RE-INTERPRETATION OF THE EARLY DEVONIAN DARTMOUTH GROUP: A TIDALLY-INFLUENCED MUDDY SHELF SEQUENCE, SOUTHWEST ENGLAND, U.K

The Dartmouth Group is an outlier of the Old Red Sandstone in southwest England, and may represent the marine equivalents. Due to the Variscan Orogeny, the unit is now predominantly slate subjected to multiple episodes of folding, thrusting, and low-grade thermal metamorphism.

Previous interpretations for the Dartmouth Group range from fluvial to lacustrine, playa, mudflat, lagoonal, or tidal flat environments. Previous workers described debrites, waterlain tuffs, tempestites, and phosphatic nodules argued to be lacustrine in origin.

This study re-interpreted the Dartmouth Group as a mud-dominated marine shelf sequence. Muds were organic–rich, massive, and bioturbated. Common small Fe-rich and phosphatic concretions have peculiar wispy, deformed, or embayed outer contacts suggesting the host sediment was compacting or creeping downslope during formation.

Almost all sandstone bodies >1 meter were recumbently folded by syndepositional slumping. Slump fold analysis suggests downslope gravity sliding. Some recumbently folded sandstones were intruded by mud diapirs and clastic dikes, suggesting the sands and surrounding muds were semi-lithified or cohesive at the time. Today the sandstone bodies outcrop as “turtlebacks” shaped by one limb of the recumbent fold. Subaqueous debrites are consistent with this evidence for gravity-sliding processes.

Internally, the sandstones exhibit tidally-influenced structures such as tidal rhythmites, flaser, wavy, and lenticular bedding, herringbone cross-stratification, and remolded foreset bedding (reactivation surfaces). These sandy deposits are interpreted as inner shelf tidal sandbars. Upper portions can include tempestites and shell hash due to wave reworking.

Recognition of the Dartmouth Group as a tidally-influenced muddy shelf sequence provides important paleogeographical constraints for the Devonian of SW England. The dominance of mud, rarity of fossils, and presence of tidal- and wave-structures in the sandstone bodies suggests the Dartmouth Group represents the distal portions of tide-dominated deltas, which the overlying Meadfoot Group may represent the equivalent proximal environments.