JOURNAL OF SEDIMENTARY RESEARCH,VOL. 71, NO. 5, SEPTEMBER, 2001, P. 692–704 Copyright  2001, SEPM (Society for Sedimentary Geology) 1527-1404/01/071-692/$03.00 A NEW TYPE OF BEDFORM PRODUCED BY BACKFILLING PROCESSES IN A SUBMARINE CHANNEL, LATE MIOCENE, TABERNAS–SORBAS BASIN, SE SPAIN KEVIN T. PICKERING 1 , DAVID M. HODGSON 1 , ELLEN PLATZMAN 1 , JULIAN D. CLARK 2 , AND CLARE STEPHENS 3 1 Department of Geological Sciences, University College London, Gower Street, London, WC1E 6BT, U.K. 2 Department of Petroleum Engineering, Heriot-Watt University, Edinburgh, EH14 4AS, U.K. 3 BP-Amoco, New Orleans, U.S.A. e-mail: ucfbktp@ucl.ac.uk ABSTRACT: The Late Miocene ‘‘Solitary Channel,’’ Tabernas–Sorbas basin, SE Spain, has been interpreted as a submarine channel fed by sediment gravity flows from the east. In this paper, the channel is reinterpreted as a lower-slope erosional channel fed by sediment grav- ity flows from the west. The channel shows cobble/pebble lag deposits, including breccias, associated with erosional phases with substantial sediment bypass, and a later infill by episodes of inclined backstepping macroforms (the primary focus in this paper), mainly comprising sands, interpreted here for the first time as channel backfill deposits. These inclined sandy macroforms, typically 2–5 m in height and 30– 40 m in length, are described in detail for the first time in this paper, and are interpreted as a new large-scale sedimentary structure. We observe that the seeding process for the inclined sandy macroforms appears to have been in the upstream depression immediately behind ridges on the surface directly overlying cohesive debris-flow deposits. The internal channel architecture is interpreted in terms of fluctu- ating relative base levels. A purely local tectonic explanation for the inclined sandy macroforms is discounted because within the bed bun- dles, dips are essentially constant across the intrachannel disconfor- mities. We speculate that the most likely overall change in base level throughout the history of the channel was driven by regional tectonic change. The higher-frequency variations were probably a consequence of fluctuations in sediment supply/caliber from the source area and/or of cycles of eustatic or regional sea-level changes. The channel was abruptly overlain by about 200 m of marls and then a heterolithic sheet-like turbidite system typical of a confined basin-floor setting. This change in depositional style represents a response to a significant over- all decrease in basin-floor gradient, in which there was a differential change in base level, shown by the coeval development of a major angular unconformity farther east (Sorbas area). The channel history is important for sequence-stratigraphic modeling because it demon- strates that a backstepping fill can be caused by an overall tectonic control on the accommodation space (initiation and abandonment). Higher-frequency source-area changes in sediment flux/caliber and/or eustatic sea level probably exert a strong influence on the detailed de- positional architecture in the channel (multiple bypass–backfill events). INTRODUCTION Submarine channels are major conduits for the transfer of sediment from shallow to deep-water environments (Clark and Pickering 1996a, 1996b, references therein). The focus of this paper is the documentation of a new type of bedform in submarine channels, which we refer to as ‘‘inclined sandy macroforms’’ or meter-scale backstepping bedforms. We speculate on the cause of these structures, which we predict will be tested by detailed, ultra-high-resolution studies of modern submarine channel fills. This paper also documents the evolution of the submarine channel, which contains the inclined sandy macroforms. The channel developed in a deep- marine basin that was tectonically active during sedimentation (Kleverlaan 1987; Clark and Pickering 1996a, 1996b; Haughton 2000). The channel fill appears to have been controlled by both relatively rapid regional syntec- tonic processes and changing base levels (source-area sediment flux/caliber variations and/or eustatic sea level). The intrachannel disconformities, sep- arating bed bundles that are several meters thick, were the product of a series of base-level changes at rates of  10 5 years. Regional differential uplift or subsidence led to overall decrease in basinal gradients and drove the complete backfilling of the channel. In general, interpretations of the erosional and depositional history of such channels tend to emphasize the importance of relative (commonly eustatic) sea-level changes rather than demonstrable tectonic control (e.g., Mutti 1985; Posamentier and Vail 1988; Rasmussen 1994; cf. fig. 8 in Pickering et al. 1995). Rasmussen (1994), however, has postulated a simple model for the tectonic control of canyon aggradation, (middle Miocene, offshore Gabon), but rejected this because there was no apparent evidence for coeval regional tectonic activity. Additional arguments used by Ras- mussen (1994) were that tectonic processes would be too slow to cause the distinct unconformity associated with canyon incision, and that such pro- cesses were unlikely to be responsible for the cyclical nature of mid-Mio- cene canyon incision and infill. GEOLOGICAL SETTING OF THE CHANNEL The Tabernas–Sorbas basin is one of a series of Neogene intramontane basins resting on the Internal Zone of the Betic Cordillera, SE Spain (Figs. 1, 2), the westernmost extension of the Tertiary Alpine Belt. The origin and development of the basins is contentious. Montenat et al. (1987) be- lieved that subsidence in these basins was due to a wide NE–SW trending left-lateral shear zone, linked to the N–S African–Iberian convergence, which produced pull-apart basins. Basin-forming E–W trending dextral strike-slip faults were postulated by Sanz de Galdeano and Vera (1992) and Stapel et al. (1996). Vissers et al. (1995) suggested that, because the southern boundaries are faulted and the northern boundaries are in uncon- formable contact with metamorphic rocks of the basement, the basins prob- ably originated as half-graben. Poisson et al. (1999) interprets the Neogene basins as lateral ramp basins oriented parallel to west-verging, deep-seated thrust faults (the Sierras being the structural culmination of these thrusts). The western part of the Tabernas–Sorbas basin contains 1 km of ma- rine sediments, in an overall transgressive–regressive sequence, from con- tinental and shallow-marine through deep-marine to fan-delta deposits (Fig. 1). The upper part of the basin infill shows a regressive–transgressive pulse associated with the Mediterranean Messinian salinity crisis. The basin sed- iments range in age from? Serravallian to Pliocene (Kleverlaan 1987). There is no precise, high-resolution biostratigraphic framework for this ba- sin despite previous attempts (e.g., Kleverlaan 1989a, Poisson et al. 1999), although the deep-marine fill passes through the Tortonian–Messinian boundary (Fig. 1). This lack of precision is due to the rapid subsidence and filling of the basin, and the occasional restriction to open marine con- ditions during the basin history. Aspects of the deep marine sedimentology of the Tabernas area of the basin have been described by Kleverlaan (1987, 1989a), Cossey and Kleverlaan (1995), Cronin (1995), Clark and Pickering (1996a, 1996b, p. 148–154), and Haughton (1994, 2000). Kleverlaan (1989a, 1994) recognized three essentially coeval late Tortonian turbidite systems within the Tabernas area: System I (Sandy System), System II (Mixed System), and System III (Solitary Channel, the focus of this paper).