James Lee Wilson

Characteristics of Carbonate-Platform Margins

Study of many varied carbonate complexes in ancient rocks results in recognition of three recurring patterns of shelf-margin facies and associated organic buildups. Bases for the classification are stratigraphic profile, shape and disposition of discrete carbonate bodies along the profile slope, and environmental interpretations through analysis of textural and biologic microfacies. Type I consists of linear belts of downslope bioclastic lime-mud accumulation, below the zone of active wave action and in places even below the photic zone. Organic boundstone may or may not be present. Crestal deposits on the profile are commonly barrier bars or islands of lime sand. Type II includes linear ramps or platforms composed of organic-reef knolls, commonly forming on gentle slopes at outer edges of shelf margins. Construction of such ramps may interfere effectively with wave action so that the knolls exist in moderately quiet water and can trap lime mud. Lime-sand particles accumulate in flanking beds around the knolls or in passes between them. Massive frame-building organisms may be somewhat rare within the knolls which are the growth form of the reef-builders. Interreef material is volumetrically much greater than the patches of boundstone framework. Crest of slope may be occupied by lime-sand shoals, bars, or even mudflats behind the wide, gently sloping platform. Type III is an organic reef rim of resistant framework, built like that of a modern Acropora reef into active wave base and forming the topographic crest of the profile. Growth form of reef organisms is zoned ecologically. Lagoonal environment develops behind the rim, with lime-sand bars and submarine shoals. Most shelf margins in the geologic record belong to types I and II. These types of carbonate shelf margins and their facies sequence are observed in four common paleotectonic situations: (1) in areas of major subsidence at outer margins of great carbonate ramps built into basins or geosynclines from positive cratonic blocks; (2) within sedimentary troughs such as isolated major carbonate banks--profiles with steep slopes and narrow marginal facies belts; (3) in areas of moderate subsidence, where the same patterns of buildups fill in intracratonic basins; and (4) even surrounding second-order positive elements on shelves. Two types of small-scale organic buildups are found on shallow shelves: lime-mud mounds and patch reefs, each with its attendant facies (e.g., core, flank, crestal, capping beds, etc.). The development of such carbonate-shelf margins is a natural consequence of the normal processes of prolific carbonate production, trapping, baffling, encrustation, organic frame construction, and inorganic and organic cementation. These processes operate in clear warm shallow water on slopes at some distance out from low-lying landmasses. Hydrography, sea-level fluctuation, organic evolution, and tectonics all operate to influence the balance of these factors and to construct particular types of shelf margins. Causal relations are not completely clear. Because the various types of shelf margins are present in all tectonic settings, rate of subsidence apparently is not so important a control as the others. Type I seems to develop with a lack of dominant framebuilders and in moderate t quiet surge. Type II would appear to develop also with moderate surge and with varied organisms as framebuilders, encrusters, or sediment trappers and in a tectonic environment of moderate subsidence. Type III develops in areas of rapid subsidence with open-sea surge or with abundant framebuilders.

Dolomitization of the Madison Group, Wyoming and Utah Overthrust Belt

Two stages of pretectonic dolomitization are recorded in the upper Madison Group of the Wyoming-Utah thrust belt. Marine to hypersaline dolomitization occurred during deposition of upper Madison carbonates and evaporites in peritidal and sabkha settings. This timing is supported by the common association of fine-grained, nonluminescent dolomite with restricted, shallow-water facies, and the relatively enriched isotopic composition of the dolomite (^dgr18O = +2.0 ^pmil; ^dgr13C = +4.0 ^pmil). A second stage of dolomitization began during exposure of the Madison shelf and continued through deposition of the overlying Amsden Formation. Regressions and transgressions during this time caused migration of marine-meteoric mixing zones across the Madison shelf and widespread dolomitization of all depositional facies within the Madison Group. The second stage of dolomitization is volumetrically dominant, has a meteoric isotopic composition (^dgr18O = -2.0 ^pmil; ^dgr13C = +2.0 to +7.0 ^pmil), and in most locations has masked the isotopic record of the first-stage, synsedimentary dolomite. In addition to replacive dolomite, dolomite and calcite cements were also formed during this diagenetic stage. These three components exhibit isotopic variations concomitant with stratigraphic position that reflect spatial gradients in ground water chemistry.

