Skeletal Calcarenites Research Articles

Experimental Vadose and Phreatic Cementation of Skeletal Carbonate Sand

ABSTRACT Some processes of and diagenesis in carbonate rocks have been simulated in the laboratory, using CO2-charged water to leach a bed of carbonate sand. Precipitation of calcite cement in a second sand body, above and below an artificial water table, was induced by CO2 evasion. Magnesian calcite was preferentially leached from the source bed, and calcite was precipitated as cement in the second sand unit. More cement was produced in the vadose zone than in the phreatic zone. Cement formed in the zone is composed of micron-sized crystals, and occurs as abundant rim cement and grain contact cement. Crystal size increases with distance from grain boundaries. Rounding of intergranular void spaces is common. Cement formed in the zone is more coarsely crystalline (50 microns) than in the and abuts directly against original skeletal grain boundaries. The texture of the cement closely resembles textures observed in cementation of skeletal calcarenites in Bermuda.

Carbonate Deposition Near Mean Sea-Level and Resultant Facies Mosaic: Manlius Formation (Lower Devonian) of New York State

Study of the Lower Devonian Manlius Formation along 250 miles of outcrop in New York State indicates that, by analogy with Recent carbonate environments, it was deposited in an environment very near mean sea-level, and having great lateral extent with very low relief. Three facies can be recognized within the Manlius: supratidal, intertidal, and subtidal, representing carbonate environments slightly above, at, and just below, mean sea-level, respectively. Small fluctuations in water level, caused by lunar tides, storms, or seasonal climatic variations, as well as local differences in sediment accumulation and erosion, resulted in sharp lateral and vertical facies changes with the result that the Manlius today has a complex internal stratigraphy and fossil distribution.The supratidal facies is characterized by non-fossiliferous, laminated, mudcracked, dolomitic, pelletal carbonate mudstone. Mudcracks and spar-filled vugs (bird's-eye structure) indicate frequent subaerial exposure; thin bituminous films separating individual carbonate laminae suggest the presence of algal mats. Dolomitization was penecontemporaneous and is inferred to have been similar to present-day supratidal dolomite formation in Florida and the Bahamas. The intertidal facies is composed of alternating thin beds of sparsely fossiliferous, pelletal, carbonate mudstone and skeletal calcarenite; individual beds commonly lie uncomformably on those below. Many primary structures are evident, especially scour-and-fill, cross-stratification, and limestone-pebble conglomerate. Many fossil occurrences have abundant individuals but only a few taxa are represented. Algal stromatolites and oncolites also are common. A few mudcracks and minor erosional relief on carbonate mudstone beds indicate intermittent subaerial exposure. The subtidal facies is a skeletal, pelletal, carbonate mudstone commonly with well-developed, tabular masses of stromatoporoids. The relatively diverse biota of this facies required continuous marine submergence; its close association and juxtaposition with rocks that clearly indicate periodic subaerial emergence further suggest that water depths were very shallow. The stromatoporoids are interpreted as having grown either as encrusting masses within tidal creeks or channels, or else as closely crowded, individual heads in front of the tidal flats of the intertidal facies. The three facies of the Manlius existed contemporaneously, retreating and advancing continuously with the result that, today, they form a complex facies mosaic. Manlius deposition as a whole took place within and marginal to a broad and shallow, though somewhat restricted, lagoon that developed on the landward side of a wide belt of crinoid meadows, represented by the laterally equivalent Coeymans Formation, which is a brachiopod-rich crinoidal calcarenite. During the Early Devonian marine transgression in the northern part of the Appalachian basin, these environments represented by the Manlius and Coeymans migrated westward with time across New York State.

