Supratidal Deposits Research Articles

The Westphalia Limestone of the Northern Midcontinent: A Possible Ancient Storm Deposit

Two very different rock types, a fusulinid lime packstone to grainstone and an ostracode coaly lime mudstone, form the vast majority of northern Midcontinent Westphalia (Stranger Formation, Douglas Group, Virgilian, Pennsylvanian) outcrops. A mixed-fossil lime wackestone is the principal Westphalia rock type in the subsurface. Petrologic characteristics such as mixed faunas, abrasion and polish of grains, grading in the form of hands of larger invertebrates in the fusulinid sandstone, lime mud rip-clasts as much as 7.5 inches in long dimension, shale pebbles, local current lineation, and the concentration of trillions of fusulinids along with many other invertebrate grain fragments within the fusulinid lime packstone to grainstone are believed to record a current-transported sediment load derived from a shallow seafloor area estimated very conservatively at 1450 square miles. The ostracode coaly lime mudstone displays well-preserved lamination, extremely limited fauna interpreted as indicative of a fresh-to-brackish water habitat, and contained plant debris. This rock type lacks physical connection with the fusulinid lime packstone to grainstone facies. Both outcropping rock types overlie deposits of different types in different areas. The subsurface mixed-fossil lime wackestone facies extends over the largest area and is westward of the outcrop facies. Although widespread, the Westphalia is much less so than most Midcontinent late Missourian-early Virgilian limestones and is completely absent from large areas within its limits of extent. Ball (1964) documented the regional depositional framework for the Douglas Group, which includes the Westphalia Limestone and showed that depositional basin margins were persistently located within the area of, and to the east-southeast of, present-day outcrops. Within this depositional framework, the mixed-fossil lime wackstone of the subsurface is believed to have been the source of most constituents of the centrally-located fusulinid lime packstone to grainstone facies as well as the lime mud of the eastward facies, the ostracode coaly lime mudstone. This fit within the regional depositional setting as well as the petrologic characteristics cited above, when compared to those of Recent storm deposits given below in the introduction, indicate that the fusulinid sand and the ostracode rock are shore zone tidal and supratidal storm deposits.

Petrology and Paleoenvironments, Robinson Member, Minturn Formation (Desmoinesian), Eagle Basin, Colorado

The Robinson Member is one of eight carbonate members of the 6,000-ft (1,824 m), predominantly sandstone, Minturn Formation (Desmoinesian) in the study area west of the Gore Range in central Colorado. The Robinson Member consists of three limestone units (lower, middle, and upper) interstratified with marine and nonmarine sandstone, siltstone, and conglomerate. Total thickness of the Robinson Member ranges from 200 to 400 ft (60-120 m). Several fusulinid biozones can be recognized in the Robinson Member. They are parallel with and equivalent to the lower, middle, and upper limestones. Fusulina pristina and Fusulina curta do not appear to range above the lower Robinson. Fusulina rockymontana and Wedekindellina coloradoensis are present throughout the Robinson. Fusulina truncatulina has been found only in the upper Robinson and Fusulina plattensis is limited to the upper Robinson and overlying Elk Ridge Member. The areal distribution and the thickness of four time-transgressive facies which make up each of the Robinson limestone units imply that the late Paleozoic Eagle basin was an asymmetric depositional basin with a relatively deep marginal trough along the east side. The basin was bounded on the east by the Ancestral Front Range highland and on the southwest by an Ancestral Uncompahgre highland. From east to west the limestone units contain (1) an oolite facies composed of superficial oolites, pseudo-oolites, and oncolites in microspar or micrite; it is interpreted to be a shallow-water, high-energy deposit on the east edge of the marginal trough; (2) a tubular Foraminifera facies composed mainly of tubular Foraminifera micrite; (3) a phylloid algae facies of biomicrite containing abundant Archaeolithophyllum, Eugonophyllum, fusulinids, and Komia; and interpreted to be a deeper water deposit near the middle of the trough; (4) a stromatolite facies of stromatolites, laminated micrite, and vuggy ostracod-bearing intramicrudite, which indicates quiet-water to supratidal deposition near the western margin of the trough. Small Solenopora bioherms are present locally in this facies The stromatolite facies grades westward into and is interbedded with siltstones and evaporites of the Eagle Valley Evaporite near the center of the Eagle basin. The overall patterns of the limestone facies and sandstone beds indicate intervals of rapid nondepositional marine transgression, followed by relatively slow regression in which a series of limestone facies was deposited. A typical sequence of lithofacies in the Robinson limestones from bottom to top is: argillaceous micrite, phylloid algae biomicrite, tubular Foraminifera biomicrite, and oosparite and/or oomicrite. Bioherms as thick as 80 ft (24 m) are in the phylloid algae and stromatolite facies. Nonphylloid algae appear to be the major components of these bioherms. However, recrystallization and dolomitization make recognition of the alga types difficult. Diagenesis of the limestones involves recrystallization, silicification, dolomitization, and intrastratal solution.

