PETROLOGY AND BIOSTRATIGRAPHY OF CARBONATE ROCKS FROM THE ?LOWER TO UPPER VISÉAN (LOWER CARBONIFEROUS) MILFORD FORMATION, CO. CARLOW, IRELAND

Preliminary study of a 255m section in the Milford drillcore from Co. Carlow, Ireland revealed an unusual sequence of shallow subtidal, peritidal and sabkha facies in ?Lower to Upper Viséan rocks.

The lower 32m (Unit 1) is cherty dolomite with thin shales and scattered crinoids and brachiopods. This is conformably overlain by 19m of unfossiliferous, cross-bedded oolitic/peloidal grainstone (Unit 2) suggesting a shallow subtidal to intertidal environment. In both of these units the original fabric was partially obliterated by planar dolomite and later overprinted by nonplanar and saddle dolomite associated with late brecciation. This interval is overlain by 81m of peritidal rocks (Unit 3) consisting of: i) alternating stromatolitic fenestral mudstone, dolomite and shale, with evaporite pseudomorphs and rhizolites, ii) ostracod-rich wackestone with detrital quartz, and iii) rare oolitic grainstone. The succeeding 123m of strata (Unit 4) displays the first appearance of biostratigraphically important taxa and consists, in ascending order, of: i) partially dolomitized oolitic/peloidal grainstone grading upward into intraclastic limestone containing late Chadian to Arundian corals (Dorlodotia pseudovermiculare, D. briarti), and foraminifera (Palaeotextularia sp. and Palaeospiropectammina mellina), overlain by ii) algal wackestone to grainstone yielding Arundian algae (Koninckopora tenuiramosa), foraminifera (Eoparastaffella simplex, Eostaffella sp.) and corals Siphonodendron martini, iii) algal-crinoid packstone and grainstone with minor ooids and peloids, containing Arundian taxa (Uralodiscus sp. and Siphonodendron sociale), and iv) cycles of fenestral mudstone and shale, algal-crinoid pack-grainstone, and peloidal grainstone containing Arundian-Holkerian forams (Glomodiscus sp. and Paraarchaediscus @concavus stage).

These data confirm that Unit 3 is early-to late Chadian while Unit 4 is late Chadian to Holkerian age. This study has recognized for the first time peritidal and sabkha deposits in Chadian rocks adjacent to the Leinster Massif in the eastern Irish Midlands and appears to be coeval with similar evaporite-bearing rocks in Co. Wexford that also are marginal to this landmass.