Bembridge Limestone Formation
The Bembridge Limestone Formation is a geological formation in the Hampshire Basin in southern England. It is the middle formation of the Solent group and was deposited in the Upper Eocene .
Type locality and occurrence
The Bembridge Limestone Formation, often just Bembridge Limestone , was named after the village of Bembridge on the east coast of the Isle of Wight . The type locality of the formation is in close proximity to Whitecliff Bay . The formation is open to the north of the Isle of Wight, for example at Cowes ( Gurnard ), at Ryde ( Binstead ) and southeast of Yarmouth . The formation here forms the frame of the Bouldnor Syncline , an east-southeast trending open syncline .
The Bembridge Limestone Formation, like the overlying Bouldnor Formation, is restricted to the Isle of Wight.
history
The first scientific descriptions and finds go back to the 19th century , e.g. B. Samuel Peace Pratt in 1831 and Richard Owen in 1846. They were followed by J. Levèvre in 1880 and EM Reid and MEJ Chandler in 1926 as relatively early descriptors. A scientific definition as formation was made in 1985 by AN Insole & B. Daley.
stratigraphy
The small Bembridge Limestone Formation, reaching a maximum of 10 to 11 meters, is the middle of the three formations of the Solent Group . It follows concordantly on marl of the underlying Headon Hill Formation . It ends with dry cracks that are concordantly covered by dark green marls of the Bembridge-Marls member of the Bouldnor Formation .
The Bembridge Limestone Formation consists mainly of light yellow to cream-colored limestone and marl that was deposited in freshwater as lacustrine facies. Subordinate also occur clays and sands and conglomerate - and Schill locations.
The Bembridge Limestone formation has no layer members ( Engl. Member ), but can still be divided into a lower and an upper Kalklage with a separating, green and black, siliciclastic pitch. The clay separation layer is gaining importance in the north of the syncline.
Lithological sequence
The lithological sequence at the type locality in the east of the island is as follows (from hanging to lying):
- Upper limestone band
- Separation tone
- Lower limestone band
Lower limestone band
The 5.5-meter-thick Lower Limestone Band is divided into two parts by an intraclast conglomerate : a 1.80-meter-thick lower section consisting of lacustrine limestone with solution structures and shell impressions of Lymnaea , followed by a 3.70-meter-thick section with initial marls and limestone marls of lacustrine limestone with shell impressions of Lymnaea and cavities filled with calcite .
Separation tone
This brackish separating layer is 1.50 meters thick and begins with a lacustrine marl layer that is full of shell remains and bone fragments . Lagunar, gray clays with broken shell remains of Corbicula obovata , Ptychopotamides vagus and Melanoida acuta lie on top . The separating layer closes with gray clays and sands with ripple marks .
Upper limestone band
The 3.75-meter-thick, lighter-looking upper limestone band consists of continuous lacustrine limestone and limestone marl, which can be divided into four layer packages by coarser clastic horizons. The first shift package carries intraclasts at the base and contains lymph nodes. In addition to Lymnaeen, the second package contains shell impressions and hydrobes . The third is characterized by lymph and planorbinen as well as by shell impressions, cavities filled with calcite and intraclast and smaller cavities. The top layer package is again separated by an intra-clast horizon.
Fossil content
At invertebrates the Bembridge Limestone formation leads many Gastropodentaxa as Filholia elliptica , Helix occlusa , Hydrobia , Lymnaea longiscata , Melania orbicularis , Melania turritissima , Melanoida acuta , Paludina globuloides , Planorbis discus , Planorbis oligyratus , Planorbis obtusus and viviparus lentus , Lamellibranchia as Corbicula obovata , Polymesoda convexa and Sinodia suborbicularis . Also ostracods are present, for example Cladarocythere apostolescui .
In the sediment there are also many seed pods (gyrogonites) of Charophytes of the species Chara tuberculata , Chara wrightii and others. There are also palm remains (taxon Flabellaria lamanonis ).
Among the reptiles , the shell remains of the turtle Trionyx should be mentioned.
Mammals were found very early, as mentioned above, mainly teeth came to light. The following taxa have become known so far:
Amphiperatherium , Anoplotherium commune , Anoplotherium secundarium , Audilophus radegondensis , Chaeropotamus cuvieri , Dichobune cervinum , Palaeotherium with the Taxa curtum , crassum , duvalii , magnum , medium , minimum and minus as well as Pterodon dasyuroides .
Cyclicality
In 1990 B. Daley & N. Edwards were the first to recognize the cyclicity underlying sedimentation in the Bembridge Limestone Formation. Based on the type locality, they differentiated 4 cycle stages (from hanging to lying), which can be characterized as follows:
- Pedagogical alteration zone
- well solidified biomicrites of a fairly high degree of purity
- Clays, marls and marl limestone
- Intraformal calcareous conglomerate bound to an erosion surface.
