Wealden petrology : The Top Ashdown Pebble Bed and the Top Ashdown Sandstone

The Top Ashdown Pebble Bed occurs immediately below the Wadhurst Clay, averages three to four inches in thickness and is well exposed over an area of about 700 square miles in Sussex, Kent and Surrey. It is thus well suited to areal petrological study, being free from the invalidating influences of vertical variation. The “Top Ashdown Sandstone” is an arbitrary horizon situated four inches beneath the pebble bed, and designed primarily as a check. The Wealden strata of the Weald contain thin, yet widespread, horizons similar to the Top Pebble Bed. Detailed petrological studies of them have been undertaken. Where possible, all information, petrological and other, is expressed quantitatively. In no case is the assumption made that sampling information necessarily represents geological reality. Sampling data are linked with the unknowable truth, which they estimate, only through probability statements based on objective statistical analysis. Throughout the paper, the chances that the author may be talking nonsense are given for most alleged facts and conclusions. The methods of modern statistics have also facilitated a more precise inquiry into sedimentary variation. The amount of variation shown by a particular sedimentary characteristic is as much an attribute of that sediment as the magnitude of the characteristic itself. In particular, the measurement and comparison of the covariation between particle-size and frequency have led to new facts and conclusions. Three major sources supplied detritus during the Ashdown episodes described. Their drainage systems entered at the north-eastern, south-western and northwestern margins of the area respectively. Each contributed pebbles of quartz; grains of quartz, muscovite, black iron ore, zircon, rutile and tourmaline ; and clay ; plus certain characteristic particles. Garnet and apatite were practically confined to the north-eastern inflow. The latter also possibly supplied much glauconite and minute proportions of staurolite, kyanite, sillimanite, corundum, topaz and stilpnomelane. The source is identified with a “North Sea-Ardennes massif”, exposing mostly sedimentary rocks of “Upper Carboniferous” type. Staurolite, kyanite and sillimanite entered mostly by the south-western inflow, chiefly as very much larger grains than the garnets and apatites, and accompanied perhaps by the bulk of the monazites and purple zircons. The source is identified with “Armorica”, exposing either metamorphic rocks and/or sediments of “Lower Greensand” type derived therefrom. Chert pebbles and ultra-fine detritals came chiefly from the north and north-west, and appear to have been accompanied by rare pink quartzite pebbles and fluorspar grains. The source is identified with the narrower western (“London”) end of the “North Sea-Ardennes massif”, exposing sedimentary rocks of “Jurassic” type. During Top Sandstone times the north-eastern inflow contributed roughly between 47 and 73 per cent more of the sediment than the south-western inflow. Later, the position was reversed. Throughout, the biggest pebbles and finest silt came from the north and north-west, and the coarsest sand from the south-west. As time went on, all three contributions increased in coarseness. North-east and south-west currents predominated, being similar in alignment to the later Wadhurst bone-beds, eroded soil-bed margins, sandstone bands, etc. They set up characteristic particle-size and frequency patterns, and scoured shallow grooves across the area, floored with coarser sediment. The water probably nowhere exceeded 24 feet in depth and was shallower in the later episode. Despite the regional patterns set up by the inflows, much of the variability in “heavy” particle-size and composition comprised small-scale patchiness. This was doubtless due to the maze of minute currents superimposed on the broad pattern. Regional changes in frequency formed higher proportions of the total variation only among those “dependent” species characterizing certain inflows. Each horizon now shows sufficient variation to account for all the “characteristic suites” which can be formulated for the entire thickness of Hastings Beds. Deposition, at first relatively rapid, came to a standstill as the increasingly coarse sediment neared water-level. Formation of the non-sequential Top Pebble Bed probably involved much decomposition of glauconite and biotite. These processes, being most intense on the northern margin of the area, may indicate that the beds were built up southwards. Both horizons are considered to represent environmental facies of one and the same youthful deltaic complex, spreading into a large subsidence-lake in the back area of the Anglo-French delta-plain. The overlying Wadhurst Clay is thought to represent the “maturity” and “old age” of the complex. The whole sequence of Wealden beds comprises pulses and cycles essentially similar to the Ashdown-Wadhurst sedimentary cycle. Each is apparently equivalent to a single delta. In a similar manner the petrography of the Wealden as a whole can only be understood in terms of the histories of the three inflows which built the deltas. Detailed study of several beds like the present enables one to observe the losing battle of the north-eastern inflow, the mounting but short-lived supremacy of “Hayward's inflow” from the north-west, and the final, but long-delayed, Aptian triumph of the south-western inflow. Post-depositional changes of the detritals now present were slight. Authigenesis of certain titanium minerals was proved. The particle-size/frequency covariation exhibited by certain minerals promises an exact test of the applicability of Boswell's suggestion that certain unstable minerals may owe their persistence, in like circumstances, to protection by overhead impermeable cover.