Postglacial alluvial terraces and an incorporated bison skeleton, Ghostpine Creek, southern Alberta

Abstract

Ghostpine Creek near Three Hills, southern Alberta, is a tributary of the Red Deer River. It has three sets of paired alluvial terraces (T-1 to T-3) in a downstream part of the valley. The rare discovery of a largely intact skeleton of plains bison (Bison bison bison) in a T-2 point bar prompted terrace mapping, 14C dating, and interpretation of the postglacial evolution of the valley. Downvalley portions of the creek began incision into the newly drained bed of glacial Lake Drumheller probably about 13 000 BP. Localized valley deepening up to 20 m, the production of erosional benches and residual spurs, and the development of partly convex-up creek paleothalwegs occurred between about 13 000 and 7600 BP, by which time basal T-1 alluvium was beginning to accumulate. Subsequent aggradation of T-1 sediment and then degradation of about 3–4 m were followed by aggradation of T-2 alluvium. These trends had taken place by 2600 BP, when the bison died and its skeletal remains were buried in uppermost sediment of a T-2 point bar. Between 2600 BP and now, the creek incised about 5 m below the former T-2 channel position and aggraded, partly synchronously, up to 3 m of T-3–floodplain alluvium. Radiocarbon-dated alluvial terrace sequences in Alberta show generally comparable trends of rapid early creek incision followed by partially overlapping episodes of net aggradation and degradation from basin to basin. However, such episodes were not closely synchronized between basins.