Early to Middle Cambrian Fluvial-Aeolian Interplay and Marine Transgression Depositional System of the Peri-Gondwana Terranes: An Example from the Amin Formation of the Haushi-Huqf Region, Oman

Abstract

The Amin Formation, the lowermost part of the Mahatta Humaid Group (Haima Supergroup), is a tight oil-producing reservoir in the interior Oman Salt Basin. It is exposed in the Huqf region of central Oman and was previously interpreted as braided fluvial, aeolian, and dry sabkha deposits. This study is an outcrop-based stratigraphic and facies analysis to furnish new insights into the Amin Formation in the Huqf region to understand the spatial and temporal distribution of the depositional facies and interpret the depositional environment. The results have shown that the formation can be stratigraphically divided into three informal members. The lowermost member is poorly exposed and comprises intervals of red to reddish-green claystones and siltstones enclosing greenish-gray, very fine-grained sandstone lenses. This member grades upward into brown, fine- to medium-grained, trough cross-bedded sandstone. The member fills relatively low topographic depression on the underlying Huqf Supergroup surface and lacks marine signatures reflecting deposition in an isolated inland freshwater lake. The middle member comprises intervals of cobble conglomerates grading upward into pebbly to granule sandstones, coarse-grained sandstones, and rarely, siltstones and claystones. The conglomerates are clast- to matrix-supported of large-scale, trough- to tabular cross-bedded, exhibiting a low relief erosional base. The interbedded pebbly to granule sandstones contain cosets of high- to low-angle trough and tabular cross-bedding with pebble lags at the bottom. The claystone, occasionally intervening in the sandstone, is yellow and parallel-laminated, enclosing lenticular-shaped bodies of siltstones. The erosional scours with lag deposits, fining upward trends, and lack of marine features indicate deposition in braided channels where the mudrocks represent overbank, floodplain, and crevasse splay. The uppermost member is fine- to very fine-grained sandstone interval with trough and tabular cross-bedding, ladder to wave ripples, and horizontal (Cruziana-like) to inclined (Psilonichnus-like) trace fossils reflecting deposition in shallow marine conditions. There are also localized transverse dunes of very fine- to fine-grained sandstones comprising cosets of medium to high-angle planar cross-bedding with current ripples reflecting their deposition as aeolian dunes. The Amin Formation can be envisaged as a complex system that is graded from an inland freshwater lake to fluvial (lower two members) and shoreface environments with subordinate coastal aeolian dunes in its upper member.