Abstract

Late Visean “Cracoean” buildups commonly formed marginal facies to rimmed carbonate shelves in northern England. Both isolated reef mounds and continuous shelf-edge tracts are represented, with original depositional (basin facing) topographies of up to 168 m. These buildups consist of bank and contiguous flank facies, and can be clearly differentiated from Waulsortian type mudmounds (with which they are frequently included) by their greater diversity of biota, their unequivocal shallow water characters and the presence of a true framework niche (a modifying rather than essential component).

The framework facies was initiated in the shallowest water settings of the buildups and produced veneers on flank and bank facies, but locally constructed more substantial frameworks over 30 m thick (possibly as much as 40 m) and covering areas in excess of 3000 m². Frame-building, exemplified here by a framestone exposed on the Stebden Hill reef mound of North Yorkshire, was the product of an encrusting community dominated by microbialite (containing Ortonella), with lithistid sponges (Haplistion and Hindia), bryozoans (Fistulipora and Tabulipora) and tabulate corals (Michelinia and Emmonsia) comprising important secondary contributors. Growth form was variable and depended on the local assembled biota, but was truly encrusting and constructional in habit and typically cavernous. Groves of small solitary rugosans (mostly Cyathaxonia, but commonly Amplexocarinia and Calophyllum) were bound upright (i.e., in growth position) into the framestone, and the tabulate coral Cladachonus and robust fenestrate bryozoan Thamniscus were incorporated in a similar way. The framework supported a unique shelly fauna dominated by attached productoid brachiopods and the “oyster-like” pseudomonotid bivalve Pachypteria. Internal sediments (typically peloidal and pelletal packstone and grainstone) contain concentrations of ostracodes (large myodocopids) and the trilobite Griffithides.

These frameworks represented a rare niche in a temporal setting typically devoid of this type of biogenic association. The general framestone construction and the associated specialized shelly fauna are analogous in many respects to Permian reef and biostromal frameworks.