Recurring Patterns of Sea Level Changes within a Palynological-Based Sequence Stratigraphy Framework of the Middle Jurassic Organic Matter-Rich Khatatba Formation, Egypt

Abstract

The Middle Jurassic was a time of marginal to shallow shelf settings dominated by organic matter-rich deposition in northern Egypt, southern Tethys. Paleoenvironmental and sequence stratigraphic reconstructions are of paramount significance for a better understanding of basin history and related reservoirs and source rock units. For this purpose, a detailed palynomorph and palynofacies analysis of the Middle Jurassic Khatatba Formation in the Dahab-Mireir Basin, north Western Desert, was conducted. A moderately to well-preserved, moderately diverse palynomorph assemblage of spores, pollen, and dinoflagellate cysts is identified. Marker dinoflagellate cysts, including Dichadogonyaulax sellwoodii, Escharisphaeridia pocokii, Gonyaulacysta adecta, Korystocysta gochtii, Pareodinia ceratophora, and Wanaea acollaris, defined a Bajocian to Callovian age. Palynofacies analysis of the Khatatba Formation revealed two assemblages, whereby PFA-1 is the most abundant in the study well and is comprised mainly of terrestrial phytoclasts deposited nearby fluvio-deltaic sources. The infrequently reported PFA-2 consists of moderate abundances of phytoclasts and AOM, deposited in an inner shelf environment. A more reliable paleoenvironmental interpretation was indicated based on three palynomorph assemblages. The spore-dominated assemblage indicated deposition in a deltaic environment, while the mixed palynomorph assemblage revealed fluvio-deltaic to marginal shallow marine conditions. The microplankton-dominated assemblage is represented by minor samples and reflects an offshore inner shelf condition. Furthermore, the Middle Jurassic relative sea level was reconstructed based on the quantitative variations in the terrestrial/marine (T:M) ratios and abundances of spores, pollen, and dinoflagellate cysts. This led to the subdivide of the Khatatba Formation into thirteen third-order transgressive-regressive sequences, which can be correlated with the global Jurassic short-term sea level changes. Active tectonics and accelerated drifting of the Eurasian Plate during the Middle Jurassic are suggested to control basin uplift/subsidence and, therefore, recurring patterns of relative sea level change.