Abstract

The Upper Carboniferous Crackington and Bude Formations of southwest England comprise laminated black mudstones, gray siltstones, thin-, medium- and thick-bedded sandstones, and contorted, silty sandstones. The thick-bedded sandstones only occur in the Bude Formation, which overlies the Crackington. The sandstones were deposited predominantly by turbidity currents in a relatively shallow basin. They are unusual for two reasons: 1) sediment coarser than fine sand is entirely absent; 2) unlike many previously described ancient-subsea-fan deposits, organized bundles of sandstones displaying asymmetric upward-thickening or upward-thinning sequences are no more common than symmetric sequences, and both types of organized bundles are much less common than disorganized (nonsequential) bundles, particularly in the Bude Formation. Sediment was supplied directly from a delta to the north, by resedimentation of delta-front sands, and slumping of upper pro-delta muds and silts to form a lower pro-delta turbidite fan. Fan growth is not readily explained by current models for single-sourced, tripartite subsea fans displaying radial growth. It developed, instead, a bipartite growth pattern, which was strongly influenced in its upper part by external variables, including grain size, distributary channel switching and other deltaic processes, active tectonism within the basin, and possibly sea-level fluctuations. The fan grew by down-slope construction of independent levee/channel complexes, each of which headed in the upper pro-delta and even possibly the delta front. Slumping and mobilization of argillaceous facies resulted in the random occurrence of contorted beds in all parts of the fan. Regionally extensive mudstones, known as key shales, within the fan deposits probably reflect large-scale, upslope, delta-lobe abandonment. The Crati and Ebro Fans of the Mediterranean Sea would appear to be good modern analogs for the Crackington-Bude fan system.

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