Paleomagnetic Isochrons, Unsteadiness, and Uniformity of Sedimentation in Miocene Intermontane Basin Sediments at Salla, Eastern Andean Cordillera, Bolivia

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Abstract
Fine-scaled stratigraphic data are combined with chronologic information provided by paleomagnetic stratigraphy to document and analyze chronostratigraphic variability of the Salla Beds, a sequence of distal floodplain and lacustrine intermontane basin sediments within the eastern Andean cordillera of Bolivia. Unsdteadiness in sediment accumulation is identified using a conceptual model of episodic accumulation in which simulated accumulation histories are constructed using estimated rates for various fluvial facies and taking into account gaps in deposition and possible effects of erosion. Variability in the Salla Beds is compared to that in sequence of Miocene fluvial sediments in northern Pakistan to evaluate controls on spatial and temporal stratigraphic variability. Sedimentation in the Salla sequence was generally steady over short time scales, but over longer intervals infrequent carbonates, associated with unusually long hiatuses, served to reduce the net sediment accumulation rate. In the Chinji sequence of Pakistan, unsteady accumulation evolved into a pattern of steadier sediment accumulation as recurring cycles of unsteadiness were superimposed on the overall sequence. The controls on sediment accumulation over time in these two sequences may be similarly related to the presence and extent of atypical and infrequent events of deposition, non-deposition, or erosion that exert a significant effect on net sediment accumulation. The discrepancy of short-term episodic and long-term net stratigraphic accumulation rates in both these sequences suggests that the decrease in net sediment accumulation rate with increasing time reflects the superimposed effects of different levels of local geomorphic and extrabasinal controls.