Scottish Silurian shorelines

Abstract

The Silurian inliers of the Midland Valley of Scotland all exhibit a regressive sequence, and despite evident facies differences between inliers, marine successions invariably pass upwards into redbeds of continental origin. Contrasting types of shoreline facies can be seen in different inliers; here the beds in the North Esk Inlier (NEI) are compared with those previously described from Knockgardner. At Knockgardner a high energy coastal environment prevailed, but in the NEI very different conditions can be identified in near-contemporaneous deposits. In the NEI thin sandstones and siltstones of the Reservoir and Deerhope formations are succeeded by the sandstones and conglomeratic beds of the Cock Rig Formation. These are overlain by the marine mudstones of the Wether Law Linn Formation and, at the top of the sequence, the continental redbeds of the Henshaw Formation.The Wether Law Linn Formation is interpreted, on various lines of evidence, as a lagoonal system. Conditions therein were initially fully marine, though within the photic zone, but subsequently the faunas were increasingly influenced by fluctuating salinity, prior to deposition of the redbeds. Such a lagoon would have required an offshore bar impounding it, which is represented by the Cock Rig Formation. These sedimentary rocks, previously interpreted as deposits of a laterally unconfined submarine channel, are now considered to be of shallow water origin. The succession closely conforms to classic models in which shoreface sands, consisting of small cross-bedded packets, are succeeded by tabular sandstone sheets representing foreshore beach deposits. Coarser and thicker beds, with herringbone cross-sets, linguoid ripples and trains of rounded pebbles, are interpreted as the deposits of tidal channels within the barrier complex. The barrier-lagoon system persisted throughout the whole of Cock Rig time and most or all of the time during which the Wether Law Linn Formation was deposited, and was either static or prograded seawards until the lagoon dried up during a final marine regression. The sedimentary and faunal evidence is consistent with this interpretation, and the contrast between the shoreline environments of the NEI and Knockgardner is striking. Brief reference is made to other inliers in the Midland Valley.

Upper Llandovery–Wenlock (Silurian) palynology of the Pentland Hills inliers, Midland Valley of Scotland

“…Influxes of more sandy material provided the substrate on which the coral faunas, that gave rise to the Deerhope Coral Beds and which characterise the formation, could thrive. Clarkson et al (2001) interpreted the overlying Cock Rig Formation as the deposits of an offshore barrier system, with the flaggy mudstones and siltstones of the Deerhope Formation deposited from suspension flows below wave base on its seaward side. On the landward side of the Cock Rig offshore bar, Clarkson et al (2001) suggested that the Wether Law Linn Formation was deposited in a broad marine lagoon, perhaps several kilometres across, in which fluctuating salinities with the occasional restoration of fully marine conditions culminated in brackish or freshwater conditions.…”

“…Clarkson et al (2001) interpreted the overlying Cock Rig Formation as the deposits of an offshore barrier system, with the flaggy mudstones and siltstones of the Deerhope Formation deposited from suspension flows below wave base on its seaward side. On the landward side of the Cock Rig offshore bar, Clarkson et al (2001) suggested that the Wether Law Linn Formation was deposited in a broad marine lagoon, perhaps several kilometres across, in which fluctuating salinities with the occasional restoration of fully marine conditions culminated in brackish or freshwater conditions. The red sandstones of the Henshaw Formation record the onset of predominantly terrestrial sedimentation in the Wenlock, with some evidence for minor marine incursions.…”

“…The Silurian successions in Midland Valley inliers ( Fig. 1) demonstrate the progressive infilling of a Llandovery to Wenlock inter-arc basin, and record a transition from transgressive marine Llandovery sequences to prograding terrestrial Lower Wenlock successions (Rolfe 1960(Rolfe , 19613 Cocks & Toghill 1973;Cameron & Stephenson 1985;Robertson 1989;Clarkson et al 2001). …”

An atypical intra-platform environment and biota from the Silurian of Gotland, Sweden

“…The above fits well with an inshore physically and chemically restricted setting, possibly a sediment-starved lagoon or analogue (cf. Clarkson et al 2001). The abundant P. dubius rhabdosomes would then indicate that pelagic graptolites occurred across the entire width of carbonate platforms and that their rarity in limestone is a result of taphonomy.…”

Bembicosoma re-examined: a xiphosuran from the Silurian of the North Esk Inlier, Pentland Hills, Scotland

“…Three-dimensionally preserved material of Bunodes from the 'Eurypterus dolomite' of Saaremaa, Estonia retains some original cuticular material and show a less pronounced sculpture (personal observations). Clarkson et al (2001) demonstrated through careful sampling, collecting and excavation that the Silurian sedimentary succession overlying the Reservoir Formation indicated a predominantly shoreline/shoreface environment, including a suite of lagoonal sediments recognised within the Wether Law Linn Formation. The exact function of this roughening of the otherwise smooth dorsal cuticular surface remains unknown.…”