Depositional Characteristics and Their Geological Implications of the Permo-Carboniferous Carbonate Rocks from Yingen-Ejinaqi and Their Surrounding Areas, Inner Mongolia, China

A study of depositional characteristics on carbonate rocks of Permo-Carboniferous was carried out by analyzing of petrology and their formation characteristics of carbonate rocks. A lot of fieldworks of the Permo-Carboniferous system in the study area indicate that, the carbonate rocks of Permo-Carboniferous are dominated by calcirudite, crinoidal limestone and micrite with massive structure and graded bedding, volcanic or pyroclastic rocks acting as pedestals and interbeds of the carbonate formations prevail in the area, which show the neritic paleoenvironmental settings of frequent tension-faulting, strong volcanism, steep topography and deeper water during the period of Permo-Carboniferous. The Permo-Carboniferous carbonate rock formations respectively on the pedestals of volcanic or pyroclastic rocks, granulites from braided river or fan deltas, mudstones originated from littoral zone and shelf comprise volcano-elevated carbonate rock formation (VEF), delta-elevated carbonate rock formation (DEF), coast-subsiding carbonate rock formation (CSF) and shelf-shoaling carbonate rock formation (SSF).The formations, all of which were formed by working together of region tectonics and change in sea level, show the distinct mechanism of construction. Among of them, the volcano-elevated formation, with volcanic or pyroclastic interbeds and prevailling in the area, originated from the relative sea level fall driven by volcano eruption resulting from tectonic extension, in the setting of long-term rise in sea level. The delta-elevated formation inferior to the volcano-elevated in development, with granulite interbeds, arose from the relative sea level fall driven by braided river or fan deltas progradation resulting from tectonic uplifting, in the context of long-term rise in sea level. The coast-subsiding and shelf-shoaling formations, subordinate to the former two, came from short-term sea level fall under the circumstances of regional steady tectonics. However, the depositional characteristics of the carbonate rock formations having the differences in lithology, depositional texture, depositional structure and paleo-fauna assemblage, are constrained by depositional environment. Besides, four kinds of deposition models for the four carbonate rock formations above, i.e. volcano-elevated carbonate platform (VEP), delta-elevated carbonate platform (DEP), coast/uplift-subsiding carbonate ramp (CSR) and shelf-shoaling carbonate ramp (SSR), are produced with distinct depositional characteristics respectively. The neritic shelf paleoenvironment of the age is favorable to development of the marine-originated source rocks. As a result of this, the carbonate rocks, the volcanic rocks, pyroclastic rocks, and the granulites of pedestals and interbeds, originated from braided river and fan deltas become the potential reservoirs of Permo-Carboniferous for oil and gas in the area.