Origin and diagenesis of mid-Bih conglomerate/ breccia beds (Permian-Triassic) in Ras Al Khaimah, northern United Arab Emirates.

摘要

The mid-Bih conglomerate/breccia is an important interval in the Bih Formation (ageequivalent to the Permian-Triassic Khuff Formation) in Ras Al Khaimah, UAE. Whether these beds represent collapse breccias, tectonic breccias or lag deposits formed during marine transgression is still a matter of debate. Fieldwork, petrography and carbon, oxygen, Strontium isotopes, fluid inclusion analyses were adopted in this research to unravel the origin and diagenetic evolution of this interval. This study shows that transgressive process have an important impact on the formation of the conglomerate/breccia beds, it is supported by: (1) the carbonate conglomerates/breccias is a stratabound interval, deposited as one single bed or two beds interbedded with massive dolostone bed; (2) presence of sedimentary structures and rip-up clasts in conglomerate/breccia pebbles; (3) presence of subrounded and rounded pebbles implying long-term depositional processes; (4) erosional basal contact between the conglomerate/breccia and underlying bed. This study proposed two probable models to interpret the origin of the conglomerate/breccia beds: (1) marine transgression model: the conglomerate/breccia beds result from reworking by transgressive processes and followed sea water dolomitization; (2) collapse-transgression model: anhydrite beds was dissolved and leads to the collapse of overlying dolostone beds in tidal flat environment. The collapse dolostone breccia was reworked by marine currents during transgressive processes and form conglomerate/breccia beds containing dolostone pebbles with variable rounding and size.;Optical microscopy, cathodoluminescence microscopy (CL), Backscattered Electron Imaging (BSEI) with Energy-dispersive X-ray spectroscopy (EDS) shows that the pebbles of conglomerate/breccia interval can be grouped into three categories: (1) finecrystalline dolostone pebbles; (2) dolopackstone/bioclastic pebbles; (3) lime mudstone pebbles. Main diagenetic processes of the conglomerate/breccia interval include formation of moldic porosity by dissolution of allochems, calcite cementation, dolomitization, recrystallization, chemical compaction, silicification and calcitization. Dolomitization of pebbles occurred in marine environment because the delta13CVPDB values and delta 13OVPDB of dolostone pebbles are within the C and O isotopic range of Permian to Triassic marine carbonates. Stable isotopes values of replacive mosaic dolomite between pebbles are also within the isotopic range of Permian to Triassic marine carbonates. The negative delta18O VPDB of drusy dolomite between pebbles implying this process the dolomitization of precursor drusy calcite cement took place during burial process. Microthermometric analyses on fluid inclusions showing fracture-filling drusy dolomite precipitated from diagenetic fluids with increasing temperature. Wide distribution of microthermometry of measured fluid inclusions indicates that some of the inclusions have been subjected to re-equilibration. Precipitation of fracture-filling drusy dolomite and calcite with high Th and salinity fluid inclusion is due to influx of hydrothermal fluids.