| Abstract: | Microfacies analyses of thin sections from over 650 samples of Asbian and Brigantian (late Viséan) platform carbonates from Ireland shows that foraminiferal assemblages are controlled by the relative abundance of echinoderms (mainly crinoids), bryozoans and calcareous algae: Koninckopora, Ungdarella and palaeoberesellids. Archaediscidae, Valvulinella and Tetrataxidae thrived in algal-poor crinoid and bryozoan thickets at or below fair-weather wavebase. Late Viséan algal limestones contain abundant Endothyridae and Palaeotextulariidae, and are interpreted to have been deposited at depths between 5 and 10 m. The morphology of the Foraminifera studied is related closely to their facies distribution. Many taxa inhabited high energy shallow marine algal meadow facies, these include: Bibradya, Cribrospira, Nevillella and Bradyina with cribrate apertures and other specialised test features that protected the protoplasm from damage, Gigasbia, Omphalotis, Globoendothyra and Pseudoendothyra with large, rotund or keeled, multilayered tests and Forschiidae with large thick-walled tests. Infaunal taxa such as the Lituotubella, Nevillella, Gigasbia and the palaeotextulariids are recorded from algal facies and appear to have been unable to survive on bryozoan and crinoidal substrates. Pseudoammodiscus and Earlandia, characterized by simple morphologies and single layered walls, and Endothyranopsis, Endothyra and Eostaffella, planispirally coiled with multilayered micritic tests, are regarded as the least facies-controlled of the taxa studied. The encrusting nature of Pseudolituotuba and Scalebrina ensured survival in a wide range of platformal palaeoenvironments, though algal meadows were not generally suitable substrates for the Pseudolituotubidae. The secondary deposits in the umbilical region of the Asteroarchaediscinae may have been an adaptation for survival in the dense crinoidal bryozoan thickets where they lived. Delicate Planoarchaediscus occurs in algal-poor wackestone to packstone facies typical of low energy palaeoenvironments possibly at depths below 20 m. Howchinia, which occurs with Fasciella (a probable alga) in intraclastic wackestone facies may also have lived in water deeper than 20 m. Major changes in foraminiferal assemblages through the Asbian and Brigantian stages of the late Viséan are attributed to changes in associated biotic constituents. Early Asbian endothyrid and forschiid dominated assemblages were controlled by the relative abundance of bryozoans, Koninckopora and palaeoberesellids. By the late Asbian, the foraminiferal assemblages were controlled by the presence or absence of palaeoberesellids, Ungdarella and to a lesser extent bryozoans. Asbian assemblages were similar, although foraminifers such as bradyinids, Cribrospira and Globoendothyra became abundant in the late Asbian. A switch to predominantly echinoderm (mainly crinoid) and bryozoan substrates in the Brigantian may have caused the major change from late Asbian endothyrid and palaeotextulariid assemblages to archaediscid and tetrataxid dominated assemblages. |
