Change from Shallow to Deep Marine Environments Recorded within Ordovician Strata, Altun Mountains, Northwestern China

Andrew D. Hanson, Bradley D. Ritts, Da Zhou, Edward Sobel

Ordovician strata in the eastern portion of the petroliferous Tarim basin, NW China, exceed 5 km in thickness. Chinese geologists have interpreted these strata as the result of deposition within a rift or aulacogen which developed on continental crust of the Tarim block between early and medial Ordovician times: these ideas are untested at the outcrop. Ordovician outcrops along the SE margin of the Tarim basin provide a rare opportunity to study depositional facies, provenance, source rock and reservoir potential.

Recent field work conducted within the Altun Mountains, along the southeastern margin of the Tarim basin and north of the Altun Tagh fault, revealed the existence of thick-bedded, fetid, stromatoporoid-bearing limestones and thick-bedded, non-fetid, brachiopod-, bryozoa-, and coral-bearing limestones of early Ordovician age. Middle and upper Ordovician strata in the Altun Mountains consist of thin-bedded siliciclastic turbidites with Bouma sequences, thin-bedded micrites punctuated with turbiditic beds containing concave down brachiopods and oolites, and coarse sandstone to conglomeratic sequences. Sedimentary sequences are variably metamorphosed and deformed but a limited number of preserved paleocureents are directed toward the north. Clasts of predominantly gneiss and marble, and a absence of any volcaniclastic component, suggest derivation from an unknown, dissected, metamorphic terrane to the south.

These data indicate a heretofore poorly documented tectonic event and record a change from a shallow marine passive margin dominated by carbonate deposition in early Ordovician times to a deep marine active margin infilled by siliciclastic deposits in medial and late Ordovician time.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995