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Karst Mapping Density in the Portsdown Chalk Formation

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Karst Mapping Density in the Portsdown Chalk Formation KARST MAPPING DENSITY IN THE PORTSDOWN CHALK FORMATION M O R E N A N H A M M E R 1 , PA U L D BURLEY 2 , H O WA R D D MOOERS 1 1Department of Earth and Environmental Sciences, University of Minnesota Duluth, Duluth, MN, 2Department of Earth Sciences, University of Minnesota, Minneapolis, MN Cranborne Chase hosts a unique geological and archaeological landscape in Satellite imagery was taken during the summer drought and by accessing this The Portsdown Chalk Formation differs relative to other surrounding south-central England. The Chase occupies chalk downlands underlain by the remotely sensed imagery in Google Earth the karst features were mapped formations by its relative lack of flint nodules or seams. The Upper Chalk in cretaceous Chalk Group plateau that extends from the northern Chalk manually as Keyhole Markup language Zipped (KMZ) files. Preliminary general is softer, smooth, and the nodules are sparse. The predominant Escarpment, southward across the counties of Dorset, Wiltshire, and Hampshire mapping of karst features was compared to other studies with similar features, though minimal, are marl seams (thin beds of clay rich chalks) that (Hopson, 2005). There is extensive archaeological evidence supporting a large features in previously mapped locations. occur no larger than 10mm in thickness which play a major factor in Neolithic population in south-central England from approximately 3600-3440 The KMZ data was imported to an ArcGIS Pro geodatabase. Fields were permeability and fracture style through the formation (Aldiss et al., 2012). developed for the confidence level at which they were mapped, coordinates, BC (Woodbridge et al., 2013) exhibiting extensive use of the chalk bedrock. This These marl seams most commonly occur in the southern part of the and other calculated geometries such as the area and perimeter. A geologic relationship was discovered at Fir Tree Field Shaft (doline identified by crop formation and have an intermittent and unconnected layering due to bedrock map provided by the British Geological Survey was intersected with field drought), which holds animal and cultural artifacts, providing a depth and inoceramid shell debris (BGS, n.d.). The marl seams are impermeable to the polygons. Fields for the formation at which the karst features sit in and time archaeologic relationship (Allen & Green, 1998). However, little to no data residing fractured chalk, often described as influencing the formation of the formation lithology was added to the geodatabase. beyond this shaft is available recording the environment that the Neolithic horizontal joints sub-parallel along the seams (Hopson et al., 2011; people were living in and how they influenced the landscape through Results Bloomfield et al., 2003). Increased fractures allow higher water cultivation and related impacts relative to the well-drained karst landscape. permeabilities and possibilities for karst processes to occur. Although soft These karst features can be potential recorders of the paleoenvironment. +1,700 karst features mapped in the Cranborne Chase. There is significant correlation between karst feature density, or occurrence, and a defined chalks seem to have incomplete propagating fractures compared to more chalk formation: 1,311 located in the Portsdown Chalk Formation; Culver brittle beds, if containing hardgrounds (which the Portsdown contains) the Chalk Formation, 234 features; Reading Formation, 146 features; and soft chalk fractures more cleanly through the bed (Adams B., et al., 1999). other units with minor assemblages included the Poole, London Clay, and However, the above factors may only be influencing the karst formation Newhaven Formations. locally because of the horizontal and vertical variability of the Chalk Group. Near the Palaeogene sequence, karst features have been identified Figure 4. Map of extensively and within local synclines (Adams, B., 1999). In many locations of the distribution the Palaeogene pyrite has been noted in unweathered locations and gypsum of karst features forming in highly weathered locations. Reproduced with the permission of the British Geological Survey ©UKRI. All rights Reserved to the geologic Figure1. Cross sectional view of the Chalk Group relative to topographic features (Adams B., et al., 1999). bedrock. Conclusions Chalk, a form of limestone, is developed by the accumulation and lithification Pyrite weathering is a major influence on the acidity of the groundwater of calcium carbonate shells from marine organisms. The Chalk Group holds and fracture dissolution as it produces sulpheric acid. When combined records of various marine environments with its preserved mud seams with calcium carbonate it produces gypsum, which has been observed in produced by regressive and transgressive phases (Adams B., et al., 1999; the Palaeogene beds. Solution pipes in the beds are allowing the sulphuric Mortimore et al., 1986; Hopson P., et