OR/18/016 Discussion of the major features and deposits

From Earthwise
Jump to: navigation, search
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).

Sea-level record in the Forth

Sea-level change in the Firth of the Forth is major control of sedimentation, particularly of the glaciomarine and post-glacial estuarine systems. Relative sea-level is controlled by an interplay of both glacial isostatic adjustment and global eustasic sea-level rise. In general, in the Firth of Forth post-LGM the rate of glacial isostatic rebound and eustatic sea-level rise result in fall in sea-level after the LGM (Boulton, Peacock, & Sutherland, 1991[1]; Shennan, et al., 2006[2]; Lambeck, 1991[3]; Peltier & Fairbanks, 2006[4]). The Devensian and Loch Lomand glaciations resulted in relatively short-lived deviations from the overall falling sea-level trend.

According to the model proposed by Shennan et al. (2006)[2], sea level in the Firth of the Forth post-LGM was between 15 and 45 m above modern sea-level, depending on the thickness of the modelled elastic lithosphere of between 96 and 71 km thick. This work also uses a number of sea-level index points from younger deposits and features to interpolate a sea-level curve for the Late Devensian, suggesting post-LGM sea level that was up to 40 m above modern sea-level.

Subsurface rockhead topography

The BGS superficial thickness model indicates significant variation in the subsurface palaeo-topography, defined by significant variation in depth of rockhead. The deepest rockhead is observed along the present River Forth path and immediately to south of it, at between 50 to 80 m depth, with some areas reaching nearly 150 m depth around Grangemouth and Stirling. The subsurface topography forms an asymmetrical valley-shaped profile, which is orientated east-west and varies from 3 to 10 km wide. Generally, the asymmetry of the subsurface valley has a gently-dipping southern flank and steeper dipping northern flank. The valley profile is widest (10 km) from Stirling to Grangemouth before narrowing (3 km) around the Forth Road Bridge and finally opening up offshore at Edinburgh. There are two points where the valley narrows, including at the Forth Road Bridge and at Stirling. At the two narrowing areas the depth of the subsurface valley is still at a maximum of 60 m, but the width is less than 3 km. In central part of the Firth of Forth, between Stirling and Grangemouth, the subsurface topography forms a subsurface valley that is up to 10 km-wide and with a maximum depth of 150 m. The geometry of the subsurface topography in this area varies from one large asymmetric valley to having multiple smaller 2 km-scale valleys.

This subsurface topography is likely to have been shaped by numerous glaciations affecting Scotland during the Late Quaternary (Lee, Busschers, & Sejrup, 2012[5]). The dynamic glacial history is likely to have strongly shaped the subsurface rockhead and top till topography. The numerous glacial intervals affecting Scotland are thought to have resulted from extreme repeated environmental conditions, which had a strong imprint on the landscape (Porter, 1989[6]).

Glacial deposits

Till and glaciofluvial deposits have been mapped and are observed in borehole records extensively across the Firth of Forth. The borehole records indicate a significant variation in thickness of till deposits across the area, from less than 5 m to nearly 60 m thick. The thickest till deposit is observed in the deepest subsurface topography, such as north-west of Grangemouth and around Skinflats village. Along the flanks of the subsurface topography in the Firth of Forth the till deposit varies from 0–30 m thick. Along the boundary of glaciofluvial and till deposits, the till varies from being described as boulder clay to including sand and gravel deposits, suggesting that further detailed work is needed to map out the boundary between till and glaciofluvial deposits.

Outcropping till in the Edinburgh district has been shown to represent three major tills (Kirby, 1966[7]). Hence, it is likely that the till preserved in Firth of Forth, particularly where interbedded with sand and gravel, represent the multiple glaciations affecting the region during the Quaternary.

Devensian raised marine and deltaic deposits

Late Devensian-aged raised marine deposits are mapped around the periphery of the Firth of Forth, and are interpreted as comparable to the Errol Clay Formation from the Tay Estuary (Peacock, 2003[8]). The borehole records indicate deposits comprising interbedded silt, clay, sand and gravel of 2–4 m thickness, at 40–30 m A.O.D west to east, and within 5 km to the north and south of the modern River Forth. Generally, the Late Devensian raised marine deposits observed form an approximately 200 m to 1 km wide surface, with occasional outcrops of approximately 3 km wide in the Grangemouth to Skinflats area.

The narrowest outcrops of Late Devensian raised marine deposits are found along the edges of the distal part of the Firth of Forth between Bo’ness and just beyond the Forth Road Bridge. This area is an approