RISING, FALLING, HIGHSTAND OR LOWSTAND: EUSTASY AT THE SILURIAN-DEVONIAN BOUNDARY

The Geologic Time Scale (2012) and the Devonian chapter in the accompanying volume depict a highstand of sea level at the Silurian-Devonian boundary (SDB). The Silurian chapter in the GTS volume shows two sea-level curves: one with sea level falling and one with sea level rising toward the SDB in the late Silurian. A brief survey of the literature reveals conflicting accounts of sea level change at the SDB: lowstand (3 papers), falling (4 papers), rising (1 paper), highstand (1 paper, and GTS 2012). Two papers describe lowstand conditions above the SDB in the lowest Lochkovian and one describes rapid transgression and highstand in the lowest Lochkovian. Two of the most widely cited papers on mid-Paleozoic eustasy do not address the SDB at all. The M.E. Johnson (2010) sea-level curve ends in the middle Přídolí. The Devonian curve of J.G. Johnson, et al. (1985) begins in the Pragian but includes a dashed curve for the late Lochkovian; the SDB and the early Lochkovian are blank. So, what was sea level doing at the SDB?

In the Appalachian Standard Succession (New York), the SDB occurs in the Green Vedder Mbr. (GVM) of the Manlius Fm. The GVM rests on the Clockville Unconformity, an erosional surface and overlying condensed bed that record subaerial exposure followed by rapid transgression and sediment starvation. The overlying, interbedded limestones and carbonaceous shales of the GVM record highstand conditions and some degree of dysoxia. The correlative Big Mountain Shale (VA, WV) of the central Appalachians is interpreted similarly. The SDB occurs within facies similar to the GVM in the basal Devonian GSSP in the Czech Republic and in Morocco, Sardinia, Catalonia, and parts of the Carnic Alps. The widespread occurrence of broadly similar facies argues for highstand conditions at the SDB, which contrasts with interpretations of regression linked with the positive δ¹³C excursion at the SDB. Occurrences of the SDB in shallower facies in other locations may reflect local tectonic effects.

Factors that confound our understanding of eustatic change at the SDB include 1) self-imposed limitations of researchers (Silurian only, Devonian only), 2) lack of detailed biostratigraphy (other than graptolites) for the Přídolí and Lochkovian stages and/or 3) the imprints of local tectonics which mask the eustatic signal in some locations.