Shell-sea Chalk Environments
Published:January 01, 1984
Pelagic carbonate oozes have been major sediment of continental slopes and the abyssal plain above calcite compensation depth since at least Jurassic times. The deposit owes its properties to the domination of nannofossil and microfossil calcite skeletons and a paucity of terrigenous material. In shallower, shelf settings, normally the pelagic constituents are diluted by land-derived muds and coarser materials and the purer carbonate oozes are restricted to local importance.
During the late Cretaceous, however, world wide transgression drowned large areas of shelf and land under epicontinental seas and extensive pelagic oozes were deposited under unusually shallower conditions to produce the well known on-shore occurrences of Cretaceous chalk. It is these rocks that form the basis of the present chapter. Their importance as reservoir rocks in several areas, not least in the North Sea, has led to the intensive investigation of the properties of chalks in recent years (e.g., Scholle, 1977a; 1977b); detailed ichnologic studies of chalks, however, have rather lagged behind.
We define “chalk” in more or less the same terms as in general use in the Deep Sea Drilling Project reports (Gealy, Winterer and Moberly, 1971, p. 17; Benson, Sheridan et al., 1978, p. 14), as a firm, partly indurated calcareous ooze or friable limestone. Its fully lithified equivalent has been termed “chalkstone” (Bromley and Gale, 1982; Scholle, Arthur and Ekdale, 1983, p. 623), in harmony with the nomenclature of terrigenous rocks (clay, claystone; silt, siltstone; etc.).
At first sight, chalks represent a monotonous lithology comprising firm, pale-colored micrites.
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Ichnology: The Use of Trace Fossils in Sedimentology and Stratigraphy
Ichnology is a fascinating field of endeavor. As with science in general, it is a process of solving mysteries–in this case, mysteries of fossil behavior. In a very real sense the ichnologist is Sam Spade or Sherlock Holmes–following footprints, searching for traces of dastardly deeds, studying artifacts, attempting to reconstruct a sequence of events from subtle clues, pursuing the identity of someone (or something) long dead. Who was the culprit? What was he/she doing? Where was he/she living, working or going? Not only intellectually intriguing, ichnology also has practical application and economic importance. In today’s frenzied quest for energy and mineral resources, exploration geologists value every tool that aids their search. Ichnologic observations and analyses can help the sedimentologist reconstruct ancient depositional environments, help the stratigrapher correlate sedimentary strata, help the paleontologist determine the nature of fossil communities, and help the geochemist determine the effect of organisms on sediment composition. This publication was written to serve as a comprehensive and intelligible introduction to ichnology for anyone with even rudimentary geologic training, whether or not that person enrolls in a formal course on the subject. The book emphasizes sedimentologic, stratigraphic and paleoecologic al aspects of ichnology.