Absolute depths of Silurian benthic assemblages

Abstract

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 modem 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 14. 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. □Absolute depth, Silurian, photic zone, calcareous algae, Benthic Assemblage, storms.

Biogeographic and bathymetric determinants of brachiopod extinction and survival during the Late Ordovician mass extinction

“…The pattern is not limited to a single region but is apparent across a broad palaeolatitudinal range (figure 2), implying that it reflects the operation of a globalscale environmental driver and that the previously noted demise of the widespread deep water (BA 5-6) Foliomena fauna [14,22] is only the most extreme manifestation of a broader selective sweep. Interpretation of this pattern is complicated by uncertainty regarding the actual depth ranges represented by BAs (which are restricted to shelf and slope settings) [26], but the sign of the depth signal is informative. Cooling alone would be expected to steepen bathymetric temperature profiles and preferentially cause the extinction of genera restricted to warmer surface waters rather than those restricted to cooler deeper water.…”

Paleobathymetry of a Silurian shelf based on brachiopod assemblages: an oxygen isotope test

“…6). Accepting the validity of the brachiopod-dominated BA scheme of Brett et al (1993), and assuming that the normal storm wave base at Anticosti was about 30 m (Jin and Copper 1999), the Pentamerus oblongus Community from locality A1356a may be assigned to BA3 with an estimated depth range of 20-40 m, whereas the Pentamerus palaformis Community from localities 1380b and A1391b, as well as the stricklandiid communities A958 and A551, may be assigned to BA4 with an estimated depth range of 60-90 m (Table 1). The Triplesia Community represents the relatively deepest habitat similar to that of Clorinda and may be assigned to BA5, with estimated depth ≥ 90 m. Considering an -70 m difference in average water depth between the BA3 and BA5 communities, water temperature is expected to decrease with depth, and brachiopod shells should become relatively more enriched in 18 O.…”

“…Brett et al (1993) made a detailed examination of the criteria and constraints for determining absolute water depth of ancient communities. Ziegler's original model of community zonation was based on brachiopod assemblages from a fine-grained siliciclastic ramp setting, which typically has a relatively uniform gradient from shoreline to continental shelf break.…”

Palaeoenvironmental reconstruction of the Upper Viséan Paprotnia Beds (Bardo Unit, Polish Sudetes) deposition on the ichnological and palaeontological investigations

“…Some of the organodetrital lime stone lenses con tain rich cal car e ous al gal re mains (Dasycladaceae), ostracods and foraminifers. Re cent dasycladales are found exclu sively in warm wa ters, usu ally at low lat i tudes (Bea dle, 1988) and oc cur in masses not deeper than 30 m, spo rad i cally ex ist up to 90 m (Flügel, 1985;Brett et al, 1993). The var i ous kinds of trace fos sils are of com mon oc cur rence (Fig.…”