Geochemistry and sedimentology of the Corallian sediments of Southern England

Chowdhury, Ahad Newaz (1980) Geochemistry and sedimentology of the Corallian sediments of Southern England. University of Southampton, Doctoral Thesis.

Abstract

Corallian rocks from outcrop on the Dorset cast, from boreholes at Baulking and Ufflngton in Berkshire and from Wallngham in Surrey were subjected to petrographtal, mineralogical, and geochemical investigation. Pronounced mineral variation exists between the studied localities: well crystallized montmoritonite shows highest concentrations In the Lower Calcareous Grit Group of Berkshire with reduced crystallinity and concentration In Dorset and at Wadingham, where it h subordinate to Wits and kaolinite. In the Baulking and Ufflmgton borehole, authigenic recites of cWoptilollte/hauluWite type, low trldymitscristobalite, cathedral biotite and apatite accompany the Montmorilbdte. This association s thought to be genetically related, of volcanic origin, formed from the decomposition of air-fall ashes both on adjacent landmasses and In the area of deposition. Upward decrease in the montmoWonhe content suggests that volcanicity took place in the early phases of Coallisn sedimentation. The volcanic phase appears to hero been contemporaneous with the shoaling which Initiated CoralWn sedimentation in Southern England suggesting a common tectonic control. The westward increase In montmorulonite content locate the volcanoes In this direction; Jun-dc -early Cretaceous volcanic rocks, reported from the South Western Approaches (1fatrison a & L, 1979) provide the probable soume Geochemtal investigation of 27 elements was mainly by X.R.F. analysis. Variation occurs both between localities and within localities, mostly reflecting clay mineral-variation. Highest concentration of most of the elements occur at Walngham, suggesting that dement-concentration was mainly controlled by the detrital clays which dominate this area. Upward increase In element concentration In all three areas, Implying an upward-incredng dominance of detrital days anda lessening volcanic clay influence. A small group of elements (Cr, Me, Ni, Pb, S, V, Zn), known to be associated with organic carbon, reach their maxima in the Dorset coast outcrops rather than at Wallagham. This may result from an analytical artefact, sampling concentrating on mud rocks which are poorly represented at Wadingham. The difference between Dorset and Berkshire relate to the greater preservation of organic carbon In the former area, where reducing conditions prevailed; Berkshire samples show abundant sips of reworking and little of postburial reducing conditions. The Upper Calcareous Grit Group is distinct from the lower Groups: It s enriched in various elements (Al, Fe, TI, Mn, Ni, Pb, Cr, Cu, V, P. Ce, Y. As, 2n), attributable to intense chemical weathering during the protracted period of deposition (It span about three ammonite zones compared to the two zones spanned collectively by the lower three Group). Coralian limestone are mostly shoal calcarenites and lagoonal micrites; alchudha are few, thin and occur with the alcaenhea Coral patch reefs are common in Berkshire. Bloturbatfon s widespread, leading at Its most intense to rubbly limestones. The Corallian Limestone display the textures of phratio disposals and lack signs of compaction, indicating that each limestone unit was almost fully Etbifed before successively younger sediments were deposited. The variation in Sr-content from subreef collie shoes to patch reefs in Berkshire Is primarily related to permeability and diagonals, including the relative timings of lithifatlon of oollta and reefs. Post reef exposure and uplift s Indicated by the development of vuggy porosity in the Coral Rag. Exceptional concentrations of Iron in the Red Beds of the Trigonla davellata Beds of the Dorset coast s a consequence of penetration of son-rich fresh waters from the Sandsfoot Clay, in the form of a phreatlc lens. Geochemlcal studies confirm a Iagoonal origin for the Sandsfoot Clay and further attests to the lagoon containing fresh warts. Element-concentration vary with rock type, Sr most notably.Detailed analysis of the tectonic and depositional events of the Mesozoic Era reveals that the was of deposition consisted of fault-bounded blocks, each showing repetition of nacre or leu similar tectonic and depostional events from at least Lower tornado time onwards. Groups of blocks associated to form persistently elevated Madf or persistently cob siding troughs, die two usually separated by other groups of blocks ailed Shelves which tended to lie close to sea-level, either above or below and subject to Intermittent accumulation and lots of sediment. The Oxford Clay, which underline Coralian rocks, s dominated by a detrital Illlte Kaolinite assemblage and demise uniform marine conditions over both Troughs and Shelves. The Corallian rocks show sharp contrasts between clay-dominated Troughs and Caboate/Quartz sands dominated Shelves, with marked shoaling in the latter are accompanied by evidence of volcanic activity to die west. Within the Shelves, individual fault-bounded blocks moved independently. For example, the Berkshire Shod coven the Oxford Block whereas deeper muddy environments occurred on the adjacent Aylesbury Block Uplifts extended to the Marls, particularly those in the west, which supplied the abundant quartz sands and occasional pebble horizon. These desital sands spread rapidly eastwards across adjacent shelves to lodge in the neared Trough. They did not teach Watling nor the East Kent Shelf. Earlier authors have Identified cycles of sedimentation, either three or four, but have never satisfactorily applied diem in Kent. Tectonic uplifts generating pulses of detritus can readily captain these cycles, far better than austatic movements of N & -level. Then tectonic movements resulted in the release of siisric h connate water which encouraged abundant sponge populations in Dorset and along the Wheatley Fault which separate die Oxford and Aylesbury Blocks; they also caused local uplifts such as the one which elevated the Oxford Block after Coral Rag was deposited and the one which warped the Winchester and Portsdown areas, preventing deposition of the Berkshire and Osmlagton Oollte Groups. A final tectonic movement downwarped the entire area and permitted deposition of the Kimmeridge Clay.

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