Quantitative Environmental Analysis of Lower Cretaceous Reef Complex: ABSTRACT

Lower Cretaceous rudist reefs control facies distribution in the subsurface Edwards and Stuart City Formations in South Texas. An outcropping rudist reef of nearly equivalent age in Mexico, and the Florida reef tract-Florida Bay Recent model, facilitate definition of the subsurface facies. Dominant facies in End_Page 466------------------------------ the Edwards and equivalent units include burrowed and algal mudstone, skeletal siltite, skeletal calcarenite, rudist reef, and planktonic foraminiferal carbonate mudstone. Comparing the Cretaceous and Recent models, a change in reef frame from rudists to corals is the principal difference but minor faunal components in back-reef sediments are similar. Rock samples are described quantitatively and compared vectorially. A reduction in the dimensions of the vector space is accomplished by factor analysis. Sample composition of the reef and associated facies is determined from the resulting rotated factor matrix. A factor score, computed by post-multiplying the transpose of the standardize data matrix by the square of the rotated factor matrix, emphasizes important rock components controlling the various facies. Thus, the number of critical components needed to outline the environments is reduced. Parameters for the analysis include components modified by textural and structural adjectives (excluding burrowed carbonate mudstone). A second factor analysis was run using only important faunal components as outlined by the factor score. Environments outlined by the two analyses are very similar notwithstanding this reduction in the number of descriptive parameters. However, micro-sedimentary structures and textures are important in environmental interpretation of facies containing extinct faunas. End_of_Article - Last_Page 467------------

Recognition of Transgressive Carbonate Sequence Within Epeiric Sea: Helderberg Group (Lower Devonian) of New York State: ABSTRACT

The regional Late Silurian-Early Devonian marine transgression of the central Appalachians is represented in New York State by a shallow-water carbonate rock sequence (Helderberg Group) which locally transgressed north and west. The resultant stratigraphic section comprises several hundred feet of fossiliferous limestone which has several distinctive sedimentary facies. Early workers interpreted each of the major facies as a separate time-stratigraphic lithologic unit or formation. However, from detailed field examination Rickard (1962) demonstrated that these formations are in fact time-transgressive toward the west and interfinger laterally with each other. Paleoecological study of the Helderberg Group supports this interpretation and shows that each of the formations represents a local sub-environment within the transgressive interval as a whole. These formations (facies) are: (1) Manlius Formation (25-50 feet), a complex of rock types interpreted to represent supratidal, intertidal, and shallow subtidal environments within a broad shelf lagoon (Laporte, 1964; 1967). (2) Coeymans Formation (20-100 feet), crinoidal-brachiopod skeletal calcarenite and carbonate siltstone which are commonly burrow-mottled toward the base of the unit but which show increasingly greater evidence of current reworking toward the top (high-and low-angle cross-stratification and sheet deposits). The Coeymans is interpreted to have been deposited in a wide belt of shallow, submerged crinoid mounds and banks which served as an effective, though discontinuous, barrier to circulation separating the more open-marine environment on the east from the restricted shelf lagoon of the Manlius on the west (Anderson, 1965). (3) Kalkberg Formation (50-100 feet), highly burrow-mottled carbonate mudstone with a very abundant, diverse, and well-preserved biota. The Kalkberg is interpreted to be a shallow-water, open-marine deposit which developed on an extensive shelf seaward from the Coeymans crinoid banks and meadows. (4) New Scotland Formation (50-150 feet), highly argillaceous and siliceous carbonate mudstone with a somewhat less diverse and abundant biota than the Kalkberg. The New Scotland is interpreted as having developed on a broad shelf like the Kalkberg (and marginal to it), but with significantly greater influx of terrigenous detritus which probably came from a distant easterly source. Lateral and vertical variations in constituent carbonate-grain types, mudstone-sparite ratios, fossil abundance and diversity, and presence of primary sedimentary structures provide criteria for recognizing the transgressive nature of the major sedimentary facies of the Helderberg Group. The inferred depositional framework, moreover, is very similar to that postulated by Shaw (1964) and Irwin (1965) for clear water sedimentation within an epeiric sea and demonstrates the predictive validity of their generalized sedimentary model. End_of_Article - Last_Page 473------------

A discussion concerning the floor of the northwest Indian Ocean - The recent sedimentary facies of the Persian Gulf region

