Heterogeneity within a sand-rieb submarine fan, Tabernas Basin, Spain S.P.J. Cossey1 and K. Kleverlaan2

1BP Exploration Inc., PO Box 4587, Houston, TX 77210, USA 2AIB 'Hoop op Welvaart', Zeeburgerdijk achter 301, Amsterdam AE 1095, The Netherlands

Location The two outcrops described here are located in the Tabernas Basin in southern Spain, c. 50 km north of Almeria.

Regional geology The two outcrops described here are in close proximity and perpendicular to one another (Fig. 23.1). They are part of a Tortonian-age sand-rich submarine fan complex within the Chozas Formation of the Tabernas Basin. The sand-rich fan complex has been mapped in great detail by Kleverlaan (1989) who named it System 1 in a time-slice reconstruction of the basin. The System 1 submarine fan is c. 6 km lang and 2 km wide with a feeder system to the north. The fan consists of a series of linear, coalesced, gravel-filled channels which feed into a sandy 'lobe' consisting of stacked, gravel to sand-filled scours. The feeder channels originate in a palaeovalley incised directly into the metamorphic rocks of the northern basin margin. Grain size gradually decreases away from this margin, with boulder conglomerates in the far north decreasing to sand and rare mudstarres in the south.

Outcrop A (*Unnamed' outcrop) (Fig. 23.2) This outcrop is exposed in a meander of the Rambla de Ia Sierra at c. latitude 37° 02' 30", longitude 2° 22' 20" and is oriented c. east-west, perpendicular to palaeocurrent direction. The outcrop is c. 250m lang and 20m high.

The lower part of the outcrop represents a mudstone-poor conglomerate sandy fan whereas the upper part represents a more mudstone-rich sequence. This vertical change occurs at a major erosional boundary on the outcrop which separates the two different styles of sedimentation. The lower unit is dominated by a graded pebbly sandstarre lithofacies, with individual units being up to 2m thick and 20-30 m wide. These units contain numerous, thin, discontinuous mudstones. The base of each bed is very deeply scoured, with erosion at the base of units and numerous dewatering and liquefaction structures providing flow conduits between units. The upper unit is dominated by fine to coarse sandstarres with individual beds being over 75 m wide. Syn-sedimentary faults cut many of the units.

On the eastern side of the outcrop, a small, vertical fault offsets the graded conglomerate beds by c. 2m. The base of the fault plane terminates in a pebbly mudstarre unit and does not appear to penetrate any older units. The pebbly mudstone unit may have been utilized as a glide plane for the block of sediment which has slid to the south (away from the viewer). This block is interpreted as a syn-sedimentary slide block and is more completely exposed in an adjacent outcrop (outcrop B), perpendicular to outcrop A.

Permeabi lity barriers/baffles

Three distinct types of features would act as permeability baffles of barriers in the context of a hydrocarbon reservoir:

1. heterolithics (thin-bedded shale/ silt and fine sand);

2. mudstone rip-up clasts and thin shale beds; 3. carbonate concretions.

The heterolithics are up to 1.5m thick and are 15-50m wide. They occur as lens-shaped bodies and may represent the abandonment stage of the channel-fill. They are sequentially laterally offset to the west in the outcrop.

The thin shale beds are c. lücm thick, between 1 and 25m wide and have a mean length of 3 m. They appear to represent drapes which have been

Atlas of Deep Water Environments: Architectural style in turbidite systems. Edited by K.T. Pickering, R.N. Hiscott, N.H. Kenyon, F. Ricci Lucchi and R.D.A. Smith. Published in 1995 by Chapman & Hall, London. ISBN 0 412 56110 7.

subsequently eroded into small remnants. Many 'nests' of mudstone rip-up d asts have been derived from these beds. The 'nests' are up to 1m thick and 10m wide and probably form elongate bodies oriented parallel to palaeoflow.

The concretions occur in two types of situations. 'Strings' of concretions between 15 and 25 m wide along which up to 50% of the rock is tightly cemented occur along grain-size boundaries between coarse and fine sediment and where there are permeability contrasts at the bases of graded units. Other concretions occur within the graded or sandy units and do not appear to localize at grain-size boundaries.

Outcrop B (Canuelo 2 outcrop) (Fig. 23.3)

Location The outcrop is adjacent and perpendicular to outcrop A and is located in the Rambla del Canuelo (a tributary of the Rambla de Ia Sierra) just to the south of outcrop A. Outcrop B is oriented parallel to palaeocurrent direction and is c. 125 m lang and 25m high.

Outcrop description The general appearance of the outcrop is a broad anticlinal feature. This is a syn-depositional fold because the first sandstone overlying the anticline thickens either side of the crest. The outcrop exposes the other dimension of the slide interpreted on the eastern end of the Unnamed outcrop. The pebbly­mudstone at the base of the outcrop is probably the same pebbly-mudstone which forms the glide-plane for the slide on both outcrops. The upthrown block of a small, normal fault on the southern end of the outcrop has been eroded prior to the deposition of the overlying sandstones. This fault is thought to be a relaxation fault formed after the southward (downslope) movement of the slide block.

The outcrop also shows a change from base to top, similar to the change observed at outcrop A. The basal part is a mudstone-poor sand-rich system with only thin discontinuous mudstones, whereas the upper part

is a more mudstarre-rich sandy system with laterally continuous heterolithic sequences.

Permeabi I ity barriers/baffles The heterolithics on this outcrop are almost all continuous across the complete outcrop, contrasting with their short dimensions in a perpendicular-to­palaeocurrent direction at outcrop A. The mean length of the thin drape mudstones is 5.2 m or almost twice the mean length measured at the Unnamed outcrop. Therefore, individual drape mudstarres and heterolithics are likely to be elongated in a parallel to palaeocurrent direction.

Reference Kleverlaan, K. 1989. Three distinctive feeder-lobe systems

within one time slice of the Tortonian Tabernas fan, SE Spain. Sedimentology, 36, 25-45.

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