The geology of the northern half of the Rhynie Basin, Aberdeenshire, Scotland

Abstract

A new geological map of the Early Devonian Rhynie Basin has been produced by traditional methods supplemented by trenching to bedrock and a ground magnetic survey. This shows that the basin margins are mostly fault-controlled and three trends are recognised: NE–SW, NNE–SSW and N–S. Three sets of open folds are distinguished with axial traces trending NE–SW, E–W and roughly NW–SE. The faults defining the basin margin and the folding may be related to basin formation within a regional strike-slip system of Early Devonian age.The stratigraphic succession comprises three mappable units: a lower mixed unit of sandstones, shales, conglomerates and andesitic lava (>700 m), a middle unit of laminated grey shale and siltstone (>300 m); and an upper unit of laminated sandstones and shales (>300 m). These correlate with the Tillybrachty Sandstone and Quarry Hill Sandstone Formations (lower unit), and the Dryden Flags Formation (middle and upper units).Small areas containing abundant chert float found outside the Rhynie SSSI may represent the surface expression of chert pods within the middle unit of laminated grey shale and siltstone, which also hosts the Rhynie cherts. The Windyfield cherts occur within the upper unit. No further centres of hydrothermal activity have been found in the northern half of the basin

A high-precision U–Pb age constraint on the Rhynie Chert Konservat-Lagerstätte: time scale and other implications

“…The sediments in question pass southeastwards (i.e. up-succession) over a few tens of metres, and with no detected break, into strata of undoubted Dryden Flags Formation parentage (specifically, the Milton Flags and Shales Member; Rice & Ashcroft 2004). This points strongly to their being part of the Dryden Flags Formation, but it remains the case that all of the other Rhynie Outlier lavas sit within the Tillybrachty Sandstone Formation.…”

“…The Milton of Noth Andesite (Rice et al 2002;Parry 2004) is a moderately altered mass of basaltic andesite which extends along the northwestern margin of the Rhynie Outlier between Milton of Noth [NJ 5029 2904] and Newnoth [NJ 5172 3020] (Rice & Ashcroft 2004;Fig. 1).…”

“…One of four stratigraphic positions may therefore be occupied by the Milton of Noth Andesite (Fig. 2): (1) and (2) well within the Dryden Flags Formation (Rice et al 2002), above or below the Rhynie cherts; (3) close to the boundary between the Dryden Flags and Tillybrachty Sandstone formations (Parry 2004); or (4) near to the base of the Tillybrachty Sandstone Formation (Rice & Ashcroft 2004). This uncertainty over the exact stratigraphic position of the Milton of Noth Andesite fortunately has no detrimental impact on the present study as both the Tillybrachty Sandstone and Dryden Flags formations have consistently yielded spores belonging to the polygonalis-emsiensis Spore Assemblage Biozone (see preceding section).…”

“…the Milton of Noth Andesite (see below). A substantial thickness (> 140 m) of tuffaceous sediment present around Longcroft [NJ 4961 2836] -the so-called Longcroft Tuffs (Rice et al 2002) -is interpreted as the erosional product of intra-Tillybrachty Sandstone Formation lava flows (Rice & Ashcroft 2004), but notably does contain air-fall material (Rice et al 2002).…”

Reply to Discussion on ‘A high-precision U–Pb age constraint on the Rhynie Chert Konservat-Lagerstätte: time scale and other implications’

“…The 40 Ar/ 39 Ar system calibrations on which these various ages are based are summarized in Table 1. The second radio-isotopic age constraint in question is a weighted Andesite has a peperitic 'contact' with sediments resembling those of the Dryden Flags Formation (the host of the Rhynie cherts) and which pass laterally (effectively up-succession) over a few tens of metresand with no proven break -into strata of undoubted Dryden Flags Formation parentage (Rice & Ashcroft 2004). Thirdly, there is neither physical nor geochronological evidence (Parry 2004, unpubl.…”