ALGAL LIMESTONES WITH PSEUDOMORPHS AFTER GYPSUM FROM THE MIDDLE JURASSIC OF SCOTLAND

Hudson, J. D.: Algal limestones with pseudomorphs after gypsum from the Middle Jurassic of Scotland. Lethaia, Vol. 3, pp. 11–40. Oslo, January 15th, 1970. Algal limestones from two horizons within the Great Estuarine Series of the Hebrides are described. That from the Mytilus Shales is a typically stromatolitic limestone formed by algal trapping and binding of micrite pellets. Calcite pseudomorphs after gypsum occur along some of the laminae. The limestones from the Lower Ostrea Beds include less regularly-stromatolitic types, and nodular algal limestones that contain tubes of calcified algae, notably Cayeuxia nodosa Anderson. The algal nodules also yield uncalcified tubes composed of organic material, that are comparable to recent Schizothrix. These algae form inter-growths similar to those recorded from the recent supratidal deposits of the Bahamas. The algal nodules are often coated with microtufa. Pellet- and spheru-lite-limestones are associated with the algae, and pseudomorphs after gypsum are again found. Isotopic data on the carbonate-carbon, and the palaeoecological implications of the algae and the pseudomorphs are discussed.

Comparison of Modern Island and Mainland Supratidal-Flat Carbonate Deposits, British Honduras: ABSTRACT

The usefulness of supratidal-flat sediments in paleogeographic reconstructions is decreased by the difficulty in distinguishing between island and mainland deposits. Holocene carbonate sediments in British Honduras form supratidal-flat deposits on shelf-margin islands and on the mainland. The vertical sequence and sedimentary structures of deposits in both areas are similar, reflecting similar modes of formation and physical settings, respectively. Constituent compositions and the nature of nearby deposits differentiate these island and mainland supratidal carbonates. Shelf-margin islands are associated with backreef coralgal sand; fragments of corals and coralline algae are included in some island deposits. Mainland deposits lack these indicators of a shelf-edge environment but contain brackish-water organisms and terrigenous material, such as quartz and clay minerals. Furthermore early diagenetic dolomite occurs only in island sediments. These distinguishing compositional differences, though diagnostic for northern British Honduras, are not absolute. Rather, they illustrate types of characteristics which may be useful environmental indicators in other areas. In the absence of compositional differences, lateral facies relations may distinguish island and mainland deposits if sufficient stratigraphic control is available. An island interpretation is favored where submarine shelf-lagoon deposits occur between discontinuous supratidal deposits which are separated by several miles, End_Page 715------------------------------ especially within a marine-to-terrestrial facies transition. Otherwise, an interpretation of frequently shifting sites of supratidal sedimentation along a mainland is favored. End_of_Article - Last_Page 716------------

Diagenetic Modification of Recent Sediments Associated with a Limestone Island: ABSTRACT

