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High‐latitude pelmatozoan–bryozoan mud‐mounds from the late Ordovician northern Gondwana platform
Author(s) -
Vennin E.,
Álvaro J. J.,
Villas E.
Publication year - 1998
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/(sici)1099-1034(1998040)33:2<121::aid-gj780>3.0.co;2-d
Subject(s) - geology , siliciclastic , gondwana , paleontology , facies , subaerial , ephemeral key , terrigenous sediment , ordovician , sedimentology , marine transgression , carbonate , glauconite , sedimentary rock , ecology , structural basin , materials science , metallurgy , biology
Mud‐mound complexes identified within the early to middle Ashgill Cystoid Limestone Formation of northeastern Spain are the first fossil build‐ups to be described in the high latitude north‐facing margin of Gondwana. Mud‐mound complexes comprise individual lenticular mounds (composed of floatstones, cephalopod‐rich mudstones and cementstones), flanks and intermound deposits (including pelmatozoan packstones and floatstones). 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 influx of terrigenous material. Finally, during the Hirnantian regression, the Iberian mixed (carbonate–siliciclastic) platform was exposed to subaerial conditions sufficiently for erosion and karstification 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. © 1998 John Wiley & Sons, Ltd.