Identification of Karst Features in the Portsdown Chalk Fm. from Aerial Photography, Dorset, UK

Identification of Karst Features in the Portsdown Chalk Fm. from Aerial Photography, Dorset, UK Morena N Hammer1, Paul D Burley2, Howard D Mooers1 1Department of Earth and Environmental Sciences, University of Minnesota Duluth, Duluth, MN 2Department of Earth Sciences, University of Minnesota, Minneapolis, MN (email: [email protected]) Introduction Cranborne Chase hosts a unique geological and archaeological landscape in south central England. The Chase occupies the chalk downlands underlain by the cretaceous White Chalk Subgroup plateau that extends from the northern Chalke Escarpment, southward across the counties of Dorset, Wiltshire, and Hampshire (Hopson, 2005). In this region, there is extensive archaeological evidence supporting a large Neolithic population in southern central England from approximately 3600-3440 BC (Woodbridge et al., 2013). During the transition from the Mesolithic to the Neolithic at about 4000 BC, there was a significant change in the tools, farming technique, and cultural tradition. Though archaeologically these transitions are significant, there is little to no data available recording the environment that the Neolithic people were living in and how they influenced the landscape through cultivation and related impacts. One site that has yielded an extensive collection of animal and cultural remains during this time period is the excavation of Fir Tree Field Shaft doline (Allen & Green, 1998). Other typical data archives that would be used for paleoenvironmental reconstruction of the Neolithic period, such as lakes or peat fens, do not exist in Cranborne Chase because of the well-drained karst landscape. These karst features developed in the chalk can be excellent paleoenvironmental archives. Chalk, a form of limestone, is developed by the accumulation and lithification of calcium carbonate shells from marine organisms. For southern England, the Chalk Group was deposited during the Late Cretaceous recording various marine environments with its preserved regressive and transgressive phases (Adams B., et al., 1999; Mortimore et al., 1986; Hopson P., et al., 2011). Within the group are the Upper, Middle, and Lower Chalk Groups that comprise chalk beds of varying composition and a young unconformity overlain by the Paleogene sequence containing pyrite amongst stilts and clays. The process of dissolution of calcium carbonate by water percolation from the surface allows for the formation of karst features in chalk. These karst features were mapped to assist in the identification of potential recorders of the paleoenvironment of Neolithic Britain. Methods During the summer of 2018, there was a significant drought in south central England that gave insight to the soil and karst interaction of Cranborne Chase. The drought enhanced the identification of karst features because the topsoil was excessively dry, being drained by the underlying dissolution chalk and passageways. Satellite imagery was taken during this summer drought. Accessing this remote sensing imagery in Google Earth the karst features of Cranborne Chase were mapped. Mohammed et al. (2013) determined that the horizontal accuracy of Google Earth is within 1.80 m (relative to the study area) when comparing points to field checked locations. Similarly, the height accuracy was within 1.7 m, therefore finding that the use Google Earth is an adequate source of high-resolution imagery for preliminary studies. Mapping was performed by visual identification of vegetation color differences and cross-checked with a DEM hillshade derivative. A geodatabase was created for the polygon features including a field of the confidence level at which they were mapped, coordinates, and other calculated geometries such as the area and perimeter. The geologic bedrock map provided by the British Geological Survey was intersected with the polygons and fields for the formation at which the karst features sit in, the formation lithology, and the link at which more information on the formation can be found was added to the geodatabase. The sites that were mapped were compared to a study completed by Sperling et al. (1977) showing a significant correlation between the mapped locations. The second phase that was not completed would include ground truthing and continued cross-checking by using sink tool calculations and experimenting with topographic index tools for anomalistic depression identification. Results There were +1,700 karst features mapped in Cranborne Chase. Of these features there was a significant correlation between their density and occurrence and a defined chalk formation. Of the +1,700 features, 1,311 (varying in size) were located in the Portsdown Chalk Formation. Other dominant units included the Culver Chalk Formation (234 karst features) and the Reading Figure 1. Map of Cranborne Chase detailing the karst feature density on the Portsdown Chalk Fm. and surrounding formations. Formation (146 karst features). Other units with minor karst feature assemblages included the Poole, London Clay, and Newhaven formations. Most visible karst features were identified on