Slumping and micro-jointing in an oligocene lagoonal limestone

Abstract

Small-scale deformational structures are described from argillaceous limestone bands and lenses thought to have been deposited in a lagoonal environment. The structures comprise small-scale folds and slumped slabs which have arisen from the contemporary movement of sediment down a gradient, possibly the sides of small channels. Extensively folded, but to a large extent unbroken micrite layers probably contained mucilaginous algae, which led to their being cohesive and competent. Other, highly disrupted folds reflect the downslope movement of layers lacking such a binding agent. Micro-joints apparent in many laminae often form conjugate sets which resulted from shear, post-dating fold formation, but caused by the same downslope movement. Cracking or small-scale faulting approximately parallel to the channel margins led to the formation of slabs of undistorted sediment which slid downslope to accumulate at the channel bottoms.