Geochemical Imprints for Provenance and Tectonic Settings of the Late Ordovician Glaciogenic Deposits in the Rub’ Al Khali Basin, Saudi Arabia

Abstract

Ancient glaciogenic deposits have lately become a subject of significant research interests due to their potentials as unconventional resources (tight gas sand reservoirs). This study uses petrographical (thin-section and QEMSCAN) and geochemical data (XRF and ICP-MS) to investigate the provenance and deduce the tectonic environments under which the sediments of the Late Ordovician Sarah Formation in the Rub’ al Khali Basin were deposited. The sedimentological description of the core samples used for the study resulted in several lithofacies which have been grouped into four facies associations (FAs) including the fluvial (FA1), glaciolacustrine delta (FA2), subglacial (FA3) and glaciofluvial (FA4). The quartz-feldspar-lithic (QFL) plots show that the FA1 is comprised of subarkose, FA2 is quartz and sublithic arenites, while FA3 and FA4 are mostly sublithic arenites. In contrast, the log cross plot of Fe2O3/K2O and SiO2/Al2O3 clustered the samples of FA1 and FA4 as dominantly sublithic arenite and subarkose. While those of FA3 samples are classified as arkose to subarkose. FA2 samples are scattered in the geochemical classification. QFL also indicates that the FA1 and a part of FA2 were derived from a craton interior while the other FAs originated from recycled orogens. The high-SiO2 concentration (average: 81%) and cross plots of Th/Co and La/Sc ratios indicate that the sediments of all the FAs were derived from felsic rocks. The averages of the chemical index of alteration (CIA; >60) and chemical index of weathering (CIW; >95) in the subglacial facies association (FA3) are the highest values reflecting the highest amount of illite clay mineral in the matrix of the diamictites. The CIA averages are almost the same in the other FAs which show a low content of clay minerals. Discrimination function (DF) diagrams of high-silica contents samples in all FAs indicate that they sediment were formed in a continental rift and partly from collision tectonic environments, while a few samples from FA2 and FA3 characterized by low-silica content were formed in a collision tectonic setting. Multidimensional DF diagrams of major elements show that most of the FAs were located in a passive margin. With varied facies types of siliciclastic successions in the Rub al Khali Basin, the geochemical approach followed in this study might help in stratigraphic correlations and prediction of paleoenvironments and potential reservoir targets in the subsurface.

AAPG Datapages/Search and Discovery Article #90360 © 2019 AAPG Middle East Region Geoscience Technology Workshop, 3rd Edition of the Siliciclastic Reservoirs of the Middle East, Muscat, Oman, October 28-30, 2019