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Ancient Fan-Delta Systems
Conglomeratic Fan-Delta Sequences, Late Carboniferous — Early Permian, Western Spitsbergen
Nine conglomeratic fan-delta front sequences are used to describe the evolution of a Carboniferous-Permian fan-delta complex in southwestern Spitsbergen. Within these sequences, three conglomerate assemblages characterize the fan-delta front deposits. The fluvial assemblage consists of texturally immature, sandy conglomerates interbedded with tightly-packed, well-sorted and matrix-poor, normally graded conglomerates. Beds are typically flat or slightly inclined, although cross-stratified sets do occur. The barrier/spit assemblage also has flat or slightly inclined strata, but is characterized by thinner, more evenly developed lamination, by ‘pebble-thick’ conglomerates, and by greater amounts of sandstone. The channel mouth assemblage is dominated by beds in which granules to pebbles line shallow troughs and grade vertically into massive or ripple cross-laminated sandstone. These beds alternate with matrix-supported disorganized units, with sandy cross-stratified units, and with inversely-to-normally graded units. This channel mouth assemblage represents high-concentration gravity flows, ranging from high-density turbidity currents to cohesive subaqueous debris flows, which occur intercalated with traction deposits of both fluvial and marine origin. Deposits of mass flows, though still not widely recognized as a common fan-delta front feature, are abundant in the studied succession.
The internal organization of individual sequences is determined by an interplay between several intrabasinal and extrabasinal controls. Although tectonism and corresponding relative sea level changes do influence fan evolution, avulsion of fluvial channels, lateral migration of barriers or shoals, aggradation of mouth bars, and modification by wave and tidal currents exert primary control on the character of the fan-delta front deposits. In the studied succession, vertical changes in the basal three sequences are explained by these primary controls superimposed on a background of constant gradual subsidence. In contrast, abrupt sea level changes or punctuated subsidence is necessary to produce the offshore facies observed at the base of the upper sequences.
We support earlier suggestions that, as in conventional deltas, fan-deltas be classified according to processes dominating the delta front. Preservation of subaqueous mouth bars suggests a subsidence and burial rate sufficiently high to protect those deposits from reworking after channel abandonment. Abundant channel mouth deposits may record a fluvially-dominated fan-delta system, but in this case, it is preferable to interpret the channel mouth deposits as fluvial breaks in the wave reworked fan-delta front. The intercalation of abundant beach, barrier, and spit deposits with fluvial strata indicates that the fan-delta system was of the fluvial-wave interaction type.
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