Publication TypeJournal Article
Year of Publication1993
AuthorsBrett, CE, Boucot, A, Jones, B
Type of ArticleJournal Article

A variety of evidence can be used to estimate the absolute depth of the well-established depth gradient of Silurian onshore-to-offshore benthic assemblages (BA 1-5); this evidence is reviewed herein. There is a fair degree of consistency between fossil benthic assemblages and the occurrence of certain primary sedimentary structures, particularly those involved in storm deposition. A second, probably stronger, line of evidence for absolute depth of Silurian benthic assemblages involves the distribution of fossils of light-sensitive organisms and of reefs. Among these, the most important are the calcified dasycladacean algae, to which the cyclocrinitids of the Middle Ordovician - Early Silurian may belong. The modern dasycladaceans have a narrowly restricted bathymetric range. Maximum depth for modern calcified dasycladaceans is about 90 m, but the vast majority occur at 30 m or less. Some Silurian occurrences of these algae are abundant, particularly in BA 3 and 4; rarely, small specimens of cyclocrinitids and receptaculitids are found in the lower, outer portions of BA 4 and even into 5. This evidence constrains much of the spectrum (BA 1-5) of Silurian fossil communities to a rather narrow depth range, within the photic zone. Based on taxonomic uniformitarianism, we would place the depth of Silurian BA 1 through 4 between 0 and about 40-60 m. The common coincidence of the lower end of storm wave base and the lower end of the photic zone near the BA 4-5 boundary also suggests that this position may represent water depths on the order of 50 m. Several other lines of evidence - including algal borings, widths of facies belts, separation of communities by basalt flows whose thicknesses are known - also support a relatively shallow depth range for BA 1-4. The absence of storm-disturbed beds over large parts of several major platforms below about the BA 3-4 boundary region suggests that major storms of the Silurian may have been far weaker than those of the present, possibly owing to a different climatic regime.

URL<Go to ISI>://WOS:A1993LF24800006