Fluvial Response To Cyclic Changes in Relative Sea Level and Sediment Supply: The Montserrat Conglomerate of Northeastern Spain
Beverly A. Burns, Paul Heller, Chris Paola, and Marzo
Rapid subsidence rates, small basin size, and subsequent uplift and dissection make the Montserrat alluvial system, of mid-Tertiary age in northeastern Spain, an ideal place to evaluate fluvial response to shoreline migration and relative sea-level change. Strong asymmetric subsidence provided a major control on the development of stratigraphic sequences and a1luvial architecture over different time scales.
Stratigraphic analysis of the Montserrat system identified three orders of sequences ranging from fifth-order cycles (104 yrs) to third-order cycles (>106 yrs). Fourth-order cycles (105 yrs) are composed of stacked fifth-order cycles. The fifth-order cycles contain large shoreline migrations, occasionally show incised (type 1) sequence boundaries but show no systematic changes in fluvial style, channel geometry, or channel-belt stacking patterns. The fourth-order cycles have nonerosional (type 2) sequence boundaries, strongly partitioned sedimentation between the proximal and distal part of the alluvial basin, and display well developed changes in channel belt stacking patterns. Alluvial architecture changes from ribbon channel bodies during the transgressive and early regressive phases to more sheet-like geometries during the late regressive phase.
We interpret that rapid back-tilted subsidence acts to accelerate the transgression by forcing sea level rise and by decreasing the sediment flux available to the shoreline. The converse is true for regression. Thus well developed changes in alluvial stacking patterns, may be most typically found in tectonically active, rapidly subsiding, basins with a back-tilted geometry.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California