Gable Creek Formation

The Goose Rock Conglomerate towers over the John Day River.

NPS / B. Lillie

Cretaceous Shoreline

The Goose Rock Conglomerate found within the boundary of the Sheep Rock unit is a portion of the Gable Creek Formation. This formation encompasses a wide swath of late cretaceous river deposits trending east-southeast. The conglomerate outcrop within John Day Fossil Beds, with its well-rounded river rocks stuck in a sand stone matrix, would have been near the mouth of a large river and looked similar to a modern delta, like the Nile delta in Egypt. Fossil wood and spores have been found in the formation within the park, but no other fossils to date. However, further down the geologic section there was a plesiosaur, a marine reptile, excavated from a similar unit of the Gable Creek Formation near Mitchell, Oregon. The overall formation portrays trends of high energy fluvial systems (river deposits) to marine alluvial fans (mudstones and siltstones).

The Gable Creek formation grades from a high energy fluvial system to low, which is shown in the rock record trend from large volcanoclastic material (volcanic in origin river cobbles, and other larger rocks) to softer marine alluvial fans. During the Cretaceous, Eastern Oregon was either part of a shoreline or under the ocean. The Goose Rock Conglomerate, with its imbricated (stacked in the direction of flow) and diverse well-rounded cobbles tell us that a high energy river emptied into an ocean basin near the National Monument approximately 88 million years ago.

 

Geologic Formations