No abstract
The acheulian site at Boxgrove contains one of the most extensive areas of in situ fauna and flintwork yet discovered in Britain. This material is found in a complex sequence of sediments which represent depositional conditions from a 42 m sea level rise to the onset of a full periglacial climate. Excavation of the archaeological horizon has been accompanied by a programme of multidisciplinary research examining site formation processes, palaeolandscape and palaeoecological development, using sedimentological and environmental reconstruction techniques. Dating of the site is tentative as no absolute dates are available at present. However, comparative analysis with other British sites would suggest a position for the Boxgrove sequence within the Middle Pleistocene. The archaeological horizon is interpreted as being deposited towards the latter part of an interglacial or an interstadial period.
Late-Pleistocene deposits in north County Mayo were deposited in three main glacigenic environments. 1 . Drumlins and basal tills were formed when ice moved from the Irish lowlands and local mountain catchments into Donegal Bay. 2. Gilbert-type deltas accumulated up to 80m I.O.D. on the lowlands and subaqueous moraines formed across minor valleys when marine-based ice grounded inland. 3. A thick drape of fossiliferous glaciomarine mud along the coastal fringe was deposited from meltwater plumes and by ice-rafting immediately outside of these ice limits. The muds contain an flphidium clavatum-dominated, low-diversity microfauna which is characteristic of cold-water conditions adjacent to glacier termini. Valves of Macoma calcarea from the mud have been 14C dated at 16 940 k 120 and 1 7 300 k 100 BP.The high-level delta complex was deposited from tidewater glaciers in a peripheral depression adjacent to the drumlin ice limits of north County Mayo. Although the field evidence cannot be used to determine former sea level history with any accuracy, it poses general problems for sea level history and isostatic effects of the last major ice sheet in the west of Ireland. Raised glaciomarine sequences commonly occur in close association with drumlin ice limits elsewhere in Ireland and represent marine transgressions prior to glacial unloading. It is suggested that the magnitudes and patterns of crustal depression are greater and geometrically more complex at the margins of ice sheets in Ireland than hitherto real ised.Journal of Quaternary Science
The Late Palaeocene Dir Group records the resumption of volcanic activity along the southern margin of Eurasia after a prolonged period of uplift and erosion. The group forms an integral part of the Kohistan Batholith and is readily divisible into two distinctly contrasting volcanic successions. The Baraul Banda Slate Formation comprises 2700 m of fore-arc sandstones and siltstones. The basin formed during the collapse of the Kohistan continental margin and was filled initially by subaerial debris and mass-flow deposits. Subsidence was rapid such that >900f the sedimentary record consists of thin-bedded sheet turbidites. These maintain a remarkable lateral continuity and indicate deposition in a restricted deep-water environment. Rare interbedded limestones have yielded a Thanetian (60.2–54.9 Ma) marine fauna of Miscellanea miscella and Actinosiphon tibeticus . In direct contrast the Utror Volcanic Formation comprises 3000 m of volcaniclastic sedimentary material, lava flows and ignimbrites. Eruptive style was predominantly explosive with fragmental material dominating the volcanic record. Lavas range in composition from 53% to 79% SiO 2 with rhyolite the predominant rock type. A basaltic andesite has an 40 Ar- 39 Ar age of 55 ± 2 Ma. Although the volcanic stratigraphy is complex preliminary investigations suggest accumulation in a predominantly subaerial ring-plain or flanking facies distal to the main focus of volcanic activity. Sedimentary thickness and the abundance of silicic lavas and pyroclastic flows imply extensional tectonic control even though the arc developed at a convergent margin. Juxtaposition of the two facies along the Dir Thrust excludes any record of shore-line or coastal processes from the volcanic stratigraphy. The inferred original geographical separation of the two environments coupled with their relationships to isotopically dated plutonic suites reveals that between Late Cretaceous and Early Eocene times the locus of magmatic activity within the Kohistan arc migrated progressively northward. The Shamran volcanic rocks although originally correlated with the island-arc Chalt Volcanic Group are now identified on the basis of a 58 ± 1 Ma ( 40 Ar- 39 Ar) age as a northern continuation of the Late Palaeocene volcanic arc.
SUMMARY Results from modern and ancient sediments suggest that a boron‐illite equilibrium, reflecting salinity, is established at the time of deposition. This condition is not appreciably changed by subsequent processes of diagenesis and lithification. Equivalent boron in thirteen samples, from two augerholes through Dovey Estuary sediments, ranges between 260 p.p.m. and 370 p.p.m. and averages 336 ± 29 p.p.m. (95% confidence limits). Salinity conditions in the present estuary vary greatly. A very significant direct correlation exists between equivalent boron and inferred depositional salinity of Dovey Estuary sediments.
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