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Ancient Fan-Delta Systems
Domba Conglomerate, Devonian, Norway: Process and Lateral Variability in a Mass Flow-Dominated, Lacustrine Fan-Delta
The Domba Conglomerate is a wedge-shaped (up to 10 m thick) conglomeratic body composed of mass flow deposits and enveloped by sandy fluvial sediments. It represents a small-radius (~ 2 km) alluvial fan which prograded into an inferred shallow body of water, probably a floodbasin-related lake. The fan body was generated during escarpment creation in response to local syndepositional faulting of the alluvial basin floor, and the inferred lake was probably formed by consequent damming of streams and change in water table.
The fan-delta wedge comprises clast-supported and subordinate matrix-rich conglomerates, while its marginal/frontal part consists of finer-grained, well-sorted, thinly layered deposits. Clast-supported conglomerate beds (Facies A) are sheet-like, ungraded or variously graded, often have clast a-axis imbrication, and represent density-modified grain flows. Matrix-rich conglomerate and pebbly siltstone beds (Facies B) have a disorganised clast fabric but commonly show coarse-tail grading, and are the product of mixing between fan gravels and lacustrine fines, mainly as cohesive debris flows. Granule sandstones and fine conglomerates (Facies C) form well-sorted, low-angle stratified, coarsening-up succession comprising thin (2–12 cm) sediment layers which have gently inclined erosional bases, distribution inverse grading, and clast alignment parallel to flow and imbrication. These formed from liquefied-flow grain flows (partly transitional to high-concentration turbidity currents) on the subaqueous fan slope, the steeper portions of which were probably subject to progressive liquefaction and ‘gravitational winnowing’ processes.
Within the main conglomerate body, both maximum particle size and bed thickness decrease downfan (respectively 2 cm and 6 cm per 100 m horizontal distance), and ungraded/inversely graded beds are transformed to normally graded ones. Vertical accretion rather than progradation was dominant on the fan. A rising lake level and progressive inundation of the fan slope are implied by the onlapping character of the delta front sequence.
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