al., 2011). Within the group are the acid to reach the unsaturated zone above and below the water table. The Upper, Middle, and Lower Chalk Groups that comprise chalk beds of varying acidic recharge in the Portsdown induces dissolution across the fracture composition and a young unconformity overlain by the Palaeogene sequence systems connected to the aquifer, producing the potential dolines and containing pyrite among stilts and clays. The process of dissolution of calcium preferred flow pathways that we are observing in our study area (Adams, carbonate by acidic water percolation from the surface allows for the B., et al., 1999; Aldiss, D., personal communication, November 30th, 2019). formation of karst features in chalk. These karst features were mapped to A database is now available holding the mapped locations of karst features assist in the identification of potential recorders of the paleoenvironment. that could be further assessed as potential paleoenvironmental recorders. This database was created with the intent of providing geographic Mapping Karst Feature Distribution Figure 5. Distribution of assistance to further archaeological and paleoenvironmental During the summer of 2018, there was a significant drought giving insight to 1400 1200 karst features reconstruction of south-central England. the soil and karst interaction. The drought enhanced the identification of karst 1000 within the 800 features because the topsoil was excessively drained by the underlying 600 relevant References: 400 formations. Adams, B., Buckley, D., Downing, R., Edmunds, W., Ellis, J., Headworth, H., Jones, H., Lowings, V., MacDonald, A., Mortimore, R., Robins, N., Shephard- dissolution chalk and passageways. 200 Thorn, E., and Stuart, M. (1999). The Chalk aquifer of the South Downs. Hydrogeological Report Series of the British Geological Survey. 0 Portsdown Culver Chalk Reading Newhaven Poole London Clay Other http://nora.nerc.ac.uk/id/eprint/12713/1/SD99001.pdf Chalk Chalk Aldiss, D. T. (November 30th, 2019). Email personal communication. Aldiss, D.T., Farrant, A., and Hopson, P. (2012). Geological Mapping of the Late Cretaceous Chalk Group of southern England: A specialized application of landform interpretation. Proc. Geol. Assoc. doi: http://dx.doi.org/10.1016/j.pgeola.2012.06.005 Discussion: The Density and Underlying Geology Allen, M. J. and Green, M. (1998). The Fir Tree Field shaft; the date and archaeological and paleoenvironmental potential of a chalk swallowhole feature. Proc Dorset Natur Hist Archaeol Soc 120. Vol 12, pp 25-38. The Chalk Group as an entire area has vast interconnectivity between vertical BGS. (n.d.). The BGS Lexicon of Named Rock Units. Retrieved November 15, 2019, from https://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=WHCK. and horizontal fractures, coupled with generally high permeability and porosity Bloomfield, J.P., Butler, A.P., Cobbing, J.E., Gallagher, A.J., Griffiths, K.J., Moreau, M., Williams, A.T., Peach, D., & Binley, A. (2003). Flow heterogeneity in the fractured Chalk aquifer of southern England. http://nora.nerc.ac.uk/id/eprint/7764/1/Extended%20abstract%20%28No%2028%29.pdf that allow for the movement of water through the soft, friable formations Cranfield University, UK. (2019). The Soils Guide. Retrieved November 20, 2019 from https://www.landis.org.uk. French, H. (2007). The periglacial Environment. Third ed. doi: 10.1002/9781118684931 (Melville, R. V. and Freshney, E. C., 1982; Adams B., et al., 1999). The Portsdown Google earth V 7.3.2.5776 (2018). Dorset, UK. 50°57’23.93”N, 1°55’58.17”W Eye Alt 1.19km and 50°42’32.33”N, 2°24’44.19”W Eye Alt 1.90km. Chalk Formation (containing the most karst features) is a part of the Upper Google 2019, http://www.earth.google.com. (November 15th, 2019). Hopson, P. (2005) A stratigraphical framework for the Upper Cretaceous Chalk of England and Scotland with statements on the Chalk of Northern Chalk Group. Between the Upper, Middle, and Lower Chalk Groups, the layers Ireland and the UK offshore sector. Keyworth, Nottingham, British Geological Survey. http://nora.nerc.ac.uk/id/eprint/3230/1/RR05001.pd Melville, R. V. and Freshney, E. C. (1982). Hampshire Basin and adjoining areas. London: H.M. Stationery Office vertically and horizontally vary in composition and thickness (Aldiss et al., Mohammed, N., Ghazi, A., and Mustafa, E. (2013). Positional Accuracy Testing of Google Earth. International Journal of Multidisciplinary Sciences and Figure 2. Imagery from the Portsdown Figure 3. Imagery of karst features in the 2012). Engineering, Vol 4, No 6. http://www.ijmse.org/Volume4/Issue6/paper2.pdf Formation in the study area of Sperling et al. Cranborne Chase study area (Google, 2018). Woodbridge, J., Fyfe, R., Roberts, N., Downey, S., Edinborough, K., and Shennan, S. (2013). The impact of the Neolithic agricultural transition in Britain: (1977) highliting similar karst features (Google, A comparison of pollen-based land-cover and archaeological 14C date-inferred population change. Journal of Archaeological Science. doi: 10.1016/j.as.2012.10.025 2018)..
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