Terrestrial Geology

 

Introduction

Quaternary scientists over last few decades and in ongoing work have done much to reveal the pattern of ice sheet retreat that once covered the British Isles. These data are shown in end moraines and other glacial landforms. Briticechrono uses a holistic approach linking the onshore and offshore geological record to constrain the style, rate and timing of decay of the last British Irish Ice Sheet. The main focus of the Terrestrial Team, led by Prof Rich Chiverrell and Postdoctoral research Dr Matt Burke (both the University of Liverpool) has been to collect sediments and rocks deposited by the last ice sheet that covered the British Isles, and use these, along with organic remains, to date (e.g. by radiocarbon analyses) the retreat of the ice sheet margins. The project will use over 200 carefully chosen sites, dating some 800 samples in order to achieve this. Terrestrial sampling strategies lies in the hands of the Transect Leaders who each have put together teams of scientists based on previous track record in the region and with samples volunteered and section by the wider research community. The terrestrial sampling will provide crucial information on the timing and rates of change across the this crucial boundary with the passage of the ice margin from marine to terrestrial near to the coastal zone and outer islands.

Work programme

The last decade has seen an unprecedented focus on mapping and recording the glacial geomorphology of Britain and Ireland aided by the availability of high quality remotely sensed data e.g. the NEXTMAP GB and Ireland Digital Terrain Models. In Briticechrono there has been little requirement for new mapping, with a few isolated exceptions for example Shetland. The collection of new geomorphological or stratigraphical data has been limited to characterising depositional contexts at key dating sites. Each transect leader compiled project sampling reporting known sites, existing quarries or sections for sampling and the specifying targets for dating based on measuring the rate of ice marginal retreat. This perhaps reflects a move from opportunistic sampling based on available exposure to identifying the geomorphological pattern of ice marginal retreat, then locating and choosing dating targets appropriately. The terrestrial campaign has been a series of land-based campaigns, led and co-ordinated centrally and conducted so that resources and manpower are optimised across all of the onshore transects. Site descriptions, sediment logging and analyses have been conducted for exposed sections, and with the field assistance from the geochonometric team, appropriate samples extracted for tephra, 14C, OSL and TCN dating. JCB (digger) hire has helped with excavation of specific sites (e.g. the Four Ashes SSSI Devensian Type Locality in Staffordshire in cooperation with Natural England).

Geophysical and geotechnical data collection

Where sediment exposures are lacking, geophysical techniques are being used to assess the sub-surface structure and stratigraphy to guide the location of boreholes and in particular on transects lacking sediment exposure in NE and NW England. Ground penetrating radar (GPR) survey is the primary tool to identify the subsurface structures across target core locations informed by the geomorphology and existing borehole data archives (BGS). We are using a high resolution GPR surveying system, GSSI Sir-3000, with 100, 200 and 400 MHz antenna with data interrogation using ground penetrating radar imaging software. The geophysical survey forms a discrete and focused (45 days) component in summer 2014 and is a precursor to borehole sampling in Autumn 2014. After the first field campaign the borehole campaign will secure materials for dating (OSL or AMS 14C) for sectors and locations lacking natural or quarry exposure. The field campaign and geophysical surveys will identify a list of borehole targets. Drilling will be accomplished using a Dando Terrier Rig operating in both percussive and rotary drill modes. A two-man drill crew will operate the rig and they will work alongside the PDRA and transect team members in the collection of borehole samples.

Types of terrestrial sampling

Optically stimulated luminescence (OSL) ages will provide a range of older-than, younger-than and coeval contexts constraining glacigenic units. An ideal situation is a glacigenic unit (i.e. till) overlying sands and gravels and which terminates downstream at a moraine with a distal sandur plain. OSL dates on the sands can be used to constrain the ice advance and maximum limit. Further OSL or 14C samples from above the glacigenic unit constrain the retreat. Field sampling has focused on lithofacies or depositional environments most conducive to bleaching or zeroing of the OSL prior to deposition. Field work has involved significant contributions from OSL leads Pros Geoff Duller and Mark Bateman, and Postdoctoral Researchers Drs Rachel Smedley and Alicia Medialdea. Field gamma spectrometry has been undertaken to calculate environmental dose rates for the OSL dating, critical for heterogeneous deposits with locally variable dose rates.

For Terrestrial Cosmogenic Nuclide (TCN) or Surface Exposure dating we are using both 10Be and 36Cl depending on the rock lithologies encountered, with 10Be in quartz being the preferred TCN concentrations are very sensitive to the factors affecting cosmic-ray flux (such as changing sample geometry as a result of post depositional movement) and TCN removal at the sample surface by erosion. Minimising the risk of poor sample selection requires sound geomorphology and a good understanding of TCN systematics. Consistency in sampling has been achieved through the participation of Geochronology Lead Dr Derek Fabel and Glasgow Postdoctoral research Dr David Small on all transect. Most of our targets are boulders (erratics or pseudoerratics) transports and eroded by the ice and deposited in stable locations. Other sample types include bedrock heavily eroded by ice, focusing on locations where we can confidence erosion has removed >2m of bedrock.

Materials suitable for measurement by AMS 14C from terrestrial sections and cores have been identified in pre- and post-glacial organic sediments/peats (e.g. kettle hole infills), providing older-than and younger-than contexts, but sites yielding both are not likely to be common. In settings where glacigenic sediments contain reworked shell fragments multiple determinations will be undertaken on such material from individual localities to identify the age range of the population; the age of the youngest material (corrected for the marine reservoir effect assuming the shells are marine in origin) will have a context constraining the age of the subsequent glacial readvance (cf. O’Cofaigh & Evans, 2007).