The Late Ordovician glacio-eustatic record from a high-latitude storm-dominated shelf succession: The Bou Ingarf section (Anti-Atlas, Southern Morocco)
Résumé
Evidences of glaciation at the end of the Ordovician are widespread in western Gondwana. Some authors consider the glaciation was restricted to the Hirnantian time, but occurrences of glacial deposits in the Lower Silurian strata of South America indicate that the Gondwana glaciers did not completely disappear after the Hirnantian glaciation. In addition, numerous studies based on palaeoecology, sedimentology, sequence stratigraphy and stable isotopes have suggested that ice sheets formed as soon as the Early/Middle Katian. The objective of this work is to investigate the high-frequency eustatic signals in a stratigraphic succession corresponding to the Katian and the Hirnantian (not, vert, similar 10 My comprising the uppermost part of the Ordovician). The studied section (Bou Ingarf section > 600 m) is located in the Central Anti-Atlas, southern Morocco. It shows an almost continuous succession of siliciclastic platform deposits. The lower package, Katian to early Hirnantian in age, corresponds to a shelf succession, within which storm dynamics prevailed. The upper package, middle to upper Hirnantian in age, comprises a glaciation-related succession made up of a suite of coastal to flood-dominated fluvio-glacial deposits. The latter have essentially filled in large palaeochannels interpreted as subglacial tunnel valleys. From high-resolution facies and sequential analysis, a curve of variation of depositional environments was established at very high, high and low frequencies. Time calibration was performed based on a high-quality biostratigraphic control mainly derived from chitinozoan biozones through the whole succession. Assuming a constant tectonic subsidence and a bathymetric model (shoreface/upper offshore boundary: 30 m; upper offshore/lower offshore boundary: 120 m), changes in facies-based water depths are converted into an eustatic sea-level curve using a one-dimensional backstripping procedure. The eustatic sea-level curve shows that the stratigraphic succession is dominated by the stacking of sequences at very high- (< 400 ka), high- (400 ka) and low- (“3rd order”) frequencies that reflect eustatic variations controlled by ice-cap to ice-sheet growths throughout the Katian and Hirnantian. Three episodes of abrupt sea-level falls (> 40 m) and coeval forced regressions were identified during the Katian. They represent glacial episodes of significant extent prior to the Hirnantian, but essentially lacking a glacial record. Strata reflecting the Hirnantian glaciation include two differentiated events and an intermediate but important transgression. The second Hirnantian glacial event, the only one associated with subglacial erosion in the Bou Ingarf area, corresponds to the Late Ordovician glacial climax characterised by a continental-scale ice sheet. Ice sheets permanently occupying the centre of the Gondwana landmass throughout the Late Ordovician may reconcile moderate Hirnantian eustatic sea-level fall amplitude (40–80 m) and palaeoglacial reconstructions that show a Hirnantian ice sheet covering the main part of western Gondwana.