Publication Date


Document Type


First Advisor

Morris, Robert C. (Robert Clarence), 1928-

Degree Name

M.A. (Master of Arts)

Legacy Department

Department of Geology


Geology--Peru; Geology; Stratigraphic--Cretaceous


The Copa Sombrero Group, a Mid-Cretaceous flysch succession up to 3700 meters thick, occurs within the Lancones synclinoriurm of northwestern Peru. The Group was deposited by a deep-sea fan system following subsidence due to major taphrogenic movements in an active arc-trench gap, specifically a fore-arc basin. Basin in filling was accomplished by intermingling of andesitic volcanic debris with lithic (chiefly metamorphic) sediment, resulting in volcanic, litharenite, and mixed petrofacies. Flute casts and tool marks on soles of graded turbidites show consistent northeast flow, suggesting deepening of the depositional axis in that direction. The distribution of the volcanic petrofacies also suggests derivation of sediment from a submarine volcanic center and distribution of this detritus down a major northeastward-building, deep-sea fan system. Near the volcanic center, thick debris flows deposited pebble- to boulder-size volcanic debris, whereas northeast ward finer grained volcaniclastics were deposited as depositional lobes of a deep-sea fan. Concomitantly with diminishing volcanic activity was a new sediment source area from rising horst blocks to the northwest. Interbasinal mixing of the two depositional lobes resulted in a mixed petrofacies that increased in importance by the Santonian. Paleozoic metasedimentary detritus increases upward and constitutes the entire clastic suite in the overlying Tablones Formation, another deep-sea fan.


149 pages




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