The Persian Gulf, which is a shallow marginal sea of the Indian Ocean, is an excellent model for the study of some ancient troughs. It is bordered on the west by the Arabian Precambrian shield and on the east by the Persian Tertiary fold mountains. Persia is an area of extensive continental deposition. It is bordered by a narrow submarine shelf. The deeper trough of the Persian Gulf lying along the Persian Coast seaward of the shelf is floored by marly sediments. East of this, the Arabian shelf is covered with skeletal calcarenites and calcilutites. To the northwest is the Mesopotamian alluvial plain and deltaic lobe. Arabia is bordered on the Persian Gulf littoral by a coastal complex of carbonate environments. Barrier islands, tidal deltas (the site of oolitic calcarenite formation) and reefs protect lagoons where calcilutites, pelletal-calcarenites and calcilutites and skeletal calcarenites and calcilutites are forming. There are Mangrove swamps, extensive algal flats and broad intertidal flats bordering the lagoons and landward sides of the islands. A wide coastal plain, the sabkha, borders the mainland. Here evaporation and reactions between the saline waters percolating from the lagoons, and calcium carbonate deposited during a seaward regression, leads to the production of evaporitic minerals including anhydrite, celestite, dolomite, gypsum and halite. Inland, wide dune sand areas pass into the outwash plains skirting the mountain rim of Arabia.

Paleoecology and Diagenesis of Key Largo Limestone, Florida

The Key Largo Limestone is a Pleistocene reef limestone that forms the upper Florida Keys. It consists of an organic framework of coral colonies and an interstitial skeletal calcarenite, both of which are homogeneous throughout the upper part of the formation. The Pleistocene reef was formed chiefly by massive and rounded coral heads, the principal frame builder being Montastrea annularis. No organic zonation is visible perpendicular to the long axis of the reef. Both a well-sorted and a poorly sorted calcarenite facies can be distinguished. The poorly sorted facies, which predominates, surrounds coral heads and is characterized by its high calcilutite content. The well-sorted facies forms channel deposits between coral buildups. In organic composition, the ancient Key Largo reef is roughly equivalent to the deep-water Montastrea annularis zones of living West Indian bank reefs and to West Indian patch reefs. It grew under semi-restricted conditions, possibly as a series of coalescing patch reefs, but more probably below the surf zone in op n water, where it spread laterally over large areas of the sea floor. The constituent composition of the two calcarenite facies indicates that aragonite and high-magnesium calcite originally were the chief mineralogic components. Fossils that initially were high-magnesium calcite have been altered in place to low-magnesium calcite through the removal of excess magnesium by fresh water in the meteoric zone; such fossils have otherwise remained intact. Aragonit, on the other hand, currently is being dissolved in the meteoric zone, and the dissolved CaCO3 is being redeposited near the site of dissolution as low-magnesium calcite cement. Many fossils that originally were aragonite now remain only as molds of sparry calcite. The rate of aragonite dissolution and calcite cementation is accelerated by artificial exposure. The diagenetic fabrics exhi ited by the Key Largo Limestone should be useful in recognizing ancient limestone units that were subjected to subaerial exposure soon after deposition.

Habitat of Oil in Carbonate Rocks: ABSTRACT

Entrapment of oil in carbonate reservoirs can be explained by analysis of (a) depositional environment, (b) diagenetic changes, (c) structural history, and (d) fluid mechanics. Favorable reservoir rocks in carbonate environments include reefs, bioherms, oolite bars, and porous skeletal calcarenite. Production of organic material in such environments (with the exception of oolite bars) is prolific, but under normal conditions a major part of the organic soft parts are destroyed by bacteria scavengers and early diagenesis, whereas skeletal parts are preserved. Early diagenesis modifies the texture and the original porosity of carbonate deposits by recrystallization, solution, cementation, and replacement. Under favorable conditions, dolomitization enhances the reservoir characteristics of the carbonate sediment. Hydrocarbons are found in cyclic carbonates which were deposited on unstable shelves and subjected to recurrent sea-level fluctuations with periodic influxes of terrigenous clastic material. Under a cyclic regime of sedimentation, a reservoir-type carbonate facies can be covered by sapropelic shale, evaporite, or basinal facies. This stratigraphic relation, in addition to providing an adequate seal, also can be suitable to preservation of organic soft parts within the reservoir facies. In the writer's opinion, cyclic sedimentation in carbonate rocks could explain in situ accumulation of hydrocarbons in carbonate rocks under certain favorable conditions. The oil generated in carbonate rocks is subject to secondary migration as a result of structural deformation. An understanding of fluid mechanics is very useful for explanation of some peculiarities of oil distribution within the carbonate traps. Tectonic setting of the carbonate shelf, relative to the stable nuclei and the mobile margins of the continents, has a profound influence on the type of trapping mechanisms likely to be found in the carbonate rocks. End_of_Article - Last_Page 1831------------

