干旱区生态环境知识资源中心

Facies models that adequately represent the diverse range of fine-grained fluvial systems are currently lacking from the literature. In this paper, the spectrum of these systems on the arid plains of western equatorial Pangea is explored, as well as the source and nature of the fine-grained sediments. Eight fluvial elements in the Early Permian Clear Fork Formation of north-central Texas represent channel systems up to 7m deep with coarse basal deposits, three types of lateral-accretion deposits and sandstone sheets, with laminated, disrupted and massive mudstones laid down in abandoned channels and on floodplains. The three fine-grained fluvial styles represent a continuum between two end-members: sustained lateral accretion of bedload composed of quartzose sediments and mud aggregates on point bars, and oblique accretion of suspended sediment on steep accretionary benches and banks with limited lateral migration. This spectrum is controlled, in part, by grain size and the proportion of suspended to bedload sediments. The presence of rarely documented swept ripples on exhumed accretion surfaces is attributed to rapid decline in water levels and downstream re-entry of overbank floodwaters into the channel. Rill casts, roots and disrupted mudstones low down in channel bodies indicate periods of near-dryness. Laterally extensive sheet sandstones were formed by episodic flows in broad, sandbed channels. The fluvial sediments were primarily intrabasinally sourced with extrabasinal sediments brought in during major floods from upland source areas or reworked from local storage in the basin, representing a supply limited system. The upward change in cement composition from mainly calcite and ankerite to dolomite and gypsum with minor celestine implies increasingly saline groundwater and progressive aridification, supporting Late Palaeozoic palaeoclimatic models. By integrating petrographic data with sedimentology, a plethora of information about ancient landscapes and climate is provided, allowing a fuller comparison between the Clear Fork Formation and modern dryland alluvial plains.