Grover E. Murray

Salt Structures of Gulf of Mexico Basin--A Review

More than 300 diapiric structures formed by the intrusion of relatively pure salt are known in Alabama, Mississippi, Louisiana, Arkansas, Texas, Nuevo Leon, Veracruz, Tabasco, and Cuba. In form, the structures are rod-like, domal, anticlinal, and ridge-like. They rise vertically, or nearly so, and increase or decrease with height. Many are capped by residual masses of anhydrite, altered in varied degrees to gypsum, sulfur, and calcite. Modern theory postulates growth resulting from density differences between the salt and surrounding sediments (1) by upward movement of the salt through the overlying sediments in response to gravitational inequilibrium, or (2) by salt structures remaining at an essentially constant level while the surrounding sediments of sedimentary rocks moved downward around them as deposition progressed. Model studies suggest that variations in overburden and faulting are primary causes of the initiation of salt movement. The probable source of the salt in Gulf Coast salt domes is the Louann Salt. It may have been as much as 5,000 ft thick and have had an original volume of 200,000 cu mi. Sediments enclosing salt stocks have varied structural configurations. The strata may be arched, they may be ruptured and pierced by the salt, they may be complexly faulted, or they may be deformed by various combinations of faulting and folding. All the salt structures in the Gulf of Mexico basin probably are of similar genesis.

Age and Correlation of Huizachal Group (Mesozoic), State of Tamaulipas, Mexico

Field studies in the Huizachal anticlinorium, Sierra Madre Oriental, near Ciudad Victoria, Tamaulipas, Mexico, disclose that the Huizachal group (redbeds), formerly undivided, contains at least two mappable sequences, herein named La Joya formation and La Boca formation. The La Boca formation consists mostly of red and green claystones, mudstones, siltstones, sandstones, and conglomerates and rests unconformably on (1) crystalline rocks of unknown age and (2) deformed Paleozoic strata lithically similar to those of the Marathon uplift (Texas). It is considerably folded and faulted and underlies either the La Joya formation (redbeds) or the overlying Zuloaga limestone (Late Jurassic) with angularity up to almost 90°. The La Joya includes (1) a lithically variable basal conglomerate, (2) a thin limestone and limestone conglomerate, (3) red mudstones, claystones, siltstones, and fine quartzose sandstones which grade upward into red, pink, or green, fine to very coarse, cross-bedded, quartzose sandstones and red, pink, or green conglomerates. The La Joya possesses conformable to low angular unconformable relations with the superjacent Zuloaga limestone; it rests with considerable angularity on the La Boca formation and on deformed sediments of late Paleozoic age. At some places it overlies crystalline rocks probably Paleozoic and Precambrian in age. Fossil plants from the upper part of the La Boca formation in Novillo Canyon indicate that this part of the group is probably late Triassic in age. The La Joya thus appears to be Jurassic in age, its general relations to the Zuloaga limestone (of definite Late Jurassic age) being such as to suggest that it might be Middle or Late Jurassic in age.

Midway Stage, Sabine Stage, and Wilcox Group

Stratigraphic nomenclature for provincial divisions of the lower Tertiary (Paleocene and lower Eocene) deposits of the Coastal Plain of eastern North America is variable in form and usage. Midway group, Wilcox group, and Sabine group, the modern geographic, class nouns most commonly applied to these deposits, are used dually as major divisions (groups) of rocks and as provincial subdivisions (stages) of the Paleocene and Eocene. Initially established as names of lithologic units, stated or implied modern usage of Midway, Wilcox, and Sabine is predominantly in a time-rock sense. Dual usage of Wilcox as a rock-unit name is increasingly common. This failure to differentiate fundamentally between rocks, their times-of-creation, and other rocks originating wholly or in part simultaneously, not only impedes interpretations of the natural geological history of the region but confuses beginners and others who may have only superficial or occasional contact with the terminology. The use of (1) Midway and Sabine stages and their companion units, Midway and Sabine ages, as provincial time-rock and time units of the early Tertiary of the Coastal Plain, based on major fluctuations of the strand line; and (2) Wilcox group for the great mass of lignitic, prominently arenaceous deposits of both Midway and Sabine ages; can clarify the nomenclatural confusion and permit conformity to standard usage of time and time-rock units elsewhere in the world. Type exposures of formations included in the Midwayan, Sabinian, and Wilcox, in Alabama, Louisiana, and East Texas are typical of the units. Additional exposures in these areas are usable as reference sections for comparison and partly for the emendation of stratigraphic data supplied by the type locality.

