Late Ordovician sedimentary environments, glacial cycles, and post-glacial transgression in the Taoudeni Basin, West Africa

摘要

The sedimentary record and worldwide palaeontological and isotopic data support the existence of short-lived, latest Ordovician (Hirnantian) ice sheets over western Gondwanaland. Located in the cratonic Taoudeni Basin (West Africa), the study areas are distributed across a 500-km-long profile ranging from ice-proximal to ice-distal depositional conditions (Hodh and Adrar areas, respectively; central and northwestern Mauritania). Glaciation-related but mainly non-glacial deposits form the Tichitt Group, which rests upon Cambrian-Ordovician rocks on top of a basin-wide erosional surface. The glacial record consists of depositional successions bound by unconformities of glacial origin in ice-proximal areas, or of sub-aerial origin in ice-distal areas. Facies associations reflect a variety of environments (braided streams, flood-dominated alluvial plains, delta plain to delta slope, tidal or storm-wave influenced shallow-marine settings). The upper bounding surface of the Tichitt Group generally corresponds to a wave-ravinement surface, overlain by uppermost Ordovician to lower Silurian shales. four, inter-regionally distributed units are vertically superimposed and laterally juxtaposed. A unit is a hundreds of kilometres long sedimentary body, up to 100 m thick, which is laterally discontinuous as it typically infills palaeodepressions or palaeovalleys. In ice-proximal areas, aggrading fluvial deposits are identified. Coarse-grained braided stream deposits, including glacial surfaces related to minor glacial advances, predominate upstream. Downstream, finer-grained flood-dominated fluvial deposits are identified. In ice-distal areas, thick fluvial-dominated delta sediments are deposited. Each of the four units, of climatic significance, and built under high accommodation conditions, records a recession stage of the northern Gondwana ice sheet following a major glacial advance. The overall backstepping of the glacial units characterises the large-scale depositional architecture. The fourth unit, characterised by glaciomarine deposits in ice-proximal areas, and non-glacial, bioturbated storm-dominated deposits in ice-distal areas, records the final retreat of the northern Gondwana ice sheet. The lower bounding surface of this later glacial unit marks a Late Hirnantian major transgressive surface of a Late Ordovician-Silurian relative sea-level rise starting before the glaciation.