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Research School of Geology and Geophysical Sciences, Birkeck College and University College London, Gower Street, London WC1E 6BT, UK
The Rough Rock is a coarse-grained fluvial sheet sandstone which outcrops across northern and central England. It is interpreted as a braided river sandstone on the basis of sedimentary structures, internal erosion and growth surfaces. This study reveals that the Rough Rock represents more than a single phase of deposition and that internal erosion surfaces may indicate anabranch avulsions or more significant river avulsions. Detailed outcrop profiles are presented to try and distinguish the styles of deposition and the nature of internal bounding surfaces and channel geometry. The Rough Rock Formation contains a very large number of first and third order bedding contacts, which represent the migration of dunes and channels respectively, there are relatively few macroforms and bars preserved. The lack of second order surfaces within many outcrops could indicate that bars in the Rough Rock rivers were very large and therefore cannot be resolved at the outcrop scale. Similarly, the rivers which deposited the Rough Rock may have had such high width: depth ratios that the dip of second order bounding surfaces are too low to be identified. The large number of third order bounding surfaces indicates channel stacking which may have occurred in two ways: (1) relocation of rivers in the same area over time, or, (2) anabranch avulsion within a braided river within a hierarchy of channels. The problems associated with differentiating between these two mechanisms are discussed.
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