Lithofacies of Lower Palaeozoic deep-marine sediments in the Isle of Man: a new map and stratigraphic model of the Manx Group

Abstract

A classification scheme for lithofacies in the Lower Palaeozoic Manx Group has been designed based on a simple assessment of the sandstone:mudstone ratio, bed thickness and sedimentary structures. The scheme was used to acquire high-resolution, standardized field data over areas of good exposure on the Isle of Man. These data were then correlated in order to produce a lithostratigraphic framework and a simplified geological map. Overall, the succession appears to young northwestwards from early Arenig, sand-rich sediments on the southeast coast to mud-rich sediments of mid-late Arenig age inland and on the northwest coast. A number of alternative structural reconstructions have been made which fit to a greater or lesser degree with limited biostratigraphic data. The favoured model involves a compromise between inferred duplication of stratigraphic units by reverse faults and repetition of similar depositional cycles or sequences within the succession. This model implies that the Manx Group is between 5400 and 9250 m thick and contains within it both a lateral facies change (from proximal to distal character east-west) and an overall fining-upwards stratigraphic signature. The sediments are interpreted to have been rapidly deposited on the west dipping margin of a basin situated between the Isle of Man and southeast Ireland, which forms an embayment on the northwest edge of Eastern Avalonia. The main depositional processes were turbidity currents and debris flows, with evidence of possible current deflection or reworking of the upper parts of turbidites by northwards flowing contour currents. During the Caledonian orogeny, the basin was inverted with the development of northeast-southwest trending thrusts and east-west dextral faults. Tentative evidence also exists for the presence of an older north-northeast-south-southwest normal fault trend, although whether it was active during the Arenig is uncertain. It is recommended that future stratigraphic work should concentrate on testing possible correlations between and within the mixed sand-mud Lonan, Port Erin, Injebreck and Creggan Mooar Formations.

Shallow geophysical and geological evidence for a regional-scale fault duplex in the Lower Palaeozoic of the Isle of Man

“…A difference in magnetic response in the order of a few nanoteslas is likely to occur across any steeply dipping geological boundary separating Manx Group units with contrasting amounts of mudstone. The most obvious of these types of boundaries are faults (Mclntyre 1980) which are likely to appear as linear edge-type anomalies similar to those observed on aeromagnetic data from the Isle of Man (Quirk & Kimbell 1997). In addition, if the fault has been subject to fluid flow, particularly hydrothermal fluid, then it may itself produce an anomalous fig.…”

“…The data used in this study comprise high resolution aeromagnetic images owned by World Geoscience Corporation, Bouguer gravity anomaly data owned by the British Geological Survey and Western Geophysical, marine 2D seismic data owned by JEBCO, new 1:10 000 geological field maps of the Manx Group (Quirk & Burnett 1999), published and unpublished mine data (Lamplugh 1903;Mackay & Schnellman 1963;Ford 1993;Cowin, pers. comm.…”

“…In only a few shear zones have kinematic indicators been observed (e.g. Quirk & Kimbell 1997;Fitches et al 1999;Holdsworth, pers. comm.).…”

“…However, it was only in 1998, when a lithofacies map had been compiled by Quirk & Bumett (1999), was it realized that there was an apparent fit between some of the important geological boundaries mapped in the field (faulted or otherwise) and the shallow geophysical lineaments originally reported by Quirk & Kimbell (1997). The objective of this paper is to compare these features in order to erect a tentative tectonic model.…”

“…1). These faults are rarely observed due to poor exposure but are necessary to explain the apparent juxtaposition of different lithostratigraphic and structural domains (Fitches et al 1999;Quirk & Burnett 1999). It was after recognizing the differences between these domains that the idea of Quirk & Burnett (1999).…”