Date of Award:

2006

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

Bradley D. Ritts

Abstract

The Ordos basin rests upon the North China Block and is one of the largest sedimentary basins in north China, with more than 15 km of Phanerozoic strata. Published estimates suggest that over 2000 m of carbonates and lesser amounts of siliciclastics were deposited on the North China Carbonate Platform (NCCP) from the Lower Can1brian through the Middle Ordovician. However, lower Paleozoic facies successions and deposystems of northwestern Ordos basin remain poorly represented in western literature. This paper constrains depositional environments, lithologies, facies relationships and sea-level history of the northwestern part of the North China block (NCB) in an effort to further document the Early Paleozoic geologic history of western Ordos basin. New stratigraphic data come largely from measured sections in the Zhuozi Shan and Helan Shan in northwest Ordos basin. Strata in the mountains of northwestern Ordos are divided into eleven lithofacies assemblages, distinguished by lithology, stacking patterns and sedimentary structures. Lithofacies assemblages in northwest Ordos are grouped into four lithostratigraphic units that make up the composite type section. Unit A is dominantly composed of shale and mudrock lithofacies, Unit B is dominantly composed of thin-bedded lime mudstone and banded and bioturbated lime mudstone to wackestone, Unit C is dominantly composed of quartz sandstone and dolostone and Unit D is dominantly composed of fossiliferous packstone. These four units were observed in all lower Paleozoic sections of the Helan Shan and Zhuozi Shan. Deposition of Middle Cambrian through lowermost Lower Ordovician strata in northwest Ordos basin occurred on a storm-influenced, mixed siliciclastic and carbonate, shallow-water ramp. Lateral trends in quartz sandstone, paleokarsts, thrombolites and section thickness suggest that accommodation space increased to the south. The depositional architecture changed in the Middle Ordovician to a carbonate shelf environment. The sea-level history of northwestern Ordos shows transgression through the Late Can1brian, regression in the Early Ordovician, followed by a Middle Ordovician transgression, corresponding with North American sea level signatures. Lower Paleozoic sections in northwestern Ordos basin are broadly similar to those previously described in western literature. However, based upon stratigraphic data, shoreline trends of the NCCP model are proved inapplicable to northwestern Ordos. There is no evidence for lower Paleozoic tectonics such as aulacogen-controlled subsidence and platform tilting as described by previous workers. The sea-level history interpreted for northwest Ordos basin is more similar to North American curves than to the North China Carbonate Platform model, suggesting a eustatic control on lithofacies stacking patterns in northwest Ordos basin.

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