Armenteros et al. (1997) showed 7 cycle stages, whereby they refined the micrite stage in 4 cycles. Using sequence stratigraphy, they interpreted these cycles as a superordinate transgression / regression cycle that, when fully developed, went through the stages of flooding (with accompanying erosion and formation of an intraformational, intraclastic calcareous conglomerate), initial deepening of the basin, gradual flattening and finally emergence with limited weathering and soil formation.
They suspected changes in sea level or climatic fluctuations as the causes of this cyclicality . A third possibility is of course pulse-like, tectonic movements , which have also been proven for the Hampshire Basin in the Paleogene.
Facies
Overall, three different types of facies can be identified in the Bembridge Limestone Formation :
- lacustrine facies
- palustrine facies
- gypsum-containing sea edge facies.
Lacustrine facies
The lacustrine facies include the marl, the marl limestone, the solidified limestone (mainly biomicrites) and the thin intraclastic limestone (intraformational limestone conglomerates). In this facies, the near-surface weathering and soil formation is only slightly pronounced, only thin, brecciated tuberous calcareous layers and small, porous cavities indicate brief emergence phases. By definition, the lacustrine facies arose in fresh water, but some layers show higher-level, brackish layers.
Palustrine facies
The palustrine facies are predominant in the west of the Isle of Wight and consist of a complex mixture of different lithologies. Including biomicrites with no signs of weathering and soil formation, limestone composed of agglomerated peloids and ooids and thin-layer crustal limestone. Overall, these lithologies indicate repeated drying out with possible pedogenic effects. The palustrine facies can be viewed as a lakustrine facies transformed by pedogenic processes. It shows the characteristics of recently and historically formed crustal limestone (English calcretes )
Sea edge facies
The sea edge facies are very limited. It is characterized by the appearance of plaster of paris . The gypsum forms small lenses in the sediment and arose secondarily after the host rock settled, most likely through the evaporation of pore water in the sediment. This also suggests the presence of highly saline solutions in the immediate vicinity, since gypsum is a relatively late precipitate.
Deposition conditions and environmental influences
The deposit area of the Bembridge Limestone Formation is likely to have been a lagoon-lacustrine basin and not a distinct inland lake. This is evident from paleontological, mineralogical and isotope findings. Northwestern Europe was located much further south in the late Eocene and lay in the fluctuating border area between the northeast trade wind belt in the south and the humid west wind zone further north, which brought climatic instability and fluctuating amounts of precipitation with it. In periods of relatively high precipitation, the water levels in the sedimentation area were relatively high and fine-grained siliciclastic sediments from the hinterland were poured out, which are now available as marl and marl limestone. In dry periods the water levels were low (er) and the basin was isolated. The sedimentation was then dominated by biogenic, evaporative and, occasionally, by pedogenic carbonate rocks. Long-term fluctuations in precipitation are confirmed by the mammalian communities found in the formation on Headon Hill and reveal alternating forest and savanna biotopes.
Age
Absolute ages for the Bembridge Limestone Formation are unknown. Biostratigraphically , the formation correlates with the mammalian biozone MP20 , magnetostratigraphically with Chron C 13n.1r . The age can thus be narrowed down roughly to the period 34.0 to 33.9 million years BP (end of the Pribonus ).
See also
swell
literature
- Daley, B. et al .: The Upper Eocene Bembridge Limestone Formation, Hampshire Basin, England . In: Gierlowski-Kordesch, E. & Kelts, KR (eds.): Lake basins through space and time . 2000.
Individual evidence
- ^ Pratt, SP: Remarks on the existence of Anoplotherium and Palaeotherium in the lower Freshwater Formation at Binstead, near Ryde in the Isle of Wight . In: Proceedings of the Geological Society of London . tape 1 , 1831, p. 239 .
- ^ Owen, R .: Description of an upper molar tooth of Dichobune cervinum, from the Eocene marl at Binstead, Isle of Wight . In: Quarterly Journal of the Geological Society of London . tape 2 , 1846, p. 420-421 .
- ↑ Armenteros, I. et al .: Lacustrine and palustrine facies in the Bembridge Limestone (late Eocene, Hampshire Basin) of the Isle of Wight, southern England . In: Palaeogeography, Palaeoclimatology, Palaeoecology . tape 128 , 1997, pp. 111-132 .
- ^ Daley, B. & Edwards, N .: The Bembridge Limestone, (Late Eocene), Isle of Wight, southern England: a stratigraphical revision . In: Tertiary Research . tape 12 , 1990, pp. 51-64 .
- ^ Daley, B. & Edwards, N .: Palaeogene warping in the Isle of Wight . In: Geological Magazine . tape 108 , 1971, p. 399-405 .
- ↑ Hooker, JJ et al .: Reconstruction of land and freshwater palaeoenvironments near the Eocene-Oligocene boundary, southern England . In: Journal of the Geological Society of London . tape 152 , 1995, pp. 449-468 .