Abstract

Upper Triassic and Jurassic sedimentary rocks from the Wallowa terrane of northeastern Oregon and western Idaho provide a unique insight into the depositional and tectonic history of this "suspect" terrane. Within the upper 3 km of the Wallowa sedimentary section, five distinctive conglomerate intervals are interpreted to record critical stages in the evolution of the terrane.

Epiclastic conglomerates of the upper Doyle Creek Formation (upper Seven Devils Group) contain abundant clasts of coarse-grained plutonic rocks which were derived from the intrusive roots of the magmatic arc during a prolonged period of volcanic quiescence in the late Karnian and early Norian. Soon thereafter, a regionally extensive carbonate platform which was rimmed by lime sand shoals built out into the Wallowa forearc basin, as indicated by carbonate conglomerates within the Martin Bridge Limestone. Two distinctive conglomerates (Excelsior Gulch and Deadman Lake units) in the conformably overlying Hurwal Formation represent discrete sedimentary responses to Late Triassic tectonism within the Wallowa terrane. Clasts in the Excelsior Gulch conglomerate establish a provenance linkage with rocks of the Baker terrane in the early Norian, and they may be directly related to movement along shear zones which developed in the fore-arc as a response to oblique plate convergence. The coeval Deadman Lake breccia is an olistostromal megabreccia generated by tectonically induced slumping of Martin Bridge and Hurwal sediments along the shelf edge of the Wallowa fore-arc basin. The Oxfordian Coon Hollow Formation contains the youngest sedimentary rocks in the Wallowa terrane. Chert-pebble conglomerates in this unit were locally derived from an uplifted accretionary prism and deposited in a trench or lower trench-slope basin during the initial stages of terrane accretion in the early Late Jurassic.

The sedimentological data presented here establish constraints on models for the tectonic evolution and accretionary history of the Wallowa terrane and provide a unique basis for comparison and correlation with other terranes, both in eastern Oregon and elsewhere in the Cordillera. These correlations, however, must consider the stratigraphic and structural complexity of modern terranes in an accretionary tectonic setting, such as the Indonesian region. Tectonic and sedimentary processes in the Solomon Islands and Sumatra fore-arc provide modern analogues for those of the Wallowa terrane.

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