Near Cerro de Pasco, Peru, thrust faulting and close folding bring an eastern facies of lower Mesozoic limestones, 9500 feet thick, close to beds on the west oe the same age and one quarter as thick. The strata are Upper Triassic and may extend into the lower Liassic. Fossils are abundant but conclusive only as to the oldest beds, of late Triassic age. Original difference in thickness of the two facies is due to abnormal accumulation in a narrow north-trending negative zone east of Cerro de Pasco, with rapidly diminishing thickness and time continuity to the west.

Tertiary strata, which have been confused with the Triassic in recent works, accumulated in low-level intermontane basins on folded and faulted Mesozoic and Paleozoic formations. Strong deformation at the end of the Cretaceous was followed, after the accumulation of Tertiary continental deposits, by thrust faulting and isoclinal folding in the early Tertiary. One member of the Tertiary sequence is a fanglomerate, study of which suggests a short period of movement between the two major ones. Minor warping is indicated toward the close of the Permian and Jurassic periods. Intrusive and extrusive igneous activity apparently closely followed early Tertiary deformation.

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