Small-scale sedimentary structures and their implications in recognizing large-scale ancient tidal bedforms. Example from Dur At Talah outcrop, Late Eocene, Sirt Basin, Libya

The Dur At Talah escarpment (150 m thick and 150 km long) is exposed at the southern side of the Sirt Basin, central Libya. This outcrop exposes an Upper Eocene succession, composed by highly bioturbated fine grained sandstones to claystones at the base (New 'dam Unit; 80-100 m thick), overlain by medium grained to microconglomeratic sandstones at the top (Sarir Unit; 60 m thick). The latter is split into two subunits of nearly equal thickness: the lower Sarir subunit, composed of medium to coarse cross-bedded sandstones; and the upper Sarir subunit, composed of very coarse to microconglomeratic sandstones. The whole succession evolves from shallow marine estuarine (the New Idam Unit) to fluvial deposits (the upper Sarir subunit). The sandstone of the lower Sarir subunit, which is the focus of this article, is previously misinterpreted as being deposited in a purely fluvial environment. However, close observations revealed that the depositional environment is largely tide-influenced. It is notably marked by conspicuous subaqueous dune cross-stratifications that bear a variety of discrete, multi-scale, sedimentary structures evidencing their deposition in tidal rather than fluvial setting. Mud drapes, tidal bundles, and perpendicularly draining and oppositely climbing ripples are largely developed. Among these structures, the most diagnostic are of millimetric to centimetric scale. As a prime aim of this article, all these sedimentary structures are described, interpreted, and discussed for the first time from this outcrop. Their style of association and the quality of their preservation provide an outstanding ancient example of tide-dominated siliciclastic systems. Such structures are rarely found together in one outcrop as they are in Dur At Talah, and they provide a significant indicators in identifying ancient bedforms of tidal origin. Evidences of subtidal and intertidal depositional environments are afforded by these structures. Criteria indicative of semidiurnal regime of the tide are also presented. These criteria are especially well-preserved in the bundled foresets of the spring tides, the neap tide record is also distinguishable but it provides no discernible structures. Moreover, sedimentary features that can be used to infer a macrotidal range during the depositional time are also afforded by a combination of these structures. Above all, this study also concludes that one of the most reliable sedimentary structures for recognizing the tidal bedforms are the ripple-scale (centimetric) sedimentary structures. These are preserved inside and at the base of the cross-sets. (C) 2014 Elsevier Ltd. All rights reserved.