|Palamakumbura, R. 2018. A new palaeogeographic model for the post-glacial marine and estuarine sediments of the Firth of the Forth, Scotland. Nottingham, UK, British geological Survey. (OR/18/016).|
This report aims to review and assess our current understanding of the sedimentary post-glacial infill of the Firth of Forth during and after the Late Devensian. The Firth of Forth has been affected by numerous glacial cycles during the Pleistocene, which has resulted in multiple glaciations, dynamic sea-level changes, isotatic rebound and a large variety of glacial and post-glacial marine and non-marine sediments (Sissons, 1975).
This report is in two parts, firstly, an overview of what is currently understood in terms of the Late Devensian sedimentary and geomorphological evolution of the Firth of Forth; and secondly, an assessment of this model based upon currently available superficial geological maps, geotechnical borehole records and topographic height data. The overall aim of this work is to understand the three-dimensional spatial distribution of superficial sediments across the Firth of Forth.
Numerous aspects of the Late Devensian geological evolution of the Firth of Forth have been the focus of various studies over the last half century. This work provides comprehensive overview of geological evolution of the entire glacial estuary system over the past 26 000 year BP. The large database of borehole records (over 8000 records) and site investigations available in the BGS records is used to determine the composition and 3D spatial distribution of the superficial sediments in the Firth of Forth to assess our current understanding of the Firth of Forth depositional system.
The Firth of Forth sedimentary and geomorphological evolution was controlled by the interaction of glacial, isotatic, eustatic sea-level and climatic processes of varying extents during the Late Devensian time.
The Late Devensian represents a relatively short time period with dynamic glacial-interglacial cycles controlling glacial events, eustatic sea-level and local climate. The interaction of these global and regional-scale processes has a strong control of the local sedimentary environments across the Firth of Forth. The impacts of these processes have a major control of the composition, spatial distribution and geotechnical properties of the superficial sediments within the Firth of Forth.
To understand the subsurface superficial geology of the Firth of Forth a number of datasets has been used including: 1:50 000 superficial (drift) maps, over 8000 borehole records and topographic datasets. To assess and analyse datasets a number of software packages have been applied including ARC GIS, BGS Groundhog and GeoVisionary.
Geological event model of the Firth of Forth
The first part of this work is to determine the major geological events that are thought to have affected the Firth of Forth during the Late Devensian are summarized in Table 1 shows the various regional geological events, the regional consequences, and the local features or deposits.
|1||Devensian glaciation||Ice cover/ice streaming||Till deposition||(Browne, Gould, & Akhurst, 2015)|
|1||Sea-level incursion||Flooding of landmass||Deposition of Errol Clay Beds||(Peacock, 2003)|
|2||Melting/collapse of the Dimlington ice sheet||Isostatic uplift||‘Main Perth Shoreline’||(Sissons & Smith, 1965)|
|3||Sea-level fall||Sea-level regression||Deposition of deltaic deposits||(Sissons & Smith, 1965); (Sissons, 1969)|
|4||Fully deglaciated during Windermere Interstadial||Fluvial/estuarine deposition||Largo Bay Member, Forth Formation (offshore) — No onshore evidence||(Gatliff, 1994; Browne, Gould, & Akhurst, 2015)|
|5||Loch Lomond re-advance||Ice rafting||Bothkennar Gravel deposition||(Peacock, 1998)|
|6||Holocene sea-level low stand||Fluvial/estuarine deposition||Claret Formation deposition||(Barras & Paul, 1999)|
|7||Holocene sea-level high stand||Sea-level transgression||Deposition of Holocene raised marine deposit forming the ‘Main Postglacial Shoreline’||(Cullingford, 1991)|
|8||Holocene sea-level fall to present level||Sea-level regression||Beach deposit overlies raised marine deposit||(Browne, Gould, & Akhurst, 2015)|
The Devensian deglaciation is represented by events one to three in Table 1. The Devensian glaciation is the oldest glacial event to have sediment preserved in the Firth of Forth and is represented by the Wilderness Till deposit (Sissons, 1975). The subsequent deglaciation is represented by a marine deposit (Peacock, 2003), an isostatically uplifted shoreline (Sissons & Smith, 1965) and overlying deltaic deposits (Sissons, 1969) (Table 1).
The Windermere interglacial is represented by event 4 in Table 1, which is represented by a silty clay infill of the Firth of Forth (Gatliff, 1994). The nature of the Windermere interglacial-aged estuarine infill of the Firth of Forth is speculative as there is no exposed section (Gatliff, 1994).
Loch Lomond re-advance
The Loch Lomond re-advance is represented by event 5 in Table 1. (Peacock, 2003) described a ‘buried gravel’ layer that is hypothesised to extend from Grangemouth to Stirling. This deposit was interpreted as an ice raft deposit associated with the Loch Lomond re-advance.
Three major events have been previously described to have affected the Firth of Forth during the Holocene (events 6 to 8 in Table 1). Firstly, the Claret Formation which is the Holocene-aged estuarine infill (Barras & Paul, 1999). Secondly, a major marine transgression is described that results in the ‘Main Postglacial Shoreline’ (Cullingford, 1991). Finally, a beach deposit is described along the peripheries of the distal part of the Firth of Forth (Gatliff, 1994).
- SISSONS, J B. 1975. The Quaternary in Scotland: a review. Scottish Journal of Geology, Vol. 10, 311–337.
- BROWNE, M A E, GOULD, D, and AKHURST, M C. 2015 In press. Geology of the Edinburgh district, Sheet description for the British Geological Survey, 1:50 000 Series Sheet 32E Edinburgh (Scotland). (Keyworth, Nottingham: British Geological Survey.)
- PEACOCK, J. 2003. Late Devensian marine deposits (Errol Clay Formation) at the Gallowflat Claypit, eastern Scotland: new evidence for the timing of ice recession in the Tay Estuary. Scottish Journal of Geology, Vol. 39, 1–10.
- SISSONS, J, and SMITH, D. 1965. VIII — Raised Shorelines Associated with the Perth Readvance in the Forth Valley and their Relation to Glacial Isostasy. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, Vol. 66, 143–168.
- SISSONS, J. 1969. Drift stratigraphy and buried morphological features in the Grangemouth- Falkirk-Airth area, central Scotland. Transactions of the Institute of British Geographers, Vol. 37, 19–50.
- GATLIFF, R. 1994. The geology of the central North Sea. No. 5. Memoir of the British Geological Survey, DN05.
- PEACOCK, J. 1998. The Bothkennar Gravel Formation (‘buried gravel layer’) of the Forth Estuary. Scottish Journal of Geology, Vol. 34, 1–5.
- BARRAS, B F, and PAUL, M A. 1999. Sedimentology and depositional history of the Claret Formation (‘carse clay’) at Bothkennar, near Grangemouth. Scottish Journal of Geology, Vol. 35, 131–143.
- CULLINGFORD, R A, SMITH, D E, and FIRTH, C R. 1991. The altitude and age of the main postglacial shoreline in Eastern Scotland. Quaternary International, Vol. 9, 39–52.