ANALYSIS AND INTERPRETATION OF A TRIASSIC-AGED REEF COMPLEX, WALLOWA TERRANE, NORTHEASTERN OREGON

The Triassic-aged outcrop of Martin Bridge Formation at Summit Point (near Halfway, Oregon) is indicative of shallow carbonate deposition on the flanks of an ancient volcanic island arc system, known as the Wallowa terrane. Microfacies analysis of hand samples resulted in the identification of three depositional environments at the locality. The first, characterized by bedded marl-like limestones, reflects deposition at the interface of a near-reef shelf and a shallow subtidal slope environment. The second, characterized by bioclastic wackestones and in situ sponge, coral, algal, and spongiomorph framestones, is a central- and flank-reef complex, considered to be the result of sea-level regression. Spatially and temporally distributed patch reefs, ranging from 1 to 7 m in height and 5 to 30 m in length, constitute the framework. The third depositional environment, characterized by a coarse neomorphic micrite, suggests sedimentation during a transgressive cycle on a distal subtidal slope. Statistical analysis of the paleontology reveals that the invertebrates occur in a determined rank order of sponges, corals, and algae, similar to other Norian-aged reefs, including ones in the Alps and at Lime Peak in the Yukon.