Cenozoic Deposits of Central Gulf Coastal Plain

Twenty thousand feet or more of Tertiary and Quaternary sediments are present in the central Gulf region of southern United States. They comprise a large, seaward-thickening, wedge-shaped sedimentary complex (Gulf Coast geosyncline) composed predominantly of deltaic deposits. Thin, relatively uniform and widespread, marine strata are present between the thick deltaic deposits and on the seaward edges of the deltaic masses. These thin, generally distinctive, marine strata are adaptable on the surface to detailed structural mapping; they also serve as key strata in core drilling, in tracing surface units into the subsurface, and in the preparation of subsurface structural maps. Fossils present in the marine units determine their position in the standard geological time scal and assist in determining the relative geographic position at the time of deposition. The thick, ladle-shaped, deltaic deposits are normally unadaptable to structural mapping; however, they are readily used in the construction of areal, facies, and isopachous maps. Landward, both marine and deltaic deposits are replaced by brackish-water and fluviatile sediments; seaward, the marine deposits are progressively of a deeper-water environment, the deltaic deposits are progressively more marine. The Tertiary is represented by four, perhaps five, epochs of deposition, which are, in ascending order, Paleocene, Eocene, Oligocene, Miocene and Pliocene (?). Each successively younger series of rocks occupies an outcrop position progressively nearer the present coastline. Similarly, each younger rock series has been downwarped less by the thick, geosynclinal sedimentary load and, therefore, has less southwest regional dip. The Midway (Paleocene), Claiborne (middle Eocene), Jackson (upper Eocene), and Vicksburg (Oligocene) groups each contain important marine units. The Wilcox (lower Eocene), Miocene, and Pliocene (?) are primarily deltaic deposits; they constitute the thickest Cenozoic sedimentary accumulations in the eastern Gulf region. The Quaternary is represented by two epochs of deposition, the Pleistocene and Recent. These deposits are characteristically fluviatile gravels, sands, silts, and clays; they border or fill alluvial valleys and were deposited during or subsequent to the Pleistocene glaciation. The outcrop patterns of the major rock divisions of the Central Gulf Coastal Plain are illustrated by areal geologic maps and stratigraphic sections. The thickness and structural configuration of each division is shown by isopachous and structural contour maps. Representative electrical logs illustrate the electrical pattern of each rock division.

Indigenous Precambrian petroleum revisited

Irrefutable evidence of fossil remains from Precambrian sediments and proved petroleum reserves in upper Proterozoic (Riphean-Vendian) strata of the Irkutsk basin, USSR, suggest that unmetamorphosed Precambrian sedimentary rocks should be a focus for hydrocarbon exploration. Since 1965, a dramatic increase in publications which document worldwide occurrences of Precambrian life forms discloses that, by the end of the Proterozoic, organic evolution had produced diversified assemblages of relatively highly developed macroorganisms and microorganisms. Some of these organisms have generated crude oil in the Nonesuch Shale of northern Michigan and kerogen (which yielded hydrocarbons) in stromatolitic carbonate rocks in Africa. Kerogen has been extracted from ~2,300-m.y. old Transvaal (Africa) stromatolitic limestone containing coccoid and complex filamentous cyanophytes (a type of algae). Also, aromatic and aliphatic hydrocarbons have been obtained from the ~2,800-m.y. old Bulawayan stromatolitic limestone of Rhodesia. Additional evidence indicates that commercial reserves of petroleum from Precambrian strata are possible. An oil discovery in Lower Cambrian rocks in 1962, at Markovo in the Irkutsk basin of the Siberian platform area, led to four noncommercial and eight commercial fields producing from Lower Cambrian and Upper Proterozoic strata. Reserves there may be as much as 80 to 100,000,000 bbl of oil in Lower Cambrian strata and about 2.2 Tcf of dry gas in the upper Proterozoic (Riphean-Vendian). Reserves at Sredne-Butuobin in the extreme northeast part of the basin have been estimated to be on the order of 10 to 25 Tcf of gas. About one-third of the gas is in Lower Cambrian strata and about two-thirds in the Proterozoic. The chemical composition and the acritarch forms of these Cambrian and P ecambrian hydrocarbons are different; therefore, the conclusion that they were generated separately and are indigenous to the containing strata appears to be valid.

