Dieses Thema hat Marian / „gnoisi“ bereits sehr ausführlich auf seiner Website abgehandelt:
http://www.petrified-wood.de/fundregionen/gb_dorset_purbeck.htm Ich werde dazu hier also nur wenige ergänzende Links, Literaturangaben und sonstige Informationen bringen.
Die Sedimente der Purbeck-Gruppe überspannen die Wende vom obersten Jura zur unteren Kreide. Bis vor kurzem wurde noch von den „Purbeck beds“ oder einer Purbeck Formation gesprochen, der aktuelle Status bzw. die aktuelle Bezeichnung ist aber „Purbeck Gruppe“. Diese beinhaltet ihrerseits drei Formationen – und zwar die Lulworth Formation und die darüber liegende Durlston Formation im Süden, im Wessex-Becken. Eine dritte, die darunter liegende Haddenham Formation, ist nur nördlich davon identifiziert.
Die beiden „Dirt Beds“ (Paläosole) mit den Kieselhölzern – auch als „basales Purbeck“ bezeichnet – gehören dabei noch in den obersten Jura (Spätes Tithonium). Früher müssen bei Steinbrucharbeiten aus diesem Paläosol massenhaft Kieselhölzer – und zwar lange Stämme in bester Erhaltung, vereinzelt sogar zu fast kompletten Bäumen zusammensetzbar – zutage gekommen sein. Eine sehr eindrückliche Beschreibung dieses (echten) „Fossil Forest“ mit teilweise aufrecht in originaler Wuchsposition stehenden Stümpfen von Cycadeoidea und Coniferophyta samt Wurzelhorizont gibt bereits
Lyell 1871.
Zitat aus Lyell 1871, 331 ff:
„ Dirt-bed or ancient Surface-soil.—The most remarkable of all the varied succession of beds enumerated in the above list is that called by the quarrymen “the dirt,” or “black dirt,” which was evidently an ancient vegetable soil. It is from 12 to 18 inches thick, is of a dark brown or black colour, and contains a large proportion of earthy lignite.
[….]
I also saw in 1866, in Portland, a smaller dirt-bed six feet below the principal one, six inches thick, consisting of brown earth with upright Cycads of the same species, Mantellia nidiformis, as those found in the upper bed, but no Coniferae.
[….]
Many silicified trunks of coniferous trees, and the remains of plants allied to Zamia and Cycas, are buried in this dirt-bed, and must have become fossil on the spots where they grew. The stumps of the trees stand erect for a height of from one to three feet, and even in one instance to six feet, with their roots attached to the soil at about the same distances from one another as the trees in a modern forest. The carbonaceous matter is most abundant immediately around the stumps, and round the remains of fossil Cycadeae.
Besides the upright stumps above mentioned, the dirt-bed contains the stems of silicified trees laid prostrate. These are partly sunk into the black earth, and partly enveloped by a calcareous slate which covers the dirt-bed. The fragments of the prostrate trees are rarely more than three or four feet in length; but by joining many of them together, trunks have been restored, having a length from the root to the branches of from 20 to 23 feet, the stems being undivided for 17 or 20 feet, and then forked. The diameter of these near the root is about one foot; but I measured one myself, in 1866, which was 3½ feet in diameter, said by the quarrymen to be unusually large. Root-shaped cavities were observed by Professor Henslow to descend from the bottom of the dirt-bed into the subjacent fresh-water stone, which, though now solid, must have been in a soft and penetrable state when the trees grew.
[….]
The dirt-bed is by no means confined to the island of Portland, where it has been most carefully studied, but is seen in the same relative position in the cliffs east of Lulworth Cove, in Dorsetshire, where, as the strata have been disturbed, and are now inclined at an angle of 45°, the stumps of the trees are also inclined at the same angle in an opposite direction—a beautiful illustration of a change in the position of beds originally horizontal.”
(siehe dazu Fig. 308–310 in untenstehendem Link)
Vereinzelt wurden solche jurassischen Kieselholzstämme aber auch in Gebieten, in welchen die Purbeck-Gesteine von jüngeren Sedimenten überdeckt sind, an der Küste angespült, so z.B. bei Bournemouth. Siehe:
http://www.southampton.ac.uk/~imw/jpg-Portland/9PT-Portland-Branksome-Tree.jpgIn einem westlichen Bereich der Purbeck Gruppe wurden im gleichen Horizont auch verkieselte Samen, Zapfen und Schachtelhalmreste gefunden (
Barker et al. 1975).
Links:
Zur Purbeck-Gruppe:
http://en.wikipedia.org/wiki/Purbeck_Grouphttp://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=PB http://www.tinkerbell.ukfsn.org/pur-strata.html http://www.southampton.ac.uk/~imw/purbfac.htm Zum “Fossil Forest”:http://www.southampton.ac.uk/~imw/Fossil-Forest.htm http://www.southampton.ac.uk/~imw/Fossil-Forest-Purbeck-Trees.htm Literatur:
• Barker et al. 1975: D. Barker, C. E. Brown, S. C. Bugg & J. Costin, Ostracods, land plants and charales from the basal Purbeck Beds of Portesham quarry, Dorset. Palaeontology, Vol. 18, Part 2, 1975, pp. 419–436.
http://palaeontology.palass-pubs.org/pdf/Vol%2018/Pages%20419-436.pdf• Lyell 1871: Charles Lyell, The Student's Elements of Geology, New York 1871.
Online: http://geology.com/publications/lyell/ch19.shtml Als pdf: https://archive.org/details/1871studentsel00lyeluoft