Controls on Focused-Flow and Dispersed-Flow Deepwater Carbonates: Miocene Agua Amarga Basin, Spain

Abstract Common models for carbonate slope or basin-margin sedimentation emphasize the contrasts between linear sediment sources and point sediment sources. Geologic mapping and high-resolution stratigraphic study of a set of Miocene outcrops, exceptional for their three-dimensional exposure and preservation of paleotopography, demonstrates that paleotopographic focus (or lack thereof) may be a dominant control on geometries and sediment character of sediment-gravity-flow deposits in line-sourced systems. In the Agua Amarga basin, SE Spain, basin topography is initially modified by subaerial exposure and erosion of volcanic basement to form paleovalley systems. After late Miocene marine inundation, topography is partially modified by infilling of high-energy, shallow-water heterozoan deposits (Units 1 and 2) that subdued topography in one area to produce a ramp-like surface, but did not significantly infill a large paleovalley located in another area. Further relative rise in sea level resulted in an extensive reefal platform that formed along a paleo-high adjacent to both features (La Rellana platform). Subsequent sediment-gravity-flow deposits shed from this reefal platform followed two pathways: down the ramp-like surface (dispersed flow), or into the large flooded paleovalley (focused flow). Deposits within each system display contrasting lithofacies architectures. These deepwater deposits (Units 3–7 in this study) consist of carbonate breccia facies (coarse facies), graded skeletal packstone facies (coarse facies), and planktonic foraminifera-rich facies (fine facies) that are interpreted as sediment-gravity-flow deposits and pelagic sediments. Relative sea-level fluctuations resulted in deepwater facies variations in the large paleovalley as a result of significant backstepping and progradation of the Rellana reefal platform on a gently sloping surface. Fining of basinal deposits and shelfward migration of the locus of deposition for sediment gravity flows is associated with sea-level rise. Coarsening of basinal deposits and basinward migration of the locus of deposition for sediment gravity flows is associated with stillstand and relative fall in sea level. Both relationships demonstrate a direct link between the locus of reefal platform growth and style of basin sedimentation. Early parts of relative rises in sea level, late parts of highstands, and falling stages led to transport of abundant coarse sediment into the basin. Focused-flow deposits were line-sourced and funneled into the axis of a large, margin-parallel paleovalley, and confined to a channel. Volume ratio of coarse facies to fine facies is estimated at 0.7. Deposits in proximal paleovalley locations display compensation geometries around positive relief on debrites, channelization, erosional truncation, lapout against paleovalley walls, and some drape as a result of lateral confinement and proximity to the sediment source. Deposits in distal paleovalley locations display increasingly tabular geometries as a result of decreasing lateral confinement and concentration of coarse-grained sediment gravity flows down depositional dip. Dispersed-flow deposits were line-sourced into the basin along a gently inclined ramp-like surface that lacked paleotopographic focus. Volume ratio of coarse facies to fine facies is estimated at 0.1. The less complex sheet-like depositional geometries and locations of these deposits result from the absence of lateral confinement and subtle changes in substrate slope. Focused-flow sediment-gravity-flow deposits that are confined by topographic elements such as channels are most likely to form sizeable accumulations of coarse facies with characteristics most appropriate for conventional oil and gas exploration. There are many paleotopographic configurations that can lead to focusing of flows, so this deepwater carbonate system may be more common than previously recognized.