Sedimentology and regional implications of fluvial quartzose sandstones of the Lee Formation, central Appalachian Basin.

摘要

Detailed sedimentological analyses, utilizing lateral profiling, demonstrate bedload-dominated fluvial deposition for sandstones of the Lee Formation. Internal elements of the sandstones consist primarily of truncated channel-fill sequences, up to 20 meters thick. The principal component of channels were downstream-accreting macroforms; channel elements often contain more than one macroform element. Reconstruction of macroforms reveals accretion primarily by superposed bedforms that migrated down a low-angle front. Macroforms episodically developed a steep slipface as indicted by giant-foreset facies. Minor components within channel deposits include lateral-accretion, minor-channel, sandy-bedform, gravity-flow, and channel-bottom elements. In general, deposition was during relatively high stage; little evidence of low-stage flow was recognized. Subordinate fine-grained facies are interpreted as levee and overbank deposits.; Unimodal paleocurrents, paucity of low-stage facies and few lateral-accretion features indicate deposition in a single-channel, low-sinuousity, system. Fluvial architecture similar to that of the Lee Formation has been previously explained by deposition in multi-channel, braided-river systems. However, the internal architecture of sandstone members is also consistent with a single-channel origin. Individual channels were temporarily confined, during which time passage of several macroforms aggraded the channel. Position of the channel in the alluvial plain was controlled by avulsion. Calculations reveal that a single-channel system may account for the sheet-like sandstone bodies of the Lee Formation. Spatial arrangement of sandstone members was controlled by tectonic processes. Episodic thrust-loading in the orogenic belt to the east and subsequent flexure of the crust in the foreland basin caused a step-wise progression of the river system towards the west.; Source area and climate were primary controls on the mature composition of the Lee Formation. Source areas, composed primarily of sedimentary rocks, were located chiefly to the northeast/north. A east/southeast source area supplied subordinate and low-grade metamorphic rock fragments. Intense weathering, associated with humid tropical climates, acted upon detritus throughout sedimentation. Lesser controls on composition were tectonics and transport/depositional processes that extended exposure of sediments to weathering. Quartzose sandstones of the Lee Formation reflect lower rates of tectonic subsidence and greater recycling of sand-sized grains during transportation and temporary deposition on the alluvial plain, relative to lithic time equivalents to the east.