The Downpatrick Formation is exposed:
1. between between Bunnahowna (grid reference 10644 34093) and the quay near the Stella Maris Hotel
2. from 1km southwest of Downpatrick Head to Rathlackan Pier, some 7km to the east, in a continuous section.
The composite section of the figure below represents the sequence from the Downpatrick Head area to Creevagh Head.
The base of the Formation is defined by the onset of marine conditions, taken to coincide with the appearance of trace fossils, notably Chondrites and Rhizocorallium (see field guide). The lower part of the Formation consists of grey and dark grey mudrock, siltstone and fine sandstone, with occasional micritic or dolomicritic limestones. A few of the limestones are finely bioclastic, with crinoid debris. Also present are sandstones, both calcareous and non-calcareous, often with rippled horizons. In many places the mudrocks contain sediment cracks, and small calcareous caliche nodules. This emergent facies is frequently found at the base of thin transgressive cycles, the mudrocks being overlain by calcareous silts with trace fossils (Chondrites, and then Rhizocorallium), the cycles being capped by a carbonate bed.
Doonbristy - the thick bed near the top is a limestone marking the base of the Moyny Point Member
Moyny Point Member
The limestones and shales of the Moyny Point Member signal the onset of fully marine conditions. Limestones, which comprise about 50% of the Member, are bioclastic packstones and wackestones, with a large component of comminuted crinoid debris. Sandstones, often highly calcareous, generally occur in tabular beds. The limestones contain tabulate corals (Syringopora and Michelinia). Foraminifera indicate a Chadian age.
Downpatrick Formation (upper part)
The upper part of the Downpatrick Formation outcrops between east of Moyny Point (at about grid ref 11530 34150) and Creevagh Head (grid ref 11775 34085). The sequence is dominated by calcareous sandstone, frequently bioturbated and with low-angle cross-stratification, and lesser proportions of siltstone and mudstone. A few limestones occur, and one of these, in the upper part of the interval, contains Lithostrotionid corals, together with Archaediscid foraminifera, indicating that the upper part of the interval is Arundian in age.
Relationship between lower Downpatrick Formation and upper Minnaun Sandstone Formation, and an environmental interpretation.
The first thick (over 1m) fully marine limestones which form the base of the Moyny Point Member in the Downpatrick Head area, and the equivalent level at Stella Maris and to the west, can be seen to have a high degree of lateral persistence. It therefore seems reasonable to use their incoming as a datum by which to compare the two sections.
A comparison of the two sequences shows some interesting relationships:
1. There is an eastward increase in the proportion of carbonate units in the 50m of sequence above the first appearance of thick limestones (from 15% to 50%).
2. The basal 110m of Downpatrick Formation to the east appears to be the lateral equivalent of a similar thickness of Minnaun Sandstone to the west. Within this "interfingering" interval it can be noted that:
a) The percentage of "thick" sandstones decreases from 50% in the west (Ceide cliffs) to 25% in the east (section west of Downpatrick Head)
b) Carbonates, micritic and unfossiliferous, are restricted to the Red Silt Member in the west, while in the east they occur throughout the interval and are both micritic and finely bioclastic in nature
3. Below this interfingering interval both sections are similar, namely being dominated by thick-bedded to massive pale sandstones.
It could be argued that interfingering does not occur, and that there is a rapid eastward thickening of the basal part of the Downpatrick Formation. However, the interfingering interpretation seems more plausible, and allows for a fairly simple palaeogeographic model to explain the two sections, as depicted in the figure below:
The upper part of the Minnaun Sandstone Formation (to the west) represents a coastal alluvial plain, river channels being separated by areas in which bedded sands and silts are deposited. The river channels probably meander, and change their positions laterally, thus the thick-bedded sands which they deposit end up with a sheet-like geometry, with internal cross cutting units and erosive bases.
To the southeast was a lagoon, probably with brackish water, due partly to the influx of fresh water from rivers, and also to the presence of a barrier bar on its seaward side, cutting off normally saline marine conditions. Periodic breaches of the offshore bar led to the establishment of marine conditions, with colonisation by a variety of invertebrates giving rise to the trace fossils we see today, and deposition of marine carbonates. The marine incursions may have been responsible for the re-working of sandstone units, giving rise to calcareous sandstones with trace fossils.
This site information was researched, written and illustrated by our esteemed colleague, Conor MacDermot RIP