This thesis investigates a sequence of 17 cycles of coarse-grained syn-rift delta progradation in the Loreto basin, Baja California Sur. The Loreto basin is a Pliocene half-graben bounded by the 30 km long easterly dipping extensional Loreto fault, with 1.5 km throw. The Loreto basin uniquely provides a temporally constrained setting that couples preserved alluvial architecture with marine depositional geometries that enables these controls to be distinguished. The depositional architecture of each cycle is reconstructed by correlating 61 close-spaced (20 m to 320 m), detailed (1:5000) sections. Of the 17 progradational cycles, the lower 11 are particularly well exposed. Temporal constraints achieved through the dating of two sequence bounding tuff horizons, give an average duration for a single cycle of delta progradation to be 14±8.6 kyr., with most cycles being <10 kyr. This duration is too short to be explained by Pliocene eustatic variations (41 kyr). A consistency in measured palaeocurrents towards the NNE suggests that lobe switching was not a significant factor in controlling delta architecture. A facies framework is established that recognises both shoal-water and Gilbert-type delta deposition in the basin. The facies framework, coupled with reconstructed depositional architectures reveals that 7 of the 11 lower cycles, and at least one upper cycle (12) exhibit a palaeoseaward transition from shoal-water to Gilbert-type deltas. This nucleation of clinoforms with initially oblique, but increasingly sigmoidal geometries is characterised by a down-transport development of forests, which increase in height from 4 m to >26 m as they are traced basinward. Each cycle is capped by a shell bed that records drowning of the delta top. The geometry, and specifically the nucleation of clinoforms in single cycles of progradation requires a continued basinward increase in bathymetry. In one case this increase can be attributed to the topography of an underlying delta cycle. However for most of the cycles the increase in bathymetry can only be achieved if the rate of tectonically-controlled accommodation creation accelerates during their deposition.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:659643 |
| Date | January 2004 |
| Creators | Mortimer, Estelle J. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/11858 |
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