Tracking the isostatic and eustatic signals in an end Ordovician–early Silurian deglacial sedimentary record (Algeria – Libya border)

Abstract

To elucidate the signature of isostatic and eustatic signals during a deglaciation period in pre-Pleistocene times is made difficult because very little dating can be done, and also because glacial erosion surfaces, subaerial unconformities and subsequent regressive or transgressive marine ravinement surfaces tend to amalgamate or erode the deglacial deposits. How and in what way can the rebound be interpreted from the stratigraphic record? This study proposes to examine deglacial deposits from Late-Ordovician to Silurian outcrops at the Algeria-Libya border, in order to define the glacio-isostatic rebound and relative sea-level changes during a deglaciation period. The studied succession developed at the edge and over a positive palaeo-relief inherited from a prograding proglacial delta that forms a depocentre of glaciogenic deposits. The succession is divided into five subzones, which depend on the topography of this depocentre. Six facies associations were determined: restricted marine (Facies Association 1); tidal channels (Facies Association 2); tidal sand dunes (Facies Association 3); foreshore to upper shoreface (Facies Association 4); lower shoreface (Facies Association 5); and offshore shales (Facies Association 6). Stratigraphic correlations over the subzones support the understanding of the depositional chronology and associated sea-level changes. Deepest marine domains record a forced regression of 40 m of sea-level fall resulting from an uplift caused by a glacio-isostatic rebound that outpaces the early transgression. The rebound is interpreted to result in a multi-type surface, which is interpreted as a regressive surface of marine erosion in initially marine domains and as a subaerial unconformity surface in an initially subaerial domain. The transgressive deposits have developed above this surface, during the progressive flooding of the palaeo-relief. Sedimentology and high-resolution sequence stratigraphy allowed the delineation of a deglacial sequence and associated sea-level changes curve for the studied succession. Estimates suggest a relatively short (<10 kyr) duration for the glacio-isostatic uplift and a subsequent longer duration transgression (4 to 5 Myr).