Ordovician Stratigraphy in Marathon Folded Belt, West Texas

Stratigraphic study of little known Ordovician exposures south and east of the Marathon uplift, West Texas, shows that a facies change exists from predominantly shaly beds of the Marathon region to more arenaceous beds in the Solitario and Dove Mountain quadrangle areas. Discovery of graptolites in a few incomplete measured sections indicates that probably Dagger Flat type of sandstone rises in the stratigraphic sequence to replace, in part, the lower Marathon limestone and shale. The upper Marathon and Alsate shale formations of the Marathon anticlinorium are represented, in the exposures considered, by sandy laminated limestones and possibly by overlying beds of massive quartzite. Typical Fort Pena and Woods Hollow formations (Middle Ordovician) of the Marathon uplift a e also present in the Solitario and Dove Mountain areas. The Maravillas is present but thin, and is overlain by about 25-40 feet of shale, probably of Late Ordovician age; the latter is here named the Persimmon Gap formation. Regional correlation shows close similarity between the sections studied and the McCurtain County, Oklahoma, exposures which also contain Early Ordovician sandy beds at approximately the same stratigraphic position. The Ordovician section of the interior (southeasternmost) folds of the Ouachita-Marathon system suggests that orogenic activity occurred in the system in Ordovician time and that this long enduring mobile belt may be considered an early and middle Paleozoic geosyncline despite its comparatively thin stratigraphic sequence.

Stratigraphy of Upper Cambrian-Lower Ordovician Subsurface Sequence in Williston Basin

Trilobites and brachiopods of Late Cambrian and Early Ordovician ages were obtained from seven wells that penetrated the Deadwood Formation in the Williston basin in Montana and North Dakota. The sections contain a basal transgressive sandstone overlain by interbedded limestone pebble conglomerate, dark shale, and light colored sandstone. On the east in North Dakota the section becomes markedly sandy. In these eastern sections it is difficult to place the Deadwood-Winnipeg boundary exactly, but in Montana there is a sharp lithic contact which represents a significant subaerial erosion break in the section. In Montana Dicellomus of Dresbachian age is present in the basal sandstone. The Elvinia, Conaspis, Ptychaspis-Prosaukia, and Saukia zones of Franconian and Trempealeauan ages occur in the overlying 287 feet. There is no evidence of a depositional break between the Upper Cambrian and the Lower Ordovician, and beds at this level carry the transitional Apoplanias faunule. An arbitrary boundary is placed at the first appearance of Symphysurina and (or) the associated Euloma. Faunas of the Lower Ordovician trilobite Zones A, B, C, D, E, and G (of Ross [1951] and Hintze [1953]) have been found. Intercalated graptolite shales permit correlation with Berrys Lower Ordovician graptolite Zones 1, 2, and 3. On the basis of the faunas the Deadwood can be correlated with formations throughout the anadian Rockies, Montana, Wyoming, Utah, Nevada, Colorado, New Mexico, Texas, and Missouri. Lithofacies exhibit many lateral changes within the basin. No persistent units can be recognized for the entire basin, but local facies members useful for mapping in smaller areas are distinguished. Reconstruction of paleogeographic conditions for the Late Cambrian shows the region as a wide, shallowly inundated coastal shelf with a west-northwesterly slope. A positive transcontinental arch between Laurentia and Siouxia forms a low sandy eastern shore or a maze of islands and shallow straits connecting with the upper Mississippi Valley region. The recurring transgressions of the middle Dresbachian, the early Franconian, and the middle Trempealeauan were not accompanied by upwarping of the land areas and may have been caused by eustatic sea-level changes. Shoaling in the Late Cambrian moved the shoreline gradually westward to western North Dakota. The Saukia zone trilobite genera died out, but dendroid graptolites persisted on the oxygenated shoals. Slow deposition of carbonate oo es, muds, and glauconites continued in the basin sites. The appearance of Euloma, Symphysurina, and Hystricurus marks the beginning of a new marine transgression which altered the shore topography and eventually carried the Zones A and B assemblages into the upper Mississippi Valley. Depositional thickening of the basal Lower Ordovician strata from the Richey to the Pine and Cabin Creek fields near the Montana-North Dakota border suggests that the axis of the earliest basin depocenter was in the direction of the Pine and Cabin Creek fields. During Zones D-E time, widespread carbonate shoals supported a gastropod-brachiopod fauna allied to assemblages of the central interior states. By Zones F-G time, the eastern sections were markedly sandy. Zone G faunas are the youngest Early Ordovicia assemblages known in the basin. The continued shoaling and eventual cessation of deposition probably occurred throughout the Williston basin during Zone H time, or definitely before the end of the Early Ordovician. Although no hydrocarbons are known in the Deadwood, it is suggested that the Deadwood shales may be source rocks. Stratigraphic entrapment of these postulated hydrocarbons may have occurred in Winnipeg strata or in sandy Deadwood facies during Early to Middle Ordovician time along the then newly formed eastern rim of the basin.

