Deep-Marine to Shelf-Margin Deltaic Sedimentation, Silurian Succession, Saudi Arabia

Abstract

A thick Silurian succession, up to 5,000 feet, fills a sag basin in eastern Saudi Arabia. It consists of deep-marine, shelf-margin to inner-shelf deltaic and fluvial deposits. The deep-marine and shelf-margin deposits are called, respectively, the Lower and the Mid-Qusaiba Sand. This study uses subsurface cores and seismic data to investigate the relation between the deep-marine and the shelf-margin deltaic sedimentation. Core logging allowed the recognition of turbidite, hybrid turbidite-debrite, hyperpycnite, mass-transport and mouth bar facies. The turbidite facies were deposited, from high- and low-concentration density flows, in deep-basin and slope fans. The hybrid facies were deposited, from transitional turbulent to laminar flows, on the fringes of the fan lobes. They consist of banded and homogeneous muddy sandstones containing floating mud clasts and sheared sand injections. The hyperpycnites were deposited, from sustained and fluctuating river-born flows, in both the deep-marine fans and shelf-margin deltas. They vary spatially in scale and stratification. Distal hyperpycnites are thin- to medium-bedded, centimeter- to decimeter-scale, and display well-developed inverse to normal grading. Medial hyperpycnites can be very thick-bedded, meter-scale, and exhibit alternating structureless and laminated/rippled divisions. Proximal hyperpycnites have internal erosional surfaces and may lack the lower inverse graded division. They are commonly hosted in slope channels, where they intercalate with trough cross-stratified sandstones, indicating alternating suspension fallout; sediment bypass; traction current reworking and bedload transport. The mass-transport deposits interfinger with slope shales and include distal debrites and proximal slumps. The mouth bar facies comprise current-rippled and laminated sandstones that record buoyancy processes in shelf margin deltas. Seismic data depict low-angle sigmoidal and relatively high-angle dipping reflectors that represent slope clinoforms. The former are associated with an ascending shelf-edge trajectory and are related to slope progradation during rising sea-level conditions. These provided accommodation for the development of the shelf-margin deltas. The latter coincide with a horizontal to descending shelf-edge trajectory that reflects slope degradation during falling sea-level conditions. These induced failure of the shelf-margin deltas, sediment bypass via slope channels and deep-basin sedimentation.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015