Difference between revisions of "OR/19/024 About the buried valley suite of data"
(Created page with "__notoc__ ==BACKGROUND== Buried valleys are ancient sub-aerial (river) or subglacial (beneath a glacier) drainage networks that are now abandoned and have become either partly...")
Revision as of 09:30, 8 January 2020
Buried valleys are ancient sub-aerial (river) or subglacial (beneath a glacier) drainage networks that are now abandoned and have become either partly or completely buried by more recent sediment. As such, buried valleys often exhibit little or no surface expression within the modern landscape.
The concealed occurrence of buried valleys can have significant and often unexpected implications for groundwater, hydrocarbon and geothermal resources. Buried valleys can also be significant stores of sand and gravel mineral resources which can act as traps for contaminants as well as pathways into groundwater aquifers.
The British Geological Survey (BGS) has recognised and identified buried valleys through its survey activities since the 1870’s (Mellard Reade, 1873), although no systematic GB dataset has ever been produced. Since the 2000’s BGS has published a Superficial Deposits Thickness Model (SDTM) which models variation in thickness of natural largely unconsolidated deposits that cover much of the bedrock across Great Britain (Lawley and Garcia-Bajo, 2009Cite error: Invalid
refs with no name must have content). Kearsey et al., (2018)Cite error: Invalid
refs with no name must have content recognised a key limitation of the SDTM methodology was that it under-represented the spatial occurrence of linear features such as buried valleys.
The British Geological Survey has initiated the development of the Quaternary Heterogeneities Program, which develops datasets that describe the distribution of non-uniform geological properties produced by geological processes that have occurred over the past 2.5 million years. The project is developing a range of datasets to consider a range of heterogeneities (defined as distinctly non-uniform characteristics). The Buried Valleys (onshore) suite of data describes one of these heterogeneities.
WHO MIGHT REQUIRE THIS DATA?
This data is for users who are seeking information about the locations of buried valleys across Great Britain.
Buried Valleys are important features for geologists, civil engineers, hydrogeologists and environmental scientists because their presence and extent is often unknown. The presence of a buried valley can often have significant and unpredictable implications for users needing to understand the elevation of the bedrock surface, the thickness of superficial deposits and the interface between the two. Equally this data will be of interest to the research community of Great Britain as buried valleys provide detailed archives of palaeoenvironmental (i.e. an environment from the geological past) and landscape change.
The dataset aims to provide users with an initial indication or where a buried valley may be present (see also 3.6 Limitations). This dataset should not be used to replace the need for detailed site investigation using boreholes or geophysical methods to help characterise the feature.
Relationship to the superficial deposits thickness model
The BGS creates the National Superficial Deposit Thickness Model (SDTM) as a licenced product (Lawley and Garcia-Bajo 2009Cite error: Invalid
refs with no name must have content). Although the Buried Valleys (onshore) data shares some of the same input data it is not meant as a replacement but compliments the existing data.
The key differences are:
- The Buried Valleys (onshore) data is presented at a coarser scale than the SDTM Model (1:250 000 compared to 1:50 000 of the SDTM).
- The all superficial deposits between 40–161 m are grouped together in the Buried Valley dataset but are in the SDTM Model.
- The Buried Valleys (onshore) data was created through an expert driven process specifically targeted at identifying buried valleys, which has removed superficial features such as drumlins and other mounds.
Please do not use this as the Buried Valley dataset as an indicator of superficial thickness. For superficial thickness please use the National Superficial Deposit Thickness Model (SDTM).
WHAT THE DATA SHOWS?
The Buried Valleys (onshore) data suite is derived from two data resources and is delivered as three separate data layers:
- Historic published buried valley centre lines (linear features);
- Historic published buried valley margins (polygons);
- Modelled Thickness of Buried Valleys (polygons).
Historic references to Buried Valleys
The two historic layers contain a compilation of published interpretations of buried valleys. This data has been gathered from 96 different publications (see Kearsey et al. 2019Cite error: Invalid
refs with no name must have content for details) which range in age from 1926–2018.
The geographic location of buried valleys identified from maps and diagrams in printed historic sources were used to create two data layers. Identified buried valley features were captured either using a centre line of the mapped feature (lines) or the margins of recorded feature (polygons).
Modelled Thickness of Buried Valleys
An alternative method which may recognise potential areas for the occurrence of buried valleys has been devised. This method identifies significant areas of superficial thickening from boreholes. A semi-automated interpretation method has been established to identify areas of significant superficial thickening. Where the centre of the feature contains >20m of superficial deposits. The source data is based on the current BGS onshore borehole dataset. This new data was created as the BGS’s digital borehole database postdates the majority of the historical references to buried valley references. The resulting methodology may be able to identify, and resolve more buried valleys when compared to those discovered through traditional geological mapping activities.