Diagenesis of Lower Coralline Limestone (Chattian), Maltese Islands: ABSTRACT

The sequence of diagenetic fabrics in the lower Coralline Limestone (Chattian) on the Maltese Islands suggests that there was one phase of early marine cementation followed by at least two periods of phreatic cementation. Cements of marine origin include fine fibrous cement on algal and foraminiferal debris, and clouded syntaxial overgrowths on echinoid fragments. A younger generation of clear overgrowths on echinoid fragments displays luminescent zones that are restricted to the lower Coralline Limestone and lower member of the Globigerina Limestone and can be correlated throughout the Maltese Islands. These clear overgrowths are phreatic and formed both before and during sediment compaction. Subsequent phreatic cementation produced fine to medium-grained, non-luminescin scalenohedral calcite crystals that postdate compaction. The final stage of phreatic cementation consists of fine to medium-grained equant void-filling spar that is non-luminescent. Vertical and lateral distribution of phreatic cements and compacted textures is irregular and discontinuous. In general, well-cemented horizons also show overcompacted textures. The relative timing of these diagenetic features indicates at least two episodes of emergence and meteoric cementation related to the development of freshwater lenses within the lower Coralline Limestone. Erosional and unconformable horizons in the overlying Miocene formations may record times of freshwater alteration corresponding to periods of eustatic lowering of sea level and emergence of the entire central Mediterranean platform. End_of_Article - Last_Page 682------------

Lower and middle Pennsylvanian Gobbler Formation: using carbonate shelf cyclicity to unravel basin history in transtensional regime

The Lower and Middle Pennsylvanian Gobbler Formation of southern New Mexico is a carbonate shelf sequence consisting of 25 to 30 limestone cycles. Cycles comprise 3-10 m-thick subtidal wackestone units capped by thin cross-bedded encrinites and pebbly lag deposits. Subaerial exposure of cycle tops, evidenced by karstic dissolution pits, brecciation, reddish oxidation crusts, and negative carbon isotope excursions, indicates episodic sea level fall. The Gobbler Formation was deposited during the initial phase of subsidence of the Orogrande basin, a north-trending pull-apart basin resulting from cratonward overstep of a broad dextral strike-slip zone transecting the southwestern margin of North America. Basin asymmetry is reflected in greater lateral continuity of cycles and facies belts on the broad, stable western shelf relative to the narrow eastern shelf, which exhibits rapid depth-related facies changes, variable cycle thicknesses, synsedimentary slumping, and a fault-bounded clastic-filled intrashelf trough. Differences in cycle thicknesses between the eastern and western shelves (4.5 m vs 6.0 m mode) record differential subsidence of opposite basin margins. Recurrence of thick, shaly units at intervals of four to six cycles indicates the existence of a longer period (2 m.y.) oscillation that may relate to pulses of basin subsidence.

