Early Silurian sea-level changes

Abstract

Of the various mechanisms suggested (Donovan & Jones, 1979), that most likely to be responsible for the eustatic sealevel changes in the early Silurian is variation in the amount of water incorporated in land-based ice-sheets. This conclusion is based first on the apparent rapidity of many of the sea-level changes (see, e.g. Loydell, 1994) and secondly on the direct evidence of glacial deposits present in the Silurian sequences of Brazil (Grahn & Caputo, 1992) and elsewhere in South America (see Hambrey, 1985).Obviously, the third (and possibly fourth) order cycles being discussed here are superimposed upon the longer term second order and first order cycles (Haq, Hardenbol & Vail, 1988) linked to global tectonic processes. It is likely, for example, that the particularly high sea-levels of the Telychian and early Sheinwoodian reflect the coincidence of interglacial 'greenhouse' conditions with an increase in the volume of oceanic ridges. Abstract -Global sea-level fluctuated markedly during the early Silurian, probably as a result of the waxing and waning of ice-sheets in the South American portion of Gondwana. The highest sea-levels of the Silurian are recorded by the Telychian upper crispus-lower griestoniensis and spiralis-lower lapworthi biozones. Other highstands occurred in the early Aeronian, during the convolutus Zone (mid Aeronian), guerichi Zone and late turriculatus Zone (early Telychian), and early Sheinwoodian. Low sea-levels characterized much of the argenteus and sedgwickii zones (Aeronian), the utilis Subzone (late guerichi-early turriculatus zones, early Telychian), the late Telychian (commencing in the mid lapworthi Zone) and, after a period of apparently only small amplitude sea-level fluctuations in the late Sheinwoodian and earliest Homerian, the mid-late Homerian, in particular the early nassa Zone. Facies (and faunal) changes in the Lower Silurian do not support the P and S model of Jeppsson and others, but are consistent with the sea-level changes proposed herein. Mid Telychian marine red beds appear to have been deposited during a minor sea-level fall immediately after a period of very high sea-levels, rather than during a transgressive episode as previously suggested. Comparison of the sea-level curve presented herein with those constructed in the past is hampered by the lack of precision currently possible in the correlation of early Silurian deep water (graptolitic) and shallow water (shelly) sequences. Improving the precision of this correlation should be a priority for future research.*

Gauging the impact of glacioeustasy on a mid-latitude early Silurian basin margin, mid Wales, UK

“…Long acknowledged as a period of falling sea levels, debate has centred on whether this peaked during the convolutus (e.g. Loydell, 1998) or sedgwickii biozones (see [4] and [5] below). Based on data presented by Wills & Smith (1922), Loydell (1998) emphasised the anoxic nature of convolutus Biozone rocks in North Wales as justification for locating the highstand in this biozone.…”

“…Loydell, 1998) or sedgwickii biozones (see [4] and [5] below). Based on data presented by Wills & Smith (1922), Loydell (1998) emphasised the anoxic nature of convolutus Biozone rocks in North Wales as justification for locating the highstand in this biozone. However, extensive recent work on rocks of this age in mid Wales show this to have been a period during which oxic and anoxic bottom conditions alternated within the Lower Palaeozoic Welsh Basin.…”

“…Others, by recognising its peak in the mid Aeronian leptotheca 464 Biozone (e.g. Loydell, 1998Loydell, , 2007Johnson, 1996Johnson, , 2010, imply a link with at least part of 465 shoaling episode E. The depiction of a discrete early Aeronian lowering of sea level by Ross 466 and Ross (1996) is replicated in the subsequent curves of Page et al (2007) and Haq and 467 Schutter (2008). It compares with a lower order progradation (Ceg 0 ) seen at Llandovery, but 468 the biostratigraphical dating of this global event is suspect (see Fig.…”

“…Such exclusion may account for the poor and inconsistent 598 record of events within this interval (see Section 8). However, it is now acknowledged that suggested to Dias-Martinez and Grahn (2007) Ross and Ross, 1996;Loydell, 1998Loydell, , 2007Nestor et al, 2003;Zhang and Barnes, 608 2002b; Page et al, 2007;Haq and Schutter, 2008;Johnson, 2010;Munnecke et al, 2010 log of glacigenic deposits (Fig. 13), a re-advance during the triangulatus 628 Biozone established an extensive, slowly down-wasting ice mass that was sustained 629 throughout much of the Aeronian, prior to an abrupt and marked retreat at or close to the base 630 of the sedgwickii Biozone.…”

“…Based on empirical associations of sedimentary facies and faunas, particularly 715 patterns of conodont extinction, Jeppsson (1990Jeppsson ( , 1998 offers support for curves that show the Rhuddanian and early Aeronian as a period of 731 elevated sea levels (e.g. Loydell, 1998) (Fig. 14).…”