IMPLICATIONS OF MASS TRANSPORT DEPOSITS (MTDS) IN THE DEVONIAN DARTMOUTH GROUP (SOUTHWESTERN U.K.): A RECORD OF SYNDEPOSITIONAL TECTONISM?

The Devonian Dartmouth Group, an outlier of the Old Red Sandstone in southwestern U.K., has been fundamentally misinterpreted as nonmarine (lacustrine, fluvial, or playa) or restricted marine (lagoonal or tidal flat) when it is better understood as a tidally-influenced, shelf-margin delta—muddy marine shelf complex. Certain deformational features have been erroneously considered part of the (post-depositional) Variscan orogeny, when these features can be better understood as syndepositional mass transport deposits (MTDs) associated with slumps, growth faults, and debrites. Slump folds are restricted to strata bound by sub-parallel, non-folded strata. Slump fold axial-plane dip directions (n=40) cluster to the southeast (124) and southwest (249) indicating paleoslopes to the south-southwest (205). Unidirectional paleocurrents (n=27) have a vector mean to the southwest (239), and bidirectional paleocurrents (n=32) are bimodal to the southeast (130) and southwest (225), together indicating paleoflow was approximately perpendicular to paleoslope. There are several types of MTDs including: (1) cohesive debrites, (2) coherent slump folds, (3) incoherent fold fragments, (4) sedimentary mélange with pervasively sheared fabrics including shattered clasts, (5) rafted blocks; (6) growth-fault rotated blocks with roll-over structures, (7) thrust-fault packages, and (8) convolute bedded or massively bedded units of probable liquefaction origin. MTDs are prominent in certain stratigraphic intervals, while the unit is otherwise dominated by bioturbated shales (now slates) interbedded with heterolithic tidalites (shales and thin sandstones with flaser, wavy, and lenticular bedding, herringbone cross-stratification, and re-activation surfaces), wave-modified turbidites, tempestites, phosphorites, and tuffs. The unit is interpreted as the distal component of a tidally-influenced marine delta, while the overlying Meadfoot Group may represent the proximal component. The prevalence of MTDs in the Dartmouth Group indicate gravity-driven processes on the continental margin (outer shelf and slope), which may have been initiated by paleoseismicity. Thus, the Dartmouth Group may provide important evidence for an overlooked component of Devonian syndepositional tectonism in this region.