Recent carbonate sediments on Ambergris Cay, British Honduras, occur as a thin veneer of supratidal and intra-island lagoonal deposits, incompletely mantling an irregular Pleistocene limestone surface. Both sediments and rock exhibit different degrees of diagenetic modification. The supratidal mud flats usually adjoin very shallow hypersaline ponds, where sediments are subjected to extremes of chemical and physical environmental conditions; the raised rims of the mud flats prevent rapid drainage after periods of heavy rainfall or sea-water flooding. Etching by rainwater and boring by algae tend to destroy or comminute sediment particles on the mud flats. Furthermore, extensive blue-green algal mats commonly are associated with a near-surface crust of dolomitized sediment. Degree of induration of this crust is related to the degree of dolomitization. Low pinnacles of Pleistocene limestone, where exposed near the dolomite crust, also have been partly dolomitized. Recent sediment particles commonly are recrystallized to cryptocrystalline carbonate, without mineralogic change, prior to burial. End_Page 611------------------------------ Storm-tossed sand and cobbles on windward beach ridges show other diagenetic effects, including disintegration caused by decay of organic matrices and by solution of particles below the fresh-water table. Conversely, cementation and pore filling in some beach-ridge sands represent incipient lithification. The extensive outcrops of Pleistocene limestone afford a study of post-lithification diagenesis affecting lithofacies which are analogous with nearby Recent sedimentary facies. Replacement of most of the component grains of this rock by low-magnesium calcite, a change not seen in Recent sediments, tends to obliterate boundaries of recognizable grains. Boring by various organisms, leaching by percolating water, and filling of pores further modify the rock's texture; however, its primary fabric remains readily recognizable. End_of_Article - Last_Page 612------------

Analogs of Recent Low-Energy Carbonate Deposits in Stony Mountain (Ordovician) and Interlake (Silurian) Formations, Montana: ABSTRACT

Reservoir rocks were studied from the Stony Mountain (Ordovician) and Interlake (Silurian) producing formations in several oil fields on the Cedar Creek anticline, southwestern Williston basin. In early Paleozoic time the basin was covered predominantly by epeiric seas in which were deposited shallow-water, intertidal, and supra-tidal carbonates of distinctive facies and fabric. These deposits now are dolomite in which intercrystalline porosity predominates. However, their delineation and extent are controlled strictly by original facies and subsequent diagenetic structures. Such facies and structures compare favorably with those of modern tidal-flat and supra-tidal deposits of Florida and the Bahama Islands, including the alteration forms which occur shoreward of the mea high-tide line. A generalized working model of facies relations was derived. This model shows the proposed environment of deposition and some of the kinds of depositional structures in which original porosity distribution was preserved. Most of the important porous structures result from a combination of organic and inorganic processes in zones of low hydrokinetic potential. These are: pelleted, laminated, and burrowed mud and silt; algal mats and stromatolites; flat-pebble conglomerate; endogenic and solution breccia; and a few cut-and-fill structures. Leaching of fossils and anhydrite in certain places has accentuated and improved pore structure. End_of_Article - Last_Page 2037------------

Oil-Bearing Analogs of Modern Calcareous Sedimentary Fabrics in Low-Energy Environment: ABSTRACT

Examples are presented of reservoir rocks taken from Stony Mountain (Ordovician) and Interlake (Silurian) producing formations in several oil fields situated on the Cedar Creek anticline, southwestern Williston basin. In lower Paleozoic time the basin was dominantly covered by epeiric seas in which were deposited shallow-water, intertidal, and supratidal carbonates of distinctive facies and fabric. These deposits are now dolomite in which intercrystalline porosity predominates. However, the delineation and extent of the latter is strictly controlled by original facies fabrics, the character of which is favorably comparable with modern tidal-flat and low-profile supratidal deposits reported for Florida and the Bahama Islands. A generalized working model of facies relationships is presented showing the proposed environment of deposition and some of the kinds of fabrics in which producible oil has been found. Most of the important oil-bearing fabrics suggest combined organic and inorganic processes in zones of low hydrokinetic potential, viz., pelleted and laminated muds and silts; burrowed, bored, and reworked muds and silts; algal mats and stromatolites; flat-pebble conglomerates; and endogenic and (or) solution breccias. Leaching of fossils End_Page 276------------------------------ and anhydrite in certain cases has accentuated and improved pore structure. End_of_Article - Last_Page 277------------