Facies Variations in Ordovician Sediments of Southeast Ireland and their Paleogeographic Significance: ABSTRACT

Abstract

A continuous section through the Ordovician belt in southeast Ireland is exposed in southwestern County Wexford. There an unconformity separates Lower Ordovician (mainly Arenigian) sediments from an Upper Ordovician (Caradocian) sequence. The thick Lower Ordovician sequence contains four facies, of which the most important is alternating siltstone and shale. Interfingered as minor facies are thick shale beds, paraconglomerate, and interbedded sandstone and shale. The siltstone and sandstone are interpreted as deposits from axial turbidity currents, with associated bottom-current activity, in an otherwise low-energy environment. Paleoslope data indicate that deposition was near the southeast flank (the Irish Sea landmass) of a northeast-trending basin. Only the paraconglomerates were derived from this border area; the dispersal pattern and petrology indicate a northeastern source for the other detrital sediments. Volcanic rocks (laterally equivalent to the more westerly Tramore limestones) are at the base of the Caradocian. An initial nearshore environment, progressively deepening, is probable for this sequence. The overlying sediments contain three facies; quantitatively, the most important is interbedded siltstone (turbidite) and shale. Paleocurrent directions indicate a southwestern source and deposition in a northeast-aligned basin. A close mineralogical similarity with the Arenigian sediments suggests that the Arenigian composed much of the source area. This facies shows an upward increase in shale, and is overlain by two facies of less importance--a pyritic siltstone and silty mudstone facies, and a black shale facies. This upward change implies diminishing supply of terrigenous material and a change in basin geometry to quiet, restricted, shallow depositional areas. These probably were near shore, because overlying rhyolite flows were extruded subaerially. End_of_Article - Last_Page 718------------