Pentre-bach Quarry, Foel Formation (Carboniferous of the United Kingdom)

Also known as Pentre-bach Quarry North and South, Foel Formation

Where: Wales, United Kingdom (53.6° N, 2.7° W: paleocoordinates 9.1° S, 0.1° W)

• coordinate stated in text

• outcrop-level geographic resolution

When: Foel Formation, Chadian (345.3 - 345.0 Ma)

• The formations in this area in stratigraphic order are: Foel Formation, Dyserth Quarry Limestone Formation, Moel Hiraddug Limestone Formation, and Gop Hill Limestone Formation

• group of beds-level stratigraphic resolution

Environment/lithology: transition zone or lower shoreface; lithified, dolomitic, peloidal, brown, silty limestone and lithified, gray, green, silty mudstone

• The sediments of the Foel Formation were deposited in a predominantly shallow-water subtidal shelf environment into which siliciclastics were continually being supplied from a nearby source area. This accounts for the ubiquitous presence of detrital quartz grains in the limestones as well as the interbedded siltstones and mudstones of terrigenous origin. PLant material in these beds was probably carried onto the shelf from a nearby land mass, either raften in, or transported by rivers draining off the land.

•The inner shelf environment was very shallow and turbulent as suggested by the presence of intraclastic oncoids, micritized and rounded bioclasts, concentrations of shells in dscrete layers, and frequent cross bedding. Together these imply that the packstones and especially the grainstones were wave- and probably storm-influenced. The stenohaline biota implies sea-water of normal salinities above the shelf.

•Periodically, however, the sedimentation on the shelf changed, with the deposition of porcellanous limestones which have a restricted fauna of ostracods and calcishperes. This suggests quiet-water, probably hypersaline and brackish, subtidal lagoonal or intertidal mudflat environments. These fine-grained lime muds would have formed in a protected area behind a barrier which could have been provided by grainstone shoals. The presence of domal stromatolites in this lithofacies would support a peritidal or restricted quiet-water subtidal lagoonal environment.

• The Foel Formation comprises an alternation of brown partly dolomitized bioclastic limestone and grey or green mudstones and siltstones with rare coal laminae or streaks. The limestones are mainly silty peloidal packstones and cross-bedded poorly-sorted intraclastic and bioclastic grainstones; many of the bioclasts have been highly micritized. Most lithotypes contain small angular quartz grains and occasionally bioclasts are coated with hematite. Jones(1921) had ealier noted that many of the limestones have a varying proportion of detrital quartz grains and iron oxide. The dolomitization is usually fabric-selective and preferentially replaces the matrix, avoiding bioclasts and peloids, although euhedral dolomite rhombs may occur within peloids.

•Rare lithotypes include a thin 15-cm thick laminated and cross-stratified sandy dolomite below the top of the formation, and a 10cm thick dark grey porcellanous limestone (micrite) containing thin valved ostracods and calcispheres. The latter lithology occurs 7m above the base of the measured section. A similar porcellanous limestone 15cm thick is found south of Foel Farm (SJ 0652 77??).

Size class: macrofossils

Primary reference: I. D. Somerville, A. R. E. Strank, and A. Welsh. 1989. Chadian faunas and flora from Dyserth; depositional environments and palaeogeographic setting of Visean strata in Northeast Wales. Geological Journal 24(1):49-66 [J. Alroy/C. Simpson/C. Simpson]more details

Purpose of describing collection: biostratigraphic analysis

PaleoDB collection 41451: authorized by John Alroy, entered by Carl Simpson on 14.07.2004

Creative Commons license: CC BY (attribution)

Taxonomic list

Strophomenata
 Productida - Monticuliferidae
 Productida - Daviesiellidae
Daviesiella llangollensis
Single 'imperfect specimen' doubtfully identified
 Productida - Rugosochonetidae
Rhynchonellata
 Athyridida - Athyrididae
 Spiriferida - Spiriferidae
Spirifer sp. Sowerby 1818
Anthozoa
 Auloporida - Syringoporidae
Syringopora reticulata tabulate coral
 Stauriida - Lithostrotionidae
Lithostrotion sp. Fleming 1828 horn coral
 Stauriida - Palaeosmiliidae
Palaeosmilia murchisoni Milne-Edwards and Haime 1848 horn coral
 Stauriida - Aulophyllidae
 Stauriida - Zaphrentidae
""Zaphrentis" spp." = Zaphrenthis
""Zaphrentis" spp." = Zaphrenthis Rafinesque and Clifford 1820 horn coral
 Stauriida - Cyathopsidae
Caninia cornucopiae Michelin 1840 horn coral
 Stauriida - Axophyllidae
 Stauriida - Lophophyllidae