Fades and cyclicity within the Oligocene-Early Miocene Panjgur Formation, Khojak-Panjgur Submarine Fan Complex, south-west Makran, Pakistan

摘要

The Makran Accretionary Belt covers vast areas of the south-western Makran, Pakistan and extends west wards into coastal Iran. In Pakistan this belt is dominated by an overall regressive succession comprising the Siahan/Hoshab shale (a valley-forming claystone/siltstone-dominant facies) and the ridge-forming, sandstone-dominant facies of the Panjgur Formation, a very thick cyclic succession of Oligocene-Early Miocene turbidites. These formations form part of the "Khojak-Panjgur Submarine Fan Complex". The Panjgur Formation comprises nine distinct facies, which are organized into seven facies associations distinguished by their constituent facies, vertical organizations and lateral geometries. Facies associa tions include: (1) mid-fan channels, (2) overbank/levee, (3) crevasse-splays, (4) inter-channel, (5) outer-fan-lobes, (6) fan-fringe, and (7) trench floor/basin-plain. The formation displays a hierarchy of 1st, 2nd and 3rd order sedimentary cycles. The 1st order cycles range in thickness from 60 m to 450 m and conceptually correspond to "sequences" or "depositional sequences" formed mainly in response to sea-level fluctuations. Their boundaries are characterized by gradual or abrupt replacement of clay-stone/siltstone facies of the basin-plain by thin- to thick-bedded mid- to outer-fan turbidites, represent ing a basinward shift of the facies. The 1st order cycles generally start with thick- to very thick-bedded sandstones of channelized mid-fan facies, passing upwards into a middle part consisting of thin- to thick-bedded sandstone and claystone facies of the outer-fan-lobes and capped by claystone/siltstone-dominant facies of the basin-plain association. These three parts of the 1st order cycles correspond to the lowstand systems tracts (LST), transgressive systems tracts (TST) and highstand systems tract (HST) of the full depositional sequence. The 2nd order cycles, which are generally thinning-up and thick-ening-up cycles, range in thickness from 4 to 65 m, and occur both as isolated and composite packages, are characterized by mid-fan channelized and outer-fan-lobe associations, respectively. We interpret the 2nd order cycles as parasequences, which formed in response to marine flooding events of relatively short duration. The 3rd order cycles are very small-scale (1-4 m thick), mostly thickening-up cycles, which generally represent compensation cycles of the outer-fan-lobes. Sedimentary logs from the measured sections display significant differences in the character of facies, facies associations and cyclicity style but exhibit sufficient similarities to demonstrate that all these sequences formed parts of the same very large submarine fan complex. The succession has been strongly deformed, as attested by the prevalence of asymmetrical/overturned folding, thrusting and imbrication, resulting in great thickening due to tectonic duplication, as well as disruption of the cycles.