Facies Architecture of a Nonmarine Half Graben: the Plio-Pleistocene Palomas Basin, southern Rio Grande Rift
Greg H. Mack and Mike R. Leeder
Several factors controlling basin architecture in marine-connected basins are also active in continental extensional basins, including episodic creation of accommodation space and changes in base level and stream gradients. However, in continental extensional basins these factors are controlled by movement on the basin-bounding fault, rather than by sea-level change. Continental extensional basins are also affected by fault linkage, catchment area growth, and changes in climate.
In the Palomas half graben of the southern Rio Grande rift, alluvial-fan and axial-fluvial sediment was deposited from Pliocene through early Pleistocene, followed by basin entrenchment. Magnetostratigraphy is available for 11 sections and a paleoclimatic record is provided by stable isotopes of pedogenic carbonate. Adjacent to the southern fault segment there were three incursions of the axial-fluvial system towards the footwall. During the earlier two (late Gilbert and late Gauss) the hanging wall-derived fans responded by depositing coarse gravel near their toes, suggesting basin tilting was the mechanism of facies shift. The youngest incursion (late Matuyama) almost eliminated the footwall fan, but cannot as yet be correlated with hanging wall-derived strata. Along the northern fa lt segment, an early Matuyama axial-fluvial incursion correlates with deposition of fine facies near the toes of hanging wall-derived fans, suggesting mechanisms other than or in addition to basin tilting were important. By late Matuyama, fans prograded from both sides of the basin, perhaps in response to increased sediment yield induced by a change to drier climate with more seasonal precipitation.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California