Characteristics of Sequence Stratigraphy and Plaeoenvironmental Evolution of Lower Cambrian strata in Hunan Province

Abstract：Lower Cambrian strata are well developed in Hunan. With the main sedimentary formations of black shale and carbonates in western and central Hunan and a suite of greywacke and carbonaceous slate with different thicknesses of rhythmic layers in southern area, the Lower Cambrian in the study area constitutes a second-order sequence, which can be further divided into six third-order sequences, corresponding to six orderly transgression-regression cycles. Impacted by palaeosedimentary environment, the depositional system tracts of the third-order sequences are formed by the vertical sequences of “CS + HST” and “TST (transgression system tract) + CS + HST”, with the major sequence boundaries being II type sequence boundary and the drowning-type sequence boundary. The Lower Cambrian sedimentary formation resulted from continental slope-ramp in the northwestern and central parts of Hunan; from northwest to southeast, with the gradual deepening of the sedimentary environment, the total thickness of the Lower Cambrian sedimentary formation becomes thinned. In the southern Hunan, in contrast, a suite of greywacke and carbonaceous slate of abyssal facies was deposited and becomes thickened remarkably. The sequence-stratigraphic framework records the complex palaeoenvironment evolution of passive margin in the Early Cambrian in Hunan and also reflects the restriction of the metallogenesis from palaeoenvironment evolution.