Correlation of the Early Devonian Rhynie chert beds between three boreholes at Rhynie, Aberdeenshire

Abstract

Synopsis Three sequences of the Early Devonian Rhynie Cherts Unit intersected by cored boreholes at the Rhynie SSSI (Site of Special Scientific Interest) are compared on the basis of both lithology and plant content. It is shown that over the 45–65 m distance covered by the boreholes general correlation can be made of the chert-bearing unit, and also sandstone and shale units. General thicknesses of units are similar in the three boreholes. However, there is no bed-to-bed correlation of the cherts or other lithologies, neither is there a correlation in the sequences of plants recorded. It is considered that the sequence was deposited in the distal regions of sinter deposition on a low-angle outwash apron from a hot spring, where the water table was close to the surface, and water was frequently ponded. Overbank flooding from a northward-flowing river system periodically inundated the outwash apron, depositing sand and mud and interrupting sinter deposition. On the basis of plant occurrence in the chert beds intersected in the boreholes it is estimated that on average 15% of the area was bare ground, 30% covered with plant litter, and 55% with growing plants. Similar environments are seen today on the distal marshy areas of outwash aprons from hot springs in Yellowstone National Park, USA.

40Ar/39Ar dating of hydrothermal activity, biota and gold mineralization in the Rhynie hot-spring system, Aberdeenshire, Scotland

“…In the RCU most of the chert beds are found within the top 17 m of strata (Trewin and Wilson, 2004) but a few thin cherts occur toward the base of the unit (Borehole 97/2). The RCU has been traced over a distance of 90 m along strike and 80 m down dip and represents distal outwash from hydrothermal springs inter-bedded with fluvial overbank flood deposits.…”

Yellowstone hot spring environments and the palaeo-ecophysiology of Rhynie chert plants: towards a synthesis

“…Positive evidence of high water tables in the clastic beds occurring between chert horizons include; the reduced nature of the sedimentary sequence and the absence of red-beds that would signify drying and oxidising conditions, the common presence of early diagenetic framboidal pyrite (sometimes observed to be replacing plant material) and preserved (but compacted and degraded) organic matter in the subsurface (e.g. Fayers 2003; Trewin and Wilson 2004). The latter again indicate reducing and saturated subsurface conditions rather than drying and oxidising.…”

“…Organic carbon preservation in the thin intervening sands suggests continued waterlogging following river sediment deposition and/or insufficient time for oxidation of organic material prior to resumption of sinter deposition. Trewin and Wilson (2004) considered that river flooding frequency, which was viewed as halting sinter deposition by dilution, was sufficiently high to prevent the development of mesophyte climax communities (but see below). The emerging consensus interpretation of this setting (e.g.…”

“…Thus the stresses endured by extant plants and the adaptations/ tolerances required provide an excellent analogue for the Rhynie plants. However, until recently comparatively little attention has been paid to the variety of microhabitats available for colonisation in the geothermal system (but see Trewin et al 2003;Trewin and Wilson 2004), because of the lack of outcrop in the Rhynie area and information on their spatial distribution. Some studies (e.g.…”

“…Channing 2001Channing , 2003Trewin et al 2003;Trewin and Wilson 2004;Channing and Edwards 2009). Given this new view of the Rhynie depositional and preservational environment there is a crucial need to re-evaluate the ecophysiological tolerances and stresses of the inhabitants.…”