High-latitude pelmatozoan–bryozoan mud-mounds from the late Ordovician northern Gondwana platform
✍ Scribed by E. Vennin; J. J. Álvaro; E. Villas
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 921 KB
- Volume
- 33
- Category
- Article
- ISSN
- 0072-1050
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✦ Synopsis
Mud-mound complexes identi®ed within the early to middle Ashgill Cystoid Limestone Formation of northeastern Spain are the ®rst fossil build-ups to be described in the high latitude north-facing margin of Gondwana. Mud-mound complexes comprise individual lenticular mounds (composed of ¯oatstones, cephalopod-rich mudstones and cementstones), ¯anks and intermound deposits (including pelmatozoan packstones and ¯oatstones). The small mounds are mainly composed of bryozoans, cystoids and crinoids, and were developed on outer ramp environments. Mound initiation depended upon the stabilization and colonization of densely packed lenses of pelmatozoan-rich sediments. In a mid-ramp setting, pelmatozoan±bryozoan meadows were episodically degraded by common wave-and storm-induced processes, the development of semi-consolidated substrates, and the periodic in¯ux of terrigenous material. Finally, during the Hirnantian regression, the Iberian mixed (carbonate±siliciclastic) platform was exposed to subaerial conditions suciently for erosion and karsti®cation to occur.
From a palaeogeographical point of view, the pattern of the Ashgill Iberian platform deposition is characterized by episodic exclusion of carbonates from most nearshore environments by a shoreline source of siliciclastic sediments. A similar interpretation to that made on the Iberian Cystoid Limestone Formation, in terms of gradual proximality± distality changes, is proposed for comparable facies types in Ashgill limestones described in southwestern Europe.