FLUVIAL RESPONSE IN A SEQUENCE STRATIGRAPHIC FRAMEWORK - EXAMPLE FROMTHE MONTSERRAT FAN-DELTA, SPAIN

Exceptional exposure of the Montserrat fan-delta system (Eocene) in no rtheastern Spain provides an excellent framework to evaluate the alluv ial response to sea-level changes over two different time scales. The alluvial system contains multiple fifth-order cycles (similar to 10(4) yr) and eight fourth-order cycles (similar to 10(5) yr). Fifth order cycles are characterized by long-distance shoreline migrations and, oc casionally, by incised basal scour surfaces but not by changes in fluv ial style, lithofacies, or channel stacking pattern. Fourth-order cycl es are composed of stacked fifth-order cycles and have non-erosional b asal boundaries. Vertical sedimentation rates and channel-stacking pat terns change significantly within fourth-order cycles. The lower parts of these cycles, referred to as the transgressive facies tract, show overall shoreline transgression, and associated alluvial deposits cont ain abundant overbank materials with isolated (ribbon) channel bodies. During this time the supply of terrigenous material to the shoreline was reduced, as indicated by sediment starvation offshore. The overlyi ng middle part of fourth-order cycles, the lower regressive facies tra ct, differs only in that shoreline is overall regressive, and there is increased elastic supply to the offshore, In the upper part of these cycles, the upper regressive facies tract, channel stacking geometries become denser and more sheet like, shoreline regression is more prono unced, and vertical aggradation rates are inferred to be reduced. Chan ges in the alluvial system during fourth-order cycles are most pronoun ced adjacent to shoreline and die away upstream over just a few kilome ters-indicating that the base-level signal decays away over the distan ce of a few backwater lengths (channel flow depth/slope), Higher-frequ ency (fifth-order) changes in relative sea level appear to produce the largest shoreline migrations, but lower-frequency (fourth-order) chan ges have more impact on the channel stacking architecture of the alluv ial systems. Observed changes in alluvial stacking pattern may be most commonly found in tectonically active, rapidly subsiding, foreland ba sins because of their back-tilted geometry. We propose a model in whic h sediment is trapped in the proximal basin during times of rapid tect onic subsidence and attendant relative sea-level rise. Progradation oc curs as erosion rates in the mountain belt increase, and rates of subs idence and relative sea level rise diminish. Changes in alluvial archi tecture reflect an increase in sediment flux towards the shoreline as less sediment is trapped upstream. Hence, changes in channel-stacking pattern coincident with transgressions and regressions likely reflect the interplay between subsidence and sediment supply in the proximal p art of the basin and are not necessarily driven by eustatic sea-level changes.