Journal of Sedimentary Research, 1997, v. 67, p. 310-320.
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Exceptional exposure of the Montserrat fan-delta system (Eocene) in northeastern Spain provides an excellent framework to evaluate the alluvial response to sea-level changes over two different time scales. The alluvial system contains multiple fifth-order cycles (~10,000 yr), and eight fourth-order cycles (~100,000 yr). Fifth-order cycles are characterized by long-distance shoreline migrations and, occasionally, by incised basal scour surfaces but not by changes in fluvial style, lithofacies, or channel stacking pattern.
Fourth-order cycles are composed of stacked fifth-order cycles and have non-erosional basal boundaries. Vertical sedimentation rates and channel-stacking patterns 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 contain 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 overlying middle part of fourth-order cycles, the lower regressive facies tract, differs only in that shoreline is overall regressive, and there is increased clastic 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 pronounced, and vertical aggradation rates are inferred to be reduced.
Changes in the alluvial system during fourth-order cycles are most pronounced adjacent to shoreline and die away upstream over just a few km --- indicating that the base-level signal decays away over the distance of a few backwaters (channel flow depth/slope). Higher-frequency (fifth-order) changes in relative sea level appear to produce the largest shoreline migrations, but lower-frequency (fourth-order) changes have more impact on the channel stacking architecture of the alluvial systems. Observed changes in alluvial stacking pattern may be most commonly found in tectonically active, rapidly subsiding, foreland basins because of their back-tilted geometry. We propose a model in which sediment is trapped in the proximal basin during times of rapid tectonic subsidence and attendant relative sea-level rise. Progradation occurs as erosion rates in the mountain belt increase, and rates of subsidence and relative sea-level rise diminishes. Changes in alluvial architecture 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 part of the basin and are not necessarily driven by eustatic sea-level changes.
(Below) Location of Montserrat fan-delta in northeast corner of Spain. The alluvial system was shed off of the rising Catalan Coastal Range, to the southeast towards the Ebro Basin to the northeast.
(Below) View of Montserrat from east. The source area was to the left, the seaway to the right. The downstream limit of coarse alluvial fan gravel of sequences threethrough 8 is outlined in yellow. Seaward of that are coastal plain facies and then shallow marine deposits. The photomosaic shown elsewhere on this site begins on the right side of this photo. Location of Montserrat Monestary is at base of sequence 7.
(Below) Photomosaic of the Montserrat Conglomerate looking northeast towards town of Monistrol. Fourth-order sequences 3 through 6 can be seen. Each fourth-order sequence begins with a transgressive facies tract, followed by an early regressive facies tract, and then a late regressive facies tract marked here, in the coastal plain, by progradation of fluvial sandstone sheet complexes. These are, in turn, overlain by flooding surfaces. Sequence boundaries are not unconformities but mark times of maximum regression.
(Below) Incised fifth-order sequence boundary placing nonmarine gravels on marine deposits.
(Below) Model for stratigraphic development of fourth-order sequences in the Montserrat Conglomerate. Sediment flux off of the uplifted mountain block is shown qs. The model shows basin filling for three phases: A. uplift rates exceed erosion rates and there is rapid flexural loading; B. uplift rates approximate erosion rates and there is reduced loading; C. uplift rates are less than erosion rates and there is isostatic rebound. Three lithofacies (proximal gravel, alluvial plain and marine deposits) and resultant relative sea-level curve are shown. Ribbon channel complexes in the alluvial plain sections are only diagrammatic. See original paper for discussion.
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