Controls of mud mound formation: The Early Devonian Kess-Kess carbonates of the Hamar Laghdad, Antiatlas, Morocco

Abstract

The origin and development of Early Devonian (late Pragian to late Zlichovian; predominantly uppermost Zlichovian as indicated by conodont faunas) mud mounds of the Hamar Laghdad area in the eastern Antiatlas, Morocco, are controlled by extrinsic and intrinsic factors. Extrinsic factors include the existence of a paleohigh (Lochkovian volcaniclastics), unidirectional currents and repeated storm events as well as sea level fluctuations. Intrinsic, biologically induced factors are the preferred growth of organisms on the top and the flanks of the mounds because of more favourable ecological conditions, and a rapid synsedimentary lithification of the steep mound flanks by interskeletal cementation of auloporid tabulate corals. The mounds developed in an epicontinental basin below the wave base but within the range of storms. The formation of the mounds started within the uppermost part of the bedded crinoid facies of the Kess-Kess Formation with the hydrological accumulation of a bioclastic pile. This elevation became settled by crinoids and high-diverse tabulate corals producing bioclastic sediment. Binding activities of the organisms were missing, calcareous algae and stromatoporoids are completely absent. Baffling by thamnoporid tabulate corals might have occurred locally but was not important for the development of the mounds. Steep slopes to the north and less steep slopes to the south may be the result of north-northwest to south-southeast trending currents, derived from orientation patterns of orthocone nautiloids in the uppermost beds of the Kess-Kess Formation. A synsedimentary cementation of the flanks, possibly triggered by submarine interskeletal cementation of patchily distributed auloporid colonies, protected the bioclastic sediment against redistribution by frequent storms (indicated by densely spaced eventstone intervals and partly also by the common >>Stromatactis<<-like structures within the mound facies). Accumulation of more parautochthonous bioclastic sediment within the mounds as compared to the intermound area, therefore, is caused by a self-sustaining system of hydrologic piling of sediment triggered by storms, preferred settlement of organisms upon these piles, producing bioclastic sediment and coeval biocementation of the growing mound flanks. This model differs from existing mud mound models in the lack or only minor significance of binding and baffling, in the lack of mound facies sequences and in the greater importance of extrinsic control factors.