integration of seismic data with lithologic, environmental-facies, biostratigraphic, radiometric, and well log information. A comparison with strata of comparable age in offshore West Africa indicates the same sequences can be recognized there. The sequences in North America and Africa are interpreted to be eustatically controlled because they occupy the same time-stratigraphic positions and display coastal onlap patterns similar to those previously recognized by us elsewhere in the world. Gulf Coast eustatic cycles and the formations occurring within them are: (1) Late Triassic--Eagle Mills Formation; (2) Early Jurassic--known only from southern Mexico and not identified in the study area; (3) Middle Jurassic (Bajocian-Bathonian)--Werner Anhydrite-Louann Salt interval; (4) Late Middle Jurassic (Callovian)--Norphlet Formation; (5) Late Jurassic (Oxfordian-Kimmeridgian)--Smackover-Buckner-Haynesville Formations; (6) Late Jurassic-Early Cretaceous (Tithonian-Berriasian)--most of the Cotton Valley Group; and (7) Early Cretaceous (Valanginian)--a restricted wedge seen only in a basinward position. West African eustatic cycles are (1) Late Triassic; (2) Early Jurassic; (3) Middle Jurassic; (4) Early Late Jurassic (Oxfordian-Kimmeridgian); (5) Late Jurassic-Early Cretaceous (Tithonian-Berriasian); and (6) Early Cretaceous (Valanginian). Further refinement may show additional sequences in Middle Jurassic and Late Jurassic intervals.