Formational Divisions of Difunta Group, Parras Basin, Coahuila and Nuevo Leon, Mexico

The Difunta Group of the Parras Basin, southern Coahuila and western Nuevo Leon, Mexico, consists of two major intertonguing lithic facies with a composite thickness of approximately 4,000 meters. Three gray, brown, red, and green, partly variegated, fine- to coarse-grained calcareous sandstone, siltstone, and mudstone units are thickest on the south in the Saltillo-General Cepeda area; they wedge out north and east in the vicinity of the Saltillo-Paredon-Hipolito railroad. Four units of interbedded, black, gray, brown, and green calcareous shales and siltstones and gray to brown sandstones and shales occur between the red units. East of the Saltillo-Paredon-Hipolito railroad they compose an as yet undifferentiated sequence. The various red and gray-brown units have been mapped westward and northward from the vicinity of Saltillo for distances of approximately 100 and 50 miles, respectively, in a region of relatively complex structure. They are, therefore, mappable lithic units and are thus defined as formations in this paper, being named in descending order: Rancho Nuevo, Las Encinas (redbeds), Cerro Grande, Las lmagenes (redbeds), Canon del Tule, Cerro Huerta (redbeds), and Cerro del Pueblo Formations. Type localities for the various formations are located along or adjacent to major roads leading south-southwest and north-northwest from Saltillo.

Geologic Occurrence of Intrusive Gypsum and Its Effect on Structural Forms in Coahuila Marginal Folded Province of Northeastern Mexico

The Coahuila Marginal Folded Province of Coahuila and western Nuevo Leon, northeastern Mexico, is characterized tectonically by numerous, widely spaced, northwesterly aligned, breached folds which form mountains. In some of these, exposed masses of gypsum-anhydrite are intrusive into overlying Late Jurassic and Early Cretaceous strata. Wide spacing and a general northwesterly alignment of these folds contrast with the closely spaced folds of the Sierra Madre Oriental which are abruptly westward at Monterrey, Nuevo Leon. In normal stratigraphic position the gypsum and anhydrite underlie the Zuloaga Limestone (= Smackover Limestone of U. S.) of Sabinasian (Late Jurassic) Age. Wells drilled on anticlinal folds (Sierra de Papagayos in coastal plain east of Monterrey, Nuevo Leon, and Sierra del Fraile in folded province northwest of that city) have encountered salt of unknown structural and stratigraphic relationships below the gypsum-anhydrite. Diapirism of the gypsum-anhydrite is clearly demonstrable in the main arroyo of Potrero Chico--the valley formed by breaching of the northern fold of the Sierra del Fraile. Here the gypsum has flowed up through and around sharply folded and fractured Zuloaga Limestone and La Casita Formation (= Cotton Valley Group of U. S.). The evaporites probably intrude the Taraises Formation (= Hosston Formation of U. S.) of Coahuilan (Early Cretaceous) Age, but direct evidence of this is not available. The gypsum in Arroyo Potrero Chico resembles metamorphic rock, being handed, oriented along flowage lines, and including elongated masses and blocks of the formations through which it passed. In thin sections, individual grains of gypsum are elongated and somewhat rectangular, with distinct physical and optical orientation. Wavy extinction, indicating strain, is common. Anhydrite is present in minor amounts in all sections examined. Although the role of evaporites in crustal deformation is well documented in many parts of the world, the problem of motivation of the various structures in the Coahuila Marginal Folded Province has not yet been resolved. We suggest: (1) tectonic (compressional) forces provided the primary energy for upward movement of the salts; (2) the folds were positioned and possibly shaped by thicker than normal evaporitic materials; and (3) gravity or density (geostatic) movements of the evaporites were secondary in the area.

