CARBONATE CHEMICAL AND PETROGRAPHIC TRENDS IN AN EDIACARAN? POST-AMALGAMATION BASIN, NW ARABIAN SHIELD

Late Precambrian history of the Arabian-Nubian shield involved closing of the Mozambique Ocean, collision and deformation to form N-S trending structures (Nabitah orogeny; 680-640 Ma), followed by later NW-trending extension and shearing to form a system of half-graben basins (so-called Najd orogeny; <640 Ma). Lithologically diverse end-Precambrian rocks of the Jibalah Group fill these fault-controlled extensional basins, including extensive carbonate deposits in lacustrine and shallow marine environments. An extensive regional unconformity (Siq) separates these late basin fill sequences from overlying Cambro-Ordovician sandstones.

We report chemical and petrographic trends in a previously unstudied Jibalah Group carbonate interval (Dhaiqa Formation) in NW Saudi Arabia of probable Ediacaran (Vendian 620-530 Ma) age. The sequence is essentially undeformed, beginning above an apparent angular unconformity separating older volcaniclastics from a thin basal shale interval and overlying ~0.5+ km carbonate pile. The carbonate interval consists largely of well-layered shallow water packstones, grainstones, and probable algal boundstones/planar stromatolites, many intervals of which are partially silicified. Distinct macroscopic organic structures (feeding traces, bioturbation?) occur in the middle of the mapped succession. Several meters of a poorly sorted pebble conglomerate abruptly bisect the carbonate sequence.

Although primary depositional textures are largely preserved, petrographic inspection shows that most intervals are partially recrystallized with a ferroan calcite component (avg Fe/Sr = 2.64). C and O isotopic compositions range from -2 to +7 and -14 to -4 per mil, respectively. These ranges are consistent with proposed Ediacaran seawater compositions (Jacobsen and Kaufmann, 1999). Sr isotopic compositions range between 0.7040 to 0.7061, values far too low for Ediacaran seawater (0.7070-0.7085). Sr concentrations are mainly 1000 to 2000 ppm. Such high values suggest minimal meteoric diagenesis. The low Sr isotopic compositions may reflect a restricted marine setting with significant hydrothermal component, diagenetic exchange with hydrothermal fluids, or deposition in a lacustrine setting with significant input from chemical weathering of mafic highlands.