Depositional system and sequence stratigraphy of Permian Amb Formation, Salt Ranges, Northwest Pakistan
Keywords:
Permain; Indus Basin; Fusulinid foraminifera; Salt Range; Amb FormationAbstract
The Permian Amb Formation consists of sandstone, limestone and shale. Based on the outcrop investigations lithofacies of the Amb Formation are documented from the Ghundi, Kalawahan and Zaluch Nala sections in Salt Range. These facies constitute, a) thick bedded fusulinid rich sandy limestone facies (AMB1), b) bioturbated limy sandstone facies (AMB 2), c) thin to medium bedded diverse bioclastic limestone facies (AMB 3), d) lime mudstone to wackestone facies interbeded with organic rich clays (AMB4), e) thin to medium bedded sandstone facies (AMB 5). The facies analysis suggests that depositional architecture of the Amb Formation in the study area is similar to a siliciclastic mixed carbonate shelf platform. The presence of reef forming organisms (bryozoans, foraminifers, echinoderms, and bivalves) support this model.
Sequence stratigraphic analysis of the Amb Formation is based on the integration of the outcrop, microfacies and foraminiferal biostratigraphic information. Based on the parafusulina kaetensis biozone a second order cycle (spanning Artinskinan) is interpreted that in turn consists of composite third order cycles, depositing transgressive and regressive (T-R) systems tract. The sequence stratigraphic analysis corroborated with fusulinid biostratigraphy and outcrop data suggests that the deposition of the Amb Formation took place in Transgressive-Regressive (T-R) systems tracts. These are TST 1-4 and RST 1-3 in different study sections. Exposure of the platform preserved unconformity clues in the form of basal lags and iron crusts in the area. Relative sea level curve is deduced from the facies interpretation and compared with eustatic sea level. This comparison shows that in long term the eustatic sea level kept still while matchup in the short term sea level changes exists between the relative and eustatic sea level. The occasional mismatch in relative and eustatic sea level is attributed to the local tectonics and interplay of sediment supply in the area.
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