![]() Using a detailed graphic-log approach, the numerous sedimentation units in the Stairway Sandstone can be grouped into six composite units. This has profoundly influenced facies distribution. These studies have revealed that the sedimentary rocks, mainly orthoquartzite and phosphatic shale, were deposited during a regressive-transgressive cycle which resulted in the migration of a single shallow-marine depositional environment across at least 40,000 square miles of the basin. For this reason, extensive field and laboratory studies were undertaken. ![]() It is therefore desirable to have a detailed knowledge of the depositional environment of this formation. The Ordovician Stairway Sandstone, a shallow-water marine formation of the Amadeus basin, central Australia, contains appreciable, though so far non-commercial, quantities of oil, gas, and phosphate. It is commonly possible to decide that an ancient body of rock was laid down in a shallow-water marine environment but in many cases it is impossible to determine precisely which shallow-water marine environment. Perhaps one of the best modern laboratories to study analogs of ancient marine epicontinental deposition is the Sahul-Arafura shelf and Gulf of Carpentaria between orogenic New Guinea and cratonic Australia. On a larger scale, systematic lithic variation along the outcrop of an entire stage of rocks may provide a regional picture of the lateral succession of ancient marine environments across an epicontinental basin. On a local scale, detailed facies mapping in undeformed rocks may allow detection of original topography that controlled facies changes. Environmental syntheses based on stratigraphic, petrographic, and paleontologic criteria can bring into focus certain aspects of ancient marine environments that are difficult to determine from the record. In contrast, calcarenite composed of fragmented, abraded, well-sorted, skeletal grains indicates water turbulence and winnowing of fines, processes which are more probable in shallow water. The presence of calcarenite composed of whole shells exhibiting little fragmentation or abrasion might indicate only local organic proliferation or lack of dilution by fine sediment. The presence of calcilutite indicates a quiet-water environment End_- that might be either shallow and protected from water agitation by a physical barrier, or deep and protected by water depth itself. Petrographic considerations also allow environmental inference. ![]() Ecologic consideration of fossil assemblages may distinguish clear-water from turbid-water, or soft-substrate from hard-substrate environments. In less distinctive marine facies, subenvironments are difficult to discriminate because visible differences may have resulted from a complex interplay of many variable factors that did not coincide to produce unique subdivisions. Recognition of marine subenvironments is possible through direct lithic analogy with distinctive modern sediments of known depositional environments, such as oolite, sea-margin carbonate laminites, and certain organism-controlled features such as reefs. Marine environments that lack significant sedimentation would be represented in the record only by a submarine paraconformity. Many unfossiliferous black shales represent a foul environment that supported no benthonic life and are inferred to be marine mainly by stratigraphic relations. ![]() Some sparse and restricted biotas may represent fully marine environments in which certain factors were unfavorable to many types of organisms. In sedimentary rocks, these environments are inferred most readily from diverse assemblages of fossils whose modern relatives are marine. Shallow-marine environments encompass a great variety of conditions from shoreline to a depth of about 600 ft. ![]()
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