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Basin Ideas on Boulder Conjecture--Microtectonics and Collapsed Limestones in Carboniferous of the Isle of Man

David G. Quirk, Jenny King, Iwan Roberts, R. A. Postance, I. Odell

A small area of Carboniferous limestone in Poyllvaaish Bay in the south of the Isle of Man caused much controversy in the latter part of the 19th century and the first half of this century due to the presence of large isolated blocks and discrete boulder beds of pale bioclastic limestone lying upon and overlapped by thin undulating beds of dark calcareous mudstone. Dickson suggested that the boulders had fallen from older beds of Upper Poyllvaaish Limestone north of an east-west paleoscarp on the north-west side of the bay. A recent detailed study has led to a new hypothesis which shows the possibility of very localized tectonics and sediment instability in a carbonate environment.

Chronostratigraphic relations indicate that a small half-graben, only 50 m across, rapidly developed at the end of Upper Poyllvaaish times, disrupting the previously stable carbonate shelf. The formative east-west normal fault, with downthrow to the south, produced a submarine scarp a few meters high. The downthrown side was tilted northward, initiating a small complementary reverse fault along the southern edge of the "microbasin." It was on this upthrown edge that a new reef began to build northward out over mudstones which were accumulating on the shallowly sloping floor of the basin. The mud, derived from north of the fault scarp, was deposited most thickly in the axis of the half-graben, leading to its gradual infill. There is no evidence of fault movement during this deposition. The out-building reef encountered steepening slope and thicker mudstones toward the scarp, eventually inducing slump faulting near the edge of the reef which caused rotated blocks of partially consolidated limestone, with preceding talus, to slide into the basin. Three such collapse events are recorded, and rubble and boulder beds close to the scarp grade into large blocks and finally unbroken reef limestone with increasing distance away from the scarp. After a slight fall in sea level, patch reefs began to build up within the "microbasin" itself (by now mostly filled) until this new limestone began to founder on the incompetent mudstones beneath the mounds, leading to folding and oversteepening of the underlying beds. Finally a thin mudstone blanketed the area before widespread volcanic activity rapidly destroyed this local sedimentary environment.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.