A HIRNANTIAN REGRESSION RECORDED IN CENTRAL SWEDEN

Ongoing re-evaluation of high-diagenetic sedimentary strata of the Caledonian foreland basin points to a shortlived but profound regression in Hirnantian times. The terminal Ordovician basin-fill in Jämtland starts with the Kogsta Siltstone Fm. It consists of shales, mud-, and siltstones yielding rare graptolites and a Hirnantian shelly fauna. The regional extent and the homogenous facies of these fine clastics indicate a highstand situation with deposition below the effective wave-base. A pronounced erosional boundary, locally with 0.5-1.0 m scouring, separates the Kogsta Siltstone from the overlying Ede Quartzite Fm. The lower few metres of the Ede Quartzite consist of thickbedded quartzites. Sedimentary structures and rapid lateral pinchout and/or amalgamation of single beds denote storm influenced deposition in a shoreface environment; i.e., a profound seaward shift of shallow water sand deposition occurs across the Kogsta/Ede boundary. Two types of conglomerates are associated to the basal Ede Quartzite; angular extraformational gravels of the Kogsta Siltstone, and intraformational, well-rounded gravels of the Ede Quartzite. The conglomerates, the lack of intermediate facies, and absence of any significant hiatus indicate that the Ede Quartzite was deposited during a forced regression rather than, as previously thought, during transgression. The quartzites are conformably overlain by thinbedded and wave rippled mixed carbonate-siliciclastic strata indicating deposition in a transgressive, wave-dominated shallow water depositional system.

In accordance to Hirnantian global palaeogeographic changes, the facies relationship between the Kogsta Siltstone and the Ede Quartzite is interpreted as related to Hirnantian interglacial-glacial cyclicity, although limited biostratigraphic data prohibit correlation to any of the three major Hirnantian regressions. Additional biostratigraphic data and evaluation of tectonic influences will permit testing of this hypothesis.