steeper hillslopes and in valleys. Discussion: Geology Behind the Karst Formations Since a correlation between our mapped locations and relationship to the Portsdown Fm. exists and is paralleled to other studies, it raises the question of what is causing this to occur across one band of the Chalk Group? The Chalk Group as an entire area has vast interconnectivity between vertical and horizontal fractures, coupled with generally high permeability and porosity that allow for the movement of water through the soft, friable formations (Melville, R. V. and Freshney, E. C., 1982; Adams B., et al., 1999). The Portsdown Chalk Fm. is a part of the White Chalk Subgroup or Upper Chalk Group in south-central England. Between the Upper, Middle, and Lower Chalk Groups (youngest to oldest, respectively), the layers vertically vary in composition and thickness (Aldiss et al., 2012) and the nomenclature varies between previous research. The Portsdown Chalk Fm. differs relative to other surrounding formations by its relative lack of flint nodules or seams. The Upper Chalk in general is softer, smooth, and the nodules are sparse. The predominant features, though minimal, are marl (thin beds of clay rich chalks) seams that occur no larger than 10mm in thickness which play a major factor in permeability and fracture style through the formation (Aldiss et al., 2012). In the Portsdown Chalk Fm., these marl seams most commonly occur in the southern part of the formation and have an intermittent and unconnected layering due to inoceramid shell debris (BGS, n.d.). The northern part of the formation is more marl free. The lowest part of the formation is grainy and contains hardgrounds (due to calcite prisms of bivalve shells, etc…) but the constituents thin out moving up the formation. Here the marl beds are sparse and are instead replaced with significant nodular flint horizons, especially evident near the Isle of Wight (Hopson et al., 2011). Altogether, the Upper and Middle Chalk have the least clay minerals and lithic fragments (Adams B., et al., 1999). Reproduced with the permission of the British Geological Survey ©UKRI. All rights Reserved Figure 2. Cross-section of the Upper, Middle, and Lower Chalk Groups (Adams, B., et al., 1999). Above the Portsdown Fm. is an unconformity cause by weathering during the sub- Paleogene with the deposition of the Paleogene sequence. This sequence includes the Reading Fm. and London Clay Fm., and the Bracklesham groups of Eocene age reaching to the base of the Culver Fm. (Adams, B., 1999; Aldiss D., et al., 2012). The Paleogene altogether is the youngest sequence consisting of pyrite and fine silts and clays laid down in a shallow marine environment (Melville, R. V. and Freshney, E. C., 1982). Further sediments were developed from Pleistocene periglacial environment from freeze-thaw and Quaternary weathering of the Reading Fm. (Adams, B. et al., 1999; French, H., 2007). It should be noted that swallowholes have been observed to form in the chalk next to regions of runoff from the Reading Fm. (Adams B., et al., 1999), possibly due to the Quaternary weathering. Under the Portsdown Fm. are the older Spetisbury and Tarrant Chalk Members (often seen as both comprising the larger Culver Fm.) containing tabular flint seams throughout (Hopson et al., 2011). The Middle Chalk Group changes vertically more than the Upper Chalk Group and Lower Chalk Group. It is also seen as the first introduction of hard, fossiliferous, nodular chalks (Adams, B., 1999). Towards the middle of the group, the chalk becomes soft, but fossils are still evident with sparse flints (Aldiss et al., 2012). The Lower Chalk Group has significant marl seams and muddier constituents throughout the chalk, coining the term Marl Chalks. The Marl Chalks are much less permeable than the beds above it. The northern part of the Lower Chalk formation thins in the marl seams and becomes more massive, referred to as the Grey Chalk. Tectonics had a hand in the cretaceous sedimentation and Paleogene deposition, however in the central and southern England there is no longer active faulting in the region (Aldiss D. T., 2013). Through remote sensing it has been found that most valleys are nearly equally spaced, suggesting that they are locations of parallel joints (Adams et al., 1999). On the Isle of Wight, it has been observed that localized faulting and folding has had a syntectonic influence on the sedimentation of the Chalk, residing beds, and its capability to retain characteristic formation features (Hopson et al., 2011). Here it has been found that the marl seams are competent to residing fractured chalk, often described as influencing nearly horizontal joints sub-parallel along the seams (Hopson et al., 2011; Bloomfield et al., 2003). Increased fractures in more brittle, marl-free beds have higher water permeabilities and possibilities for karst processes to occur than the less cleanly fractured beds containing marl seams. Although soft chalks seem to have incomplete propagating fractures compared to more brittle beds, if they contain hardgrounds (as in the Portsdown) the soft chalk fractures more thoroughly.

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