The Paleozoic?-Triassic Adigrat Sandstones in Northern Ethiopia: some new sedimentological and stratigraphical considerations

The Adigrat Sandstones consist of a continental siliciclastic sedimentary unit, more than 700 m thick. It accumulated in a widespread Gondwanaland province that included the whole Horn of Africa, part of southeastern Africa and the southwestern Arabian peninsula. Although the unit could probably be related to the Karroo rifts, its age is mostly debated, for the absence of significant constraints. In literature this formation has been referred to both the late Ordovician-early Silurian and to the late Permian?-Triassic to early-middle Jurassic. Its paleogeographic setting is controversial as well: it has been framed as close both to a shallow marine realm (on the base of trace fossils) and an alluvial plain environment (on the base of microfloras and relationships with an overlying shallow marine limestone unit). In Tigray (northern Ethiopia), the Adigrat Sandstones lie unconformably on both the glacio-fluvial deposits of the Enticho Sandstones and the Edaga Arbi Glacials (Paleozoic) and, in places, directly on the Precambrian crystalline basement. The Adigrat Sandstones are capped by carbonates, evaporites and shales belonging to the Antalo Limestones Group, that marks a main Jurassic marine transgression. Other authors consider the Adigrat Sandstones as lying conformably on the Edaga Arbi Glacials, and interpret their lowermost portion, at least for the Eritrean area, as deposited in a shallow marine environment. Here we show some preliminary field data collected within a research project; the main aims of which are: a) to better constraint the stratigraphic setting of the Adigrat Sandstones in the geological/regional framework of northern Ethiopia; b) to define their sedimentological and stratigraphical architecture; c) to elaborate a consistent stratigraphic and paleogeographic model, and d) to frame this latter in the Horn of Africa evolutionary context at the beginning of the Gondwana’s break up. The above targets will be achieved through the integration of detailed field mapping, photoaerial interpretation, sedimentological, petrographical and stratigraphic investigations on key outcrops. The Adigrat Sandstones are composed of sandstones with subordinately gravels, siltstones and shales. Sandstones are siliciclastic and very mature in composition, characterized by large amounts of subrounded quartz clasts (mainly monocrystalline undulate quartz) and very subordinate feldspars (quartzarenites and minor subarkoses). Remnants of quartz overgrowths further emphasize an intense sedimentary polycyclic history. Selected outcrops in the Adigrat area show these deposits consist of large scale, channelized sandstone bodies up to 15-20 m thick. Single bodies show a well developed, fining-upward trend from the channel lag gravel to the bar top fine sand deposits. Channel lag gravels are up to 2 m thick and are made of well rounded, imbricated medium to coarse pebbles with scattered cobble-sized mudclasts. Bar deposits consist of large scale, inclined beds, which are as much as 2 m thick and consist mainly of medium- to coarse-grained sands containing a variable amount of pebbles. Sandstones are mainly cross-stratified, with set up to 2 m thick. Ripple cross-lamination is common in the upper part of the bar deposits, where 1-2 m deep cross-cut channels can occur