Late Albian-Cenomanian paleoenvironmental and paleoclimatic reconstructions in northeastern Gondwana: Palynological perceptions

Abstract

The late Albian-Cenomanian (100.5–93.9 Ma) is considered a greenhouse world without permanent ice caps and with high eustatic sea levels. The high sea-level led to the flooding of low-lying coastal plains and the formation of extensive shallow shelf seas. Depositional conditions and ecosystems at these marginal and shallow shelf settings were affected by changes in environmental conditions, such as climate and vegetational ecosystems in surrounding regions. This study conducts palynomorph and palynofacies and lithofacies analysis of fifty-nine cutting samples from the North Qarun-1x well in the Gindi Basin (North Western Desert, Egypt) to assess the evolution of depositional and paleoenvironmental conditions as well as paleo-vegetation pattern in response to changes in climate and sea level. The sediment succession comprises the upper Albian to Cenomanian upper Kharita and Bahariya formations and Abu Roash G Member. Biostratigraphic analysis of palynomorph composition led to define seven interval zones of upper Albian-Cenomanian. At this time, Egypt was inundated by the southern margin of the Tethys Ocean, which led to the deposition of thick siliciclastic and carbonate facies in a marginal marine depositional environment. Statistical cluster analysis of particulate organic matter (POM) allowed three palynofacies assemblages (PFA-1 to PFA-3) to be recognized, indicating oscillating depositional conditions that shifted between fluvio-deltaic to marginal shallow marine, proximal, and distal inner neritic shelf environments. PFA-1 comprises the highest proportions of total phytoclasts, indicating fluvio-deltaic to marginal shallow marine conditions. PFA-2 is dominated by moderate abundances of AOM and phytoclasts, characterizing a proximal inner neritic shelf environment. PFA-3 consists of a slightly higher concentration of AOM compared to phytoclasts, indicating a distal inner neritic shelf environment with diminished terrestrial input and increased abundances of marine microplankton. Organic palynomorphs are abundant in mid-Cretaceous strata of northern Egypt. Abundant records of humidity indicators, mainly fern spores, along with other gymnosperms (mainly Taxodiaceae and Araucariaceae) compared to low aridity indicators of gnetalean Elaterates and Ephedroids along with non-gnetalean Classopollis suggest predominantly warm and humid conditions. Two intervals in the lower part of the Bahariya Formation and in the middle part of the Abu Roash G Member showed moderate to high abundances of aridity indicators and thus, warm arid to semi-arid climate events during the early and late Cenomanian.