ABSOLUTE DEPTHS OF SILURIAN BENTHIC ASSEMBLAGES

A variety of evidence can be used to estimate the absolute depth of th e well-established depth gradient of Silurian onshore-to-offshore bent hic 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 th ose 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 Siluria n may belong. The modern dasycladaceans have a narrowly restricted bat hymetric range. Maximum depth for modern calcified dasycladaceans is a bout 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; r arely, small specimens of cyclocrinitids and receptaculitids are found in the lower, outer portions of BA 4 and even into 5. This evidence c onstrains much of the spectrum (BA 1-5) of Silurian fossil communities to a rather narrow depth range, within the photic zone. Based on taxo nomic uniformitarianism, we would place the depth of Silurian BA 1 thr ough 4 between 0 and about 40-60 m. The common coincidence of the lowe r 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 - includi ng 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 ov er large parts of several major platforms below about the BA 3-4 bound ary region suggests that major storms of the Silurian may have been fa r weaker than those of the present, possibly owing to a different clim atic regime.