Astronomically forced sequence development of terrestrial massive sand-sheet, inland sabkha and palustrine units, Lower Miocene Hadrukh Formation, eastern Saudi Arabia

摘要

The Lower Miocene (lower Aquitanian) Hadrukh Formation (30-65 m thick), Al-Lidam area, Eastern Province, Saudi Arabia, was cored to examine sequence development within a non-marine siliciclastic-prone coastal plain in a semi-arid distal foreland setting. Facies are dominated by structureless quartz sands (terrestrial sand-sheets/shrub-coppice dunes or nebkhas), along with gypsiferous/calcareous quartz sands and gypsites (inland sabkhas), brecciated and sheet-cracked sandy carbonates/calcareous sands (palustrine), and some coarse lithic-clast quartz sands (fluvial) and associated paleosol-capped red and green-gray mudrock (interfluves). However, they lack the intercalated eolian laminated and cross-bedded sands of this hyper-arid region today, reflecting semi-arid Early Miocene conditions.Spectral analysis of core gamma-ray logs shows astronomical forcing within the eccentricity, obliquity and precessional bands. Tuning indicates the Hadrukh Formation is a composite sequence (similar to 1.3 myr duration) containing three (400 kyr) major sequences and eight minor sequences. Fluvial facies are locally developed at the base and near the top of the composite sequence. Massive sand sheet facies dominate the sequences, and sandy evaporitic facies and palustrine sandy carbonates are best developed in upper parts of sequences but interfinger within short distances with other facies. Subsidence and differential warping caused thickness and facies variation but the succession shows a strong astronomical signal. Astronomical forcing controlled semi-arid vs. more humid phases of deposition, as well as the position of base level, elevation of the ground-water table and presumably density of shrub cover. The three major sequence boundaries developed during 400 kyr eccentricity minima and cooling, while minor sequence boundaries formed during short eccentricity (100 kyr) cooling events.