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Mixed Carbonate-Siliciclastic Sequence Development and Chemostratigraphy On a Distal Foreland During Miocene Glaciation, Eastern Saudi Arabia
Alkhaldi, Fawwaz Muhammad
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The Miocene of the Lidam area, Eastern Province, Saudi Arabia, was studied to examine the interaction of glacio-esustasy during moderate Antarctic glaciation, within a small back bulge basin on the slowly subsiding distal Arabian foreland, distal from the active Zagros fold-thrust belt. Low subsidence rates of 1 to 4 cm/k.y generated the long-term accommodation, which were considerable slower than those in the proximal foredeep in Iran. Deposition of the siliciclastics was driven by lowered sea levels, and moderately humid to arid climate. Rising sea levels pushed the siliciclastics updip allowing mixed siliciclastics and carbonates to form downdip, under semi-arid climate and locally hypersaline conditions. Maximum transgression slightly predated the Middle Miocene climatic optimum when prograding siliciclastics migrated across the platform. Falling triggered siliciclastic deposition under semi-arid climate. Sequences appear to relate to long-term obliquity (~1.2 m.y. cycles) and long-term eccentricity (400 k.y.) cycles. The succession contains numerous missing beats reflecting the updip position of the study area, and sea level changes of tens of meters that frequently exposed the platform. Siliciclastic units commonly are incised into muddy sediments beneath sequence boundaries. Multiple exposure surfaces occur within Hadrukh brecciated palustrine carbonates. Within Dam carbonates, parasequence boundaries commonly are capped by tidal flat laminites (some of which are incipiently brecciated). High frequency negative excursions of ∂¹³C within the succession appear to relate to near-surface diagenesis by soil gas depleted in ∂¹³C beneath sequence boundaries. Positive C isotope excursions in the Lidam Miocene section can be tied to similar excursions in Qatar and UAE, where Sr isotope dates constrain the ages of the units. The overall C isotope profile at Lidam shows depleted values early in the Miocene to heavy values in the Middle Miocene, becoming lighter again in the late Miocene. The profile appears to follow the long-term global ∂¹³C curve. Incursion of meteoric groundwaters into the study area was driven by the long-term global sea level changes. Oxygen isotopes are surprisingly light, extending down to -12.5 ‰VPDB. The very light δ¹⁸O values of the meteoric waters may be explained by rainfall associated with enhanced Miocene Indian monsoons, and with far travelled air mass trajectories migrating across north Africa and from the polar region.
- Doctoral Dissertations