The anatomy of a deep water mud-mound complex to the southwest of the Dinantian platform in Derbyshire, UK




An elongate Waulsortian mud-mound complex developed at Dovedale on a ramp to the southwest of a developing carbonate platform in Derbyshire during Chadian (early Viséan) times. The complex occupied an area of approximately 6 km2 and grew to a maximum relief of 80 m with longitudinal and transverse valleys developed near the southern margin.

Five mound associated facies have been identified: mound core, mound flank (fine), mound flank (coarse), intermound (fine) and intermound (coarse). The mound core facies is a massive skeletal wackestone with comminuted sponge debris, foraminifera, ostracodes and crinoid debris set in a matrix of clotted micrite. The mound flank sediments display moderately inclined bedding surfaces. While the mound flank (fine) contains sponge debris, the mound flank (coarse) is dominated by articulated crinoid columnals, and includes algal-encrusted micritized intraclasts and coarse peloids. The well-bedded intermound (fine) facies is bituminous and micritic while the intermound (coarse) facies is composed of skeletal-peloidal-intraclast grainstones which locally contain calcified algae.

Although the fauna is diverse, the density of colonization by metazoans was low and the supply of macrofossil debris modest. The clotted micrite texture is interpreted as the product of micro-organisms which precipitated and trapped fine-grained sediment. The mud-mound complex is dominated by the bathymetric assemblages B and C proposed by Lees, Hallet & Hibo which on their model of the Belgian Waulsortian, indicate depths of between 220 and 280 m. Intercalation of assemblages B/C and C/D on the northern margin of the complex is interpreted as the result of local storm disturbance. A deep water drift is postulated to explain the NW-SE alignment of the complex which probably fitted the ‘export model’ of Bosence, Rowlands & Quine. Beneath the sediment surface, phreatic flow eroded unlithified sediments and developed interconnected cavities which were filled by cement and sediment relatively eariy. Mound instability triggered the opening of fissures which filled with crinoid debris, peloids, indurated lithoclasts and micrite.