STABLE ISOTOPE CHEMOSTRATIGRAPHY OF A BIOTURBATED MIDDLE JURASSIC CARBONATE SEQUENCE, CENTRAL SAUDI ARABIA: PALEOTEMPERATURE RECONSTRUCTION, DEPOSITION ENVIRONMENT AND DIAGENETIC INTERPRETATIONS

Oxygen and carbon isotope data obtained from Middle Jurassic burrowed carbonate (Dhruma Formation) outcrops in central Saudi Arabia, are used to reconstruct paleotemperature, and assess the depositional setting and diagenetic records. The nine lithofacies identified in the studied outcrop sections, are grouped into nine depositional sequences (HFS’s). The (HFS1) representing the top of the D2 is defined by burrowed wackestone (MF1) and packstone facies (MF2). The overlying D3 (HFS’s 2 to 4), is delineated by cyclic deposition of the MF1, MF2, grainstone (MF3), and oolitic grainstone (MF4) facies. The D4 (HFS’s 6 to 9) is characterized by cyclic deposition of burrowed to laminated oolitic wackestone (MF5), mudstone (MF8) and dolomitized mudstone to wackestone (MF9) capped by either MF2, MF3 or oolitic-skeletal packstone (MF6). The δ¹³C and δ¹⁸O show variable distribution trend in the studied sequence. The δ¹³C record exhibits a dramatic negative shift at the top of the HFS 1 and continues as a long-time negative excursion in HFS’s 2 and 3. A gradual positive excursion in HFS 4, is followed by a decrease within HFS 5 interval and a positive shift in HFS 6. A sharp return to negative values at the base of HFS 7 persisted to HFS 9. The oxygen isotope values show a positive trend from HFS1 to HFS 3, overlain by a gradual negative trend from the base of HFS4 through the basal part of HFS6 while a sharp negative excursion marked the base of HFS7. Negative excursions near the tops of the regressive phases of the HFS’s are likely in response to eustatic sea-level fall, particularly at the niortense and garantiana zonal boundary (D2-D3 contact), and within the zizag zone (the D4 dolomitic mudstone to wackestone facies). The δ¹³C and δ¹⁸O bivariate plot reveals that most of the samples in the studied sequence, are within the northwest Tethyan marine isotope region while samples from the basal part of the D4, plot within the burial diagenesis zone. Negative excursions of δ¹⁸O recorded in the sequence show the effect of meteoric diagenesis. In spite of the pervasive carbonate diagenesis, the reconstructed paleotemperatures (21 to ~45^oC; mean: 29^oC) from the δ¹⁸O data indicate that the Jurassic epicontinental sea over the Arabian Plate was warm, as expected from its paleolatitude position near the equator.