Stratigraphy and geochemistry of the Yoredale rocks between Shap and Appleby

Abstract

Detailed mapping of the Carboniferous strata of Yoredale facias in north Westmorland has shown that the Middle Limestone Group succession comprises nine well developed cyclothems. The major limestones at their bases are clearly recognisable throughout the region but thin calcareous horizons, found within the clastic successions of four of the earliest cycles, are of more local distribution. Correlations of the limestones with those in the Westmorland Pennines are suggested and involve the equation of the Iron Post Limestone with the siliceous upper leaf of the Newby Mill (= Four Fathom) Limestone. The Upper Limestone Group sediments, lacking well marked calcareous horizons, cannot satisfactorily be divided into cyclothems. However, high in the sequence, the Bewley Castle Limestone has been shown to represent marine strata in the Crag Limestone cyclothem of the Pennines. A decrease in the thickness of the Middle Limestone Group strata, of the order of 10%, has been detected in the western parts of the area as compared to the region east of the Lyvennet. This is consistent with the regional picture of minimal subsidence in west Cumberland and, taken in conjunction with the close similarity in the successions on either side of the Pennine Faults, it indicates that, at this time, a large paxt of northern England behaved as a distinct tectonic unit. In comparison with the rapidly subsiding trough to the north this was a stable region, to the whole of which the term 'Alston Block' may usefully be extended. Its structural unity was broken only after Carboniferous deposition ceased, by the initiation of the Pennine Faults; there is evidence that the latter had no earlier history as hinge-lines affecting deposition. Of particular mineralogical interest are the Grayber Limestone, the upper leaf of which invariably is rich in glauconite, and the argillites which, irrespective of their position in the cycle, have kaolinite as a common component. This is a product of diagenesis and is believed to be due to the deep penetration of weathering under the tropical conditions of immediately post-Carboniferous times. A comparison between pale and dark limestones suggests that finely divided pyrite has a greater influence upon colour than does organic carbon. Little mineralogical variation is present through the major limestones but certain trace elements, most notably manganese and strontium, are present in greatly varying amounts. Manganese is closely associated with iron- and magnesium-bearing carbonates and may total 1% in some dolomitised limestones. Studies of strontium variation in the Little Strickland Limestone strongly suggest that its distribution reflects primary differences in the proportions of aragonite and calcite accumulating on the sea floor. Such variation may prove to be a useful tool in aiding the understanding of the micro-environments of limestone deposition. Chemical and mineralogical data support stratigraphical evidence which indicates that the bulk of the succession was laid down in relatively quiet shallow marine waters whose salinity varied from normal during times of limestone deposition to brackish at higher levels in most of the cycles. Only occasionally, where there is a sheet sandstone with a sharply erosional base, may a fluvial environment be said to have been dominant. A widespread tectonic control, probably depending ultimately upon isostatic forces, is considered to be the basic cause of the cyclic repetition. Uplift in post-Carboniferous times led to strong erosion under a tropical climate which became increasingly arid. Its effects remain clearly recognisable in the reddening of the Carboniferous rocks which may extend to a depth of as much as 1000 feet below the New Red Sandstone unconformity. Two stages of reddening have been recognised; they were separated by a period of local dolomitisation consequent upon the extension into the Vale of Eden of the Zechstein Sea. It is postulated that the Vale underwent rapid sinlcing during the time of accumulation of the New Red Sandstone and that thick deposits did not spread far beyond their present limits. Deep wadi-like channels with a general northerly trend have been recognised and, in the vicinity of Appleby, upstanding remnants of Carboniferous strata, isolated by erosion from the main upland mass, can be proved to exist. The conclusion that this main erosion area lay to the south and west is supported by facies. changes in the basal New Red Sandstone, Brockram giving way to pebbly Penrith Sandstone in a northerly and easterly direction.