Indigenous Precambrian Petroleum

Accumulated evidence indicates that (1) the major part of chemical and organic evolution occurred during the 3-5 × 109 years of the earths history preceding the Paleozoic; (2) the basic elements constituting petroleum existed in the early phases of the earths history; (3) unmetamorphosed Precambrian lithic types are similar to younger ones; and (4) the population of the later Precambrian seas was relatively rich and varied, though hard skeletal parts are notably absent in these rocks and, in all probability, were not widely developed. As petroleum is now generally considered of organic origin and is widely disseminated and integral part of most sedimentary rocks, should we not consider unmetamorphosed Precambrian strata to be prospective for petroleum?

SEDIMENTARY VOLUMES IN GULF COASTAL PLAIN OF THE UNITED STATES AND MEXICO PART III: VOLUME OF MESOZOIC AND CENOZOIC SEDIMENTS IN CENTRAL GULF COASTAL PLAIN OF UNITED STATES

Gulfward-dipping Mesozoic and Cenozoic sediments underlie approximately 145,000 square miles in the Central Gulf Coast between Texas on the west and Georgia on the east. Pre-Cretaceous, Coahuilan, Comanchean, and early Gulfian rocks are predominantly red-bed elastics in the eastern portion of this area; westward and downdip marine facies predominate. Extensive marine deposits comprise the middle and late Gulfian; arenaceous facies predominate in the east; argillaceous and calcareous facies are prevalent westward and downdip. Tertiary deltaic sediments center in Louisiana and Mississippi; eastward and down-dip marine deposits prevail. Fluviatile and deltaic Quaternary deposits occur as a surficial mantle over much of the Central Gulf Coast; offshore these deposits are replaced by marine facies. Stratigraphic studies indicate that major Jurassic and Lower Cretaceous sedimentary units are typically elongate-lenticular while those of the Upper Cretaceous and Cenozoic are ladle-shaped. During Jurassic and Cretaceous times the major source of sediments was apparently eastern United States. In the Cenozoic, appreciable quantities of material appear to have come from western United States. Regional isopachous maps illustrate variations in thickness of Mesozoic and Cenozoic Stratigraphic units. In the emerged (onshore) portion of the plain, deep wells prove the presence of at least 265,000 cubic miles of sediments. Interpolations based on regional studies indicate that the total volume of Mesozoic and Cenozoic deposits in this area will exceed 300,000 cubic miles. The known volumes of sediments that accumulated in the emerged (onshore) portion of the Central Gulf Coast during these times are: Few data are available on volumes of sediments in the submerged (off-shore) portion of the plain, which has an area of approximately 140,000 square miles. Conservatively, it is estimated that at least 200,000 cubic miles of Mesozoic and Cenozoic deposits is present. On the other hand, some evidence indicates the total may exceed 500,000 cubic miles.

Difunta Strata of Tertiary Age, Coahuila, Mexico: GEOLOGICAL NOTES

The youngest strata of the Difunta group of the Parras basin, southeastern Coahuila and western Nuevo Leon, Mexico, are shown to be of lower Paleocene age by the presence of Cimomia sp. cf. C. haltomi (Aldrich) and Hercoglossa n. sp. aff. H. fricator. However, the bulk of the Difunta beds contain definite Cretaceous fossils. Previous work had uncovered no Tertiary fossils. Field investigations demonstrate a maximum thickness of 11,000 ft. which can be broken down into 2 gross lithic units: 1) marine sand and shale sequence (fossiliferous) and 2) a series of red bed tongues of sands, silts, and shales (unfossiliferous). In general, the structure is a series of E-W-trending anticlines and synclines. The new data indicates that this portion of the Parras basin was: 1) actively negative until sometime in the Paleocene (Midwayan); 2) undergoing continuous deposition from the late Gulfian (Upper Cretaceous) into Paleocene time, spanning the Mesozoic-Cenozoic time boundary; 3) deformed and uplifted sometime between late Paleocene and late Tertiary time.