Sequence Stratigraphy of An Extensional Basin, Horneien Basin (Devonian), Western Norway
Donna S. Anderson and Timothy A. Cross
Strata of linked alluvial fan, lake and braidplain facies tracts accumulate across the differentially subsiding northern margin of the extensional Hornelen Basin during the Middle Devonian. Cycles in the alluvial fan facies tract reflect the expansion and contraction of the adjacent lake and braidplain facies tracts at three frequencies and corresponding stratigraphic scales ranging from 4 to 100 meters of thickness. The cyclic strata accumulated at such high rates that erosional surfaces are rare. In the lake and alluvial fan facies tracts, disconformities are absent; cycles at all scales are symmetrical and record sediment accumulation during full cycles of decreasing and increasing accommodation to sediment supply ratio (A/S). With an absence of true unconformities, st atigraphic sequences must be identified by turnaround positions from increasing-to-decreasing or decreasing-to-increasing A/S within conformable strata. The former occurs at the time of maximum lake expansion in the basin and the latter at the time of maximum alluvial fan and braidplain facies tract expansion.
High sediment accumulation rates in the alluvial fan facies tract correspond to high subsidence rates immediately adjacent to the faulted northern basin margin. Fluctuations in lake levels modulated the creation and destruction of accommodation space within the approximately linear and continuous subsidence background. Analysis of ten stacked megacycles (100 m thick) also shows that a differential subsidence hinge between the northern basin margin and the basin center limited the lateral expansion of the alluvial fan facies tract toward the basin center.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California