Recognition of Ancient Shallow Marine Environments

Modern shallow marine environments encompass a great variety of conditions from shoreline to a depth of about 600 feet. In sedimentary rocks, these environments are most readily inferred from diverse assemblages of fossils whose modern relatives are marine. Some sparse and restricted biotas may represent fully marine environments in which certain factors were unfavorable to many types of organisms. Many unfossiliferous black shales represent a foul environment that supported no benthonic life and are inferred to be marine mainly by stratigraphic relations. Marine environments that lack significant sedimentation would be represented in the record only by a submarine paraconformity.

Recognition of marine subenvironments is possible through direct lithic analogy to distinctive modern sediments of known depositional environments, such as oolite, sea-margin laminated calcilutites, and organism-controlled features such as reefs. 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. Ecologic consideration of fossil assemblages may distinguish clear-water from turbid-water, or soft-substrate from hard-substrate environments. Petrographic considerations also allow environmental inference. For example, calcilutite indicates a quiet-water environment that might be either shallow and protected from water agitation by a physical barrier, or deep and protected by water depth itself. 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. In contrast, calcarenite composed of fragmented, abraded, well-sorted skeletal grains indicates water turbulence and winnowing of fines, processes that are more probable in shallow water.

Environmental syntheses based on stratigraphic, petrographic and paleontologic criteria may bring into focus those aspects of ancient marine environments (such as water depth) that are difficult to determine from the record. On a local scale, detailed facies mapping in undeformed rocks may allow detection of original topography that controlled facies changes. On a larger scale, systematic lithic variation along outcrop of an entire stage of rocks may provide a regional picture of the lateral succession of ancient marine environments across an epicontinental basin. 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.