干旱区生态环境知识资源中心

Nonmarine volcano-sedimentary successions in the Late Neoproterozoic Hamid Basin were studied in order to examine the distinctive characteristics of accumulation during syn-eruptive and inter-eruptive periods in a depocenter associated with active volcanism and both extensional and dextral strike-slip tectonics. In particular, the syn-rift fill in this area comprises a wide range of compositions and transport and depositional processes in which lava flows coexist with pyroclastic and epiclastic deposits in the same accumulation space. Seven different accumulation units were identified in the syn-rift fill: (1) polymictic alluvial fan units, (2) fluvial braid plain units, (3) lacustrine units, (4) coherent volcanic bodies/shallow intrusion units, (5) pyroclastic fall units, (6) phreatomagmatic volcanic units, and (7) pyroclastic density current units. These deposits are organized into several stratal packages with contrasting geometries. Analysis of these units and the relationships between them provided insights into the evolution of the syn-rift sedimentary environments and permitted identification of different stages of effusive activity, explosive activity, and relative quiescence, determining syn-eruptive and inter-eruptive rock units. These units provide important clues to the distribution of, and temporal changes in, accommodation space, and hence the configuration and structural evolution of the Hamid Basin. Two accumulation stages were defined. The underfilled stage occurs when the material supplied to the depocenter during the eruptive events is not enough to level the existing topography, allowing the development of high-gradient alluvial systems during the next inter-eruptive period. The overfilled stage occurs when extensive pyroclastic density current deposits choke the accumulation space during syn-eruptive periods, causing low-gradient sedimentary systems to develop during the subsequent inter-eruptive periods. The Hamid Basin is thus interpreted to have been hybrid in nature, influenced by the dynamic changes of the basin-margin faults, which were either normal or strike-slip.