Carbonate-Evaporite Cycles in Lower Duperow Formation of Williston Basin: ABSTRACT

The Duperow (Frasnian) sediments of the Williston basin consist of approximately 12 regular cycles. Stratigraphic and petrographic studies were made of the lower 300 ft (91 m) of these strata using cores, cuttings, and radioactivity logs from selected locations in North Dakota, Montana, and Saskatachewan. Each cycle consists of three members. The lower member consists of either dark-brown, burrowed, lithoclastic-bioclastic brachiopod-crinoid limestone with a mud matrix, or a stromatoporoid boundstone. A middle member consists of brown lime mudstone with a restricted microfauna of ostracods and calcispheres interbedded with unfossiliferous pelletoid beds or laminated lime mudstone. Bedded anhydrite and gray-green, silty, very fine-grained dolomite displaying intertidal and supratidal sedimentary structures cap each cycle. End_Page 870------------------------------ The Duperow cycles are widespread and constituent beds only 10-15 ft (3-5 m) thick can be traced completely across the Williston basin. Deposition occurred within a vast back-reef lagoon lying south of the Woodbend reef platform of southern Alberta and stretching to a sandy shore in South Dakota and northern Wyoming. This lagoon was periodically and rapidly flooded with normal marine water, permitting organisms to flourish; the sea then gradually shallowed as sediments filled the basin. Desiccation produced extensive tidal flats and evaporitic sabkhas and was perhaps responsible for some dolomitization of the carbonates on shelves outside the basin. The cause of such cyclic sedimentation might have been slow, steady subsidence of the basin with a superimposed climatic rhythm that may have speeded up reef growth and periodically choked off seawater from the basin. Perhaps this process operated coincidentally with sporadic eustatic sea level fluctuation or with abrupt periodic subsidence of the whole basin. The low bathymetric relief that permitted rapid flooding certainly would have aided in development of such cycles. End_of_Article - Last_Page 871------------

Sedimentologic Studies in Jurassic of Tunisia: ABSTRACT

Modern sedimentologic interpretation of the Tunisian Jurassic is based on excellent field studies by Tunisian and French geologists from 1955 to 1972. Jurassic facies north to south across Tunisia were formed where the African craton sloped into the southern edge of the Tethyan Sea and to the south in a restricted marine and evaporitic basin on the African shield itself. Thickness of the system varies abruptly off banks and into starved basins between 300 and 1,000 m. The Lias of central Tunisia forms a broad and typical carbonate platform separating the pelagic facies of the north from the major interior evaporite basin. A great north-south escarpment through southern Tunisia beautifully exposes these evenly bedded, restricted marine carbonate rocks and gypsum. Through the Middle and Late Jurassic the northern starved basin and slope facies (respectively radiolarian shales and Ammonitico Rosso) expanded into central Tunisia. Carbonate banks and patch reefs developed along the north-south axis west of the Sahel and its extension in the Jurassic ranges from Zaghouan to Bou Kornine uplifts. These probably rimmed the western margin of an ancestral Pelagian block. Bathonian slope deposits here consist of debris flows near Tunis, and the Kimmeridgian of Jebel Zaghouan shows a local reefy facies grading abruptly into turbidites and pelagic limestones to the north and west. These abruptly changing facies indicate a moderately unstable (rifting?) margin with intermittent reef growth. The Bathonian debris flows record a tectonic pulse which can be correlated with marked changes in thickening and elimination of strata along the Tebaga-Djeffara lineament, an important structural feature separating the southern evaporite basin from the northern unstable platform and basinal area. End_of_Article - Last_Page 553------------

Northeast Yucatan, Mexico--A New Area Opens for Study of Carbonate-Evaporite Sediments

ABSTRACT Carbonates and evaporites being deposited in the coastal regions of the northeastern Yucatan Peninsula include shallow-shelf carbonate sands and gravels, eolianites, coral reefs, carbonate mud, dolomitic mud, gypsum, and salt. Recently improved roads and transportation facilities in northeastern Yucatan now make areas which merit carbonate-evaporite research reasonably accessible. In addition, this region is particularly suited for instructive field observation.

Depositional Facies Across Carbonate Shelf Margins: ABSTRACT

Nine depositional facies, in idealized sequence across a typical carbonate shelf margin, include basinal, tidal-shelf, basin-margin, platform-foreslope, organic-reef, platform-edge-sand, open-marine-platform, restricted-marine platform, and platform-evaporite facies. Each facies is characterized by sedimentologic parameters, prevailing rock types, color, microfacies, sedimentary structure, terrigenous content, and distinctive biota. Descriptions of this very widespread sequence should aid in location of reservoir rock. End_of_Article - Last_Page 1793------------