Mapping palaeoshorelines of river-dominated deltas in lacustrine ramp settings: Application of sedimentological analyses to the Triassic Yanchang Formation (Ordos Basin, China)

Abstract

In the preserved successions of clastic lacustrine systems receiving substantial sediment input from fluvial sources, the boundary between delta-plain and delta-front facies belts marks the position of ancient lacustrine shorelines. Placing and predicting the position of this boundary in nearshore lacustrine deposits of the Triassic Yanchang Formation (Ordos Basin, China) is of considerable applied significance, since sandstone deposits of this succession host numerous geological resources; yet, delta-top and delta-front deposits differ fundamentally in terms of sandstone fraction, sandbody geometries, and petrophysical properties. However, established sedimentological criteria for shoreline identification relevant to marine environments are only partially applicable to this lacustrine setting. For example, in lacustrine systems, (i) water salinity can be considerably lower, (ii) tidal processes are negligible, (iii) wave activity is often limited and of local significance (e.g., distal and lateral delta-front fringes; transgressive periods of delta-lobe reworking), (iv) seasonal fluctuations in water chemistry and biogenic productivity may be common, and (v) the rate, frequency and magnitude of oscillations in base level differ from marine counterparts. Collectively, these factors hinder the differentiation of delta-plain and delta-front deposits in the Yanchang Formation. The establishment of effective criteria with which to make such distinction is important for palaeogeographic restorations, and for resource exploration and exploitation. On the basis of detailed sedimentological descriptions of well cores and outcrop exposures of the Yanchang Formation in the southeastern Ordos Basin, sedimentary facies accumulated as part of different sub-environments and facies belts are studied. Delta-plain deposits can be readily differentiated from those of delta-front origin on the basis of criteria relating to sediment texture, sedimentary structures, palaeontological content, well-log profiles, and vertical facies successions. These criteria are summarized and applied to better characterize the deltaic setting and to reconstruct the evolution of lacustrine shorelines during accumulation of the Yanchang Formation, particularly for the northeastern area of the basin. A key outcome of this study is the development of a broadly applicable model for the recognition of sub-environments in river-dominated lacustrine deltaic successions. A generalized workflow is established to demonstrate how techniques in sedimentary facies analysis can be employed to reconstruct siliciclastic lacustrine delta shorelines, and thereby predict reservoir characteristics.