Keeping Pace: Continental (Nonmarine) Sequence Stratigraphy in a High Accommodation-Sediment Supply Regime: Hornelen Basin (Devonian), Norway

Anderson, Donna S., Dept. Geology and Geological Engineering, Colorado School of Mines, Golden, CO

Abstract

High sediment supply that keeps pace with high subsidence in a tectonically active continental basin yields a basin-fill succession that is highly cyclic, yet has few sequence stratigraphic surfaces and displays modified systems tract architecture. This setting represents a midpoint in a continuum of accommodation-to-sediment supply scenarios that affect sequence stratigraphic architecture.

In the Hornelen Basin of western Norway, the Devonian Old Red Sandstone is a thick continental basin-fill (25 km) deposited in a relatively small basin (900 sq-km area). Interfingering and alternating deposits of three facies tracts, alluvial fan, braidplain, and lacustrine, are arranged in hierarchical groups of cycles; the largest scale cycle is approximately 100 m thick. Cycles of all scales are most easily recognized by lacustrine flooding "events" that are marked by an initial flooding surface and culminate in an interval of "deepest" lake facies at turnarounds from "deepening to shallowing." At these times, the lacustrine facies tract reaches maximum extent across the basin. In the largest scale cycle, these turnarounds represent condensed intervals, but the time of turnaround is represented as rock instead of a surface. Similarly turnarounds from "shallowing to deepening" are represented by an interval of rock instead of an erosive, master sequence boundary. This turnaround is recognized as the most basinward progradation of the alluvial fan and braidplain facies tracts. At this time in the largest scale cycle, the alluvial fan and braidplain facies tracts cover the basin to maximum extents, and the lacustrine tract virtually disappears. At these stratigraphic positions in the braidplain facies tract, channel scour discontinuously reaches up to 1 m depth over 100 m laterally, instead of practically no scour at other stratigraphic positions. Similarly at locations where braided rivers lap onto alluvial-fan toes, alluvial-fan surfaces are almost completely preserved; commonly, fluvial deposits are draped over clasts on alluvial-fan tops. As sedimentation keeps pace with subsidence in this basin, few stratal discontinuities form.

The largest-scale cycles, each composed of several smaller scale cycles of alluvial fan-braidplain progradation, show architecture reminiscent of systems tracts observed in marine shoreface regimes. However, only two systems tracts are recognizable: highstand and transgressive. A lowstand systems tract does not form because little erosion and bypass accompany the time of maximum alluvial fan-braidplain progradation (the time of sequence boundary formation).