High-Frequency Carbonate Cycles of the Middle Cambrian Oolitic Shoal Sequence Set on the North China Platform: Evidence and Significance of Milankovitch Controlled Events

摘要

A series of syngenetic and petrodynamical high-frequency asymmetric meter-scale cyclic sequences of oolitic limestones and microfacies in oolitic barrier shoal systems of the Cambrian in North China are identified, By applying the Fischer Plot method of accommodation temporal analysis, 2 Ill-order, 10 IV-order, 19 V-order, 76 VI-order, 187 VU-order cycles in the Middle Cambrian oolitic shoal formations are found on the basis of meter-scale cycles and their stacking patterns. Generally, the thickest oolitic shoal formation in a complete III-order cycle is formed in long-term regression, and deposited on average 2.5-3.5 m for 0.1 Ma, and 0.5-0.7 m for 0.02-0.04 Ma. Through comparison with Milankovitch long (0.4 Ma) and short (0.04 Ma) cycles, it was found that the III-order may correlate to the Oort cycle (3-4 Ma), and the V-order and VU-order cycle to the Milankovitch long cycle and Milankovitch short cycle respectively. There are three epochs for the maximum thickness values of oolitic shoal deposits in the III-order regression, which are concordant with the IV- or V -order sea-level changes, occurring roughly from the minimum sea-level to the following rising epoch. On the contrary, oolitic shoal reaches a maximum thickness from falling sea-level to the minimum point. But the rising sea-level, especially the maximum sea flooding, usually terminates or restricts the development of oolitic shoal. Very extensive oolitic shoal environment is often formed on platforms accompanied by shoal island, back-barrier lagoon, and tidal environment. The formation of asymmetric high-frequency sea-level cycles in the Middle Cambrian oolitic shoal sequences is the control of Milankovitch events. It may provide a method for making short-term isochronous lithofacies-palaeogeographic maps and for study of detailed sedimentological evolution history through the long Milankovitch cycle (0.4 Ma) correlation.