The La Paz Basin, northern Bolivian Andes : late Miocene - Pliocene continental sedimentation
Dating from the late Miocene to c.2.7Ma in the Pliocene, the La Paz Formation is a poorly consolidated basin fill, extensively incised at La Paz but with poor exposure. This study is the first to incorporate detailed vertical logging (15 sections totalling >1400m), facies analysis and sedimentary analogues into speculative reconstruction of depositional systems. The Formation records deposition from multiple, coarse-grained Piedmont distributary fluvial systems, flowing Southwest from the Cordillera Real. A highly aggradational setting preserved evidence of vertical accretion from high-magnitude, highly concentrated gravel channel belt flooding, conditioned by a seasonal semi-arid climate and high sediment flux. Combined channel belt and floodplain successions (including immature alluvial palaeosols) record variations in the style of channel belt bifurcation, in channel stability and in the timescale of channel belt coexistence, identifying mainly medial and proximal distributary zones. Large-scale alluvial architecture was heavily determined by individual system flow and sediment flux, dependent on catchment size and lithology. There was little relationship between architecture and immediate basin-margin faulting (of uncertain nature); systems maintained their basin location, and a balance persisted between accommodation space and sediment influx. These findings suggest that the systems had antecedent drainages. Footwall-sources systems were only temporarily active after faulting. A new model is proposed in which creation of accommodation space was coupled to sediment flux because both were driven by differential uplift of the Cordillera. Tectonic and climatic controls are inextricably linked in the depositional record due to this uplift. Late Miocene basin opening is uncertain but may have involved strike-slip fault activation. Subsequently, the basin fill expanded outward, onlapping proximal and distal margins, possibly under decreasing climatic extremity. These changes suggest gently declining differential uplift superimposed on ongoing absolute uplift, which may have enhanced regional climatic trends.