Lateral persistency of precession-driven floodplain cycles and their relation to fluvial sandbodies

Abstract

Accurately predicting the subsurface architecture is essential in finding potential reservoirs for subsurface engineering purposes. The architecture of the deposits is determined by both autogenic (internal) and allogenic (external) controls on the fluvial system and its deposition of sediments. However, the extents to which each of the two controls have an influence and how they interact in fluvial systems, is not sufficiently known for predicting the architecture of the deposits. Allogenic astronomical cycles have a trend which can be extracted from the deposits, leaving autogenic variability. This thesis means to find the lateral and vertical persistency of externally-driven floodplain deposits and their relation to the fluvial sands. Photogrammetry panels of the Early Eocene Willwood Formation (Bighorn Basin, USA) have been interpreted, a formation in which previous studies have found a pattern matching precession, one of the astronomical cycles. The formation consists of two floodplain facies in successions, together with a sandstone channel facies. Previous researches focused on the floodplain successions stratigraphically and only limited on the sandstones; this thesis means to find the lateral and vertical persistency of the floodplain successions and how these sandstones relate to the successions. The two floodplain facies, overbank deposits and heterolitics, form successions with an average thickness of 6.9m and standard deviation of 1.3m, measured over a maximum distance of 3.0km parallel and 2.8km perpendicular to the paleoflow direction. The longer the distance along which the successions are measured, the larger the range and standard deviation of an individual succession is and the more the average thickness converges to the average thickness of 6.9m. The average thickness over the 28 successions indicates a cyclicity period of 20.9kyr, matching precession cyclicity. The sandstones are subdivided into two classes based on their thickness: minor and major. The minor sands are observed to occur in the heterolitics layer of the floodplain successions. The average major sandbody thickness is 14.0m with a standard deviation of 3.9m, based on 13 bodies. Though most being multistory, it is thicker than results from previous research in the area. Opposed to the minor sands, there is not sufficient data and information on the major sands to confirm a depositional model. For the floodplain successions steps have been made to predict their behavior; once there is sufficient data to determine the role of the sandstones, prediction of the full alluvial architecture is enabled.