ENVIRONMENT RECONSTRUCTION FOR A PART OF THE GLEN ROSE LIMESTONE, CENTRAL TEXAS

SUMMARYAn approximately 50 ft. stratigraphic section of the Lower Cretaceous Glen Rose Limestone was sampled at 24 separate localities in central Texas from the standpoint of reconstructing the depositional environment. Among these samples 199 representative specimens were selected and subjected to point‐count analyses, X‐ray analyses, and insoluble residue determinations. Statistical analysis of the accumulated data with an IBM 7090 computer yielded the following facies: Corbula facies—characterized by a relative abundance of thick‐shelled ostracods, thin‐walled miliolid foraminifers, and the small pelecypod Corbula martinae; steinkern facies—characterized by large mollusc steinkerns, cellular mollusc shells and the foraminifer Orbitolina; mudstone facies—typified by less than 10% sand‐sized grains; and mixed particle facies—characterized by worn skeletal and nonskeletal carbonate grains. The mud‐stone facies subsequently was divided into two subfacies: (1) lime mudstones—characterized by delicate skeletal constituents and lime mud, and (2) marly mud‐stones—consisting of a mixture of lime mud and terrigenous clay‐sized material. Similarly the mixed particle facies was divided into four subfacies: skeletal calcarenites—characterized by sand‐sized skeletal debris; skeletal wackestones—consisting of sand‐sized skeletal particles floating in a mud matrix; nonskeletal calcarenites—characterized by nonskeletal carbonate grains; and nonskeletal wackestones—consisting of nonskeletal carbonate grains floating in a mud matrix. In addition limey sandstone, dolomite, and stromatolite facies were distinguished on the basis of relatively obvious textures and compositions.The attributes of these facies as evidenced by the 199 statistically analyzed specimens were then used to assign each of the additional samples (350) to a particular facies and to identify the distribution of these facies in the field. A reconstruction of the depositional environment was made for each facies, and the following depositional history was interpreted from the resulting facies pattern.The lowermost beds of the unit, consisting largely of stromatolites and nonskeletal calcarenites, are interpreted as representing deposition in very shallow, probably intertidal, waters, Following the deposition of these beds, the depth of water increased

Fabric Studies of Virgil and Wolfcamp Bioherms, New Mexico

Discrete and interconnected masses of dark crystalline calcite make up more than one-half of the Wolfcamp bioherms and the lower one-third of the Virgil bioherms in the Sacramento Mountains. The remainder of the rock consists of light calcite mud with stromatolitic algal deposits, platy algae, and pockets of skeletal calcarenite. Large calcite crystals have a fibrous radiate appearance. The dark masses occur in four distinctive shapes: (1) irregular globular elements from 1 to 3 cm., (2) irregularly horizontal or ribbon-like tabular bodies up to 3 cm. thick and 15 cm. long, (3) nearly vertical finger-like elements up to 6 cm. long and 2 cm. wide, and (4) three-dimensional networks extending over large areas of vertical outcrop and locally forming nearly 90 per cent of the rock. The contacts between the dark calcite masses and the surrounding sediments are very sharp, prominently displaying botryoidal upper surfaces and rounded configurations suggesting organic growth. These crystalline masses are the foss...