中国科学院南京地质古生物研究所机构知识库

Detailed microfacies analysis and sequence stratigraphic architecture of the Bajocian- mid-Callovian Samana Suk Formation exposed in the Hazara Basin, Lesser Himalaya of Pakistan, were examined. The formation reveals deposition on a carbonate ramp under variable low to high energy environments, such as carbonate mud-flats, lagoon, barrier/shoals, and open-marine setting. The stacking pattern of facies association depicts the change in depositional style in response to sea-level fluctuations. Vertical distribution of recognized facies allows dividing the Bajocian- mid-Callovian strata into one major sequence of second-order hierarchy with superimposed four cycles of third-order genetic sequences (DS1-DS4), bounded above and below by type-one unconformity. The succession comprised of the Bajocian- early Callovian Transgressive System Tract (TST) and early-mid Callovian Highstand System Tract (HST) differentiated by maximum flooding surface. It is reasonable to compare at least Bajocian- early Callovian retrogradational cycle (TST) with long-term global eustatic reconstruction suggesting eustatic change as a primary control on regional shoreline movement. Later, the sea level tends to drop toward the middle of Callovian, forming a prograding Highstand System Tract (HST). These HST deposits record a significant retreat of shoreline toward the termination of mid-Callovian, which ultimately led to regional uniformity above the Samana Suk Formation. This cycle indicates deflection from global sea-level signatures ascribes to regional tectonics in the region. For the first time, this study signifies the importance of regional tectonics overwhelming the global eustatic record of middle Jurassic carbonates in the region.