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Ancient Fan-Delta Systems
Mass-Flow Conglomerates in a Submarine Canyon: Abrioja Fan-Delta, Pliocene, Southeast Spain
In the Early - Middle Pliocene, a conglomeratic fan-delta (Abrioja Formation) prograded approximately 15 km southeastwards in a confined canyon in the Almeria Basin. The canyon was 2 - 4 km wide and 100 - 200 m deep. It probably originated from rifting of the area between the metamorphic basement blocks of the Sierra de Gador and the Sierra Alhamilla.
The Abrioja fan-delta sequence, which filled the canyon is as follows (from top to bottom): 1) Alluvial fan and coastal plain deposits with paleosols; 2) Sediments of a transition zone (beach, nearshore and delta front); 3) Upper delta slope sediments, with tangential foresets of pebbly sandstones; 4) Lower delta slope sediments, with parallel bedded conglomerates, sandstones and pebbly mudstones; 5) Pro-delta sediments, with heavily bioturbated muds.
Detailed study of the subaqueous part of the fan-delta has shown that the gravel and sand were transported mainly by sediment gravity flows. The various facies described from the subaqueous fan-delta reflect different kinds of mass flow behaviour. Slumping in the upper part of the delta slope was probably the main sediment source for the conglomeratic beds of the lower delta slope, which were deposited in troughs downslope from the slump scar. The slump scars have been filled again with sediment from the transition zone.
From several conglomerate beds, some important structures have been described: a) Asymmetric conglomerate beds show a type of back-set bedding. These ‘backsets’ are indicated by either a diffuse, centimetre thin sandy zone or by a preferred pebble alignment. They are interpreted as shear zones due to downslope transmitted compression. The compression is probably due to resedimentation at the upslope-side of the gravel body. b) The head of a structureless, polymict gravel flow deposit shows straight troughs, of ca. 50 cm in cross-section filled with pebbly sandstone, issuing from this body. c) At the base and on the top of some conglomerate beds, evenly-stratified sandstones may have originated from flow surges, rather than from (basal) shearing. The sediment for the flow surges may have been progressively winnowed by liquefaction and gravity from the margins of a gravel flow, a process which was studied from experiments described here, and has been named gravity-winnowing.
Conglomerate beds in the canyon are covered by (pebbly) mudstones which were probably deposited during periods of ‘normal’ sedimentation conditions.
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