Large-Scale Cycle Architecture in Continental Strata, Hornelen Basin (Devonian), Norway

ABSTRACT Nine large-scale stratigraphic cycles, each about 100 m thick, along the northern margin of the Hornelen basin, western Norway, record systematic expansions and contractions of alluvial-fan, braidplain, and lake facies tracts. The braidplain facies tract occupies the basin center, from time to time expanding toward the margins, and constitutes the deposits of axial or longitudinal drainage from an eastern source. The alluvial-fan facies tract, which comprises the deposits of high-gradient transverse drainages, occupies a narrow belt close to the fault-bounded basin margin. The lake facies tract is between the alluvial-fan and braidplain facies tracts. Physical correlation of strata shows that the large-scale cycles are symmetric in all facies tracts. The symmetry of the large-scale cycles, in which the base-level rise and fall hemicycles of each cycle are of approximately the same thickness, indicates that sediment accumulated in all environments during the entire base-level cycle. Absence of regional unconformities (sequence boundaries) also indicates approximately continuous sediment accumulation in the basin during base-level cycles. The base-level fall-to-rise turnaround is represented by an interval of strata, rather than by a sequence boundary, at the maximum expansion of the alluvial-fan facies tract. Condensed sections are also absent. They are represented mainly by intervals of lake strata at base-level rise-to-fall turnarounds. Strata in the base-level fall hemicycle (decreasing accommodation) accumulated as the braidplain and alluvial-fan facies tracts expanded from the basin center and the northern basin margin, respectively. Expansions of these two facies tracts filled the basin, while the lake facies tract contracted. Strata in the base-level rise hemicycle accumulated as the alluvial-fan and braidplain facies tracts contracted sourceward, respectively toward the northern and eastern basin margins, during times of increasing accommodation. This sourceward retreat of the alluvial-fan and braidplain facies tracts was coincident with expansion of the lake facies tract. At the rise-to-fall turnarounds, alluvial-fan and braidplain sediments are stored near their respective sources toward the basin margins, and the centers of mass of these facies tracts are at basin-margin positions. During base-level fall time, the centers of mass of the alluvial-fan and braidplain facies tracts migrate basinward. The alluvial-fan and braidplain facies tracts usually expand toward each other simultaneously, and then move away from each other simultaneously. This in-phase relationship is replaced by a reciprocal stacking pattern when the two facies tracts abut each other near base-level fall-to-rise turnarounds. When the two facies tracts are in contact, the higher-gradient alluvial-fan facies tract initially blocks expansion of the braidplain. As the alluvial-fan facies tract retreats toward the basin margin and fan-margin gradients are reduced, however, the braidplain expands across former fan margins. In the reciprocal configuration, the two facies tracts move in tandem in the same direction. This reciprocal pattern plus changes in aggradation-to-progradation ratio of the alluvial-fan facies tract suggest that alluvial-fan morphology changes during base-level cycles. Large-scale cycle stacking patterns show significant changes in basin-fill architecture through time, including syndepositional structural changes accompanied by changes in stratal geometries, a change from ephemeral to permanent lakes, permanent increase in alluvial-fan gradients, and permanent reduction of alluvial-fan radii and volume at the northern basin margin. Consideration of these changes and the largely in-phase relationships between facies tracts sourced by two separate sediment supplies suggest that an interplay of climate, self-regulatory, and tectonic factors controlled sedimentary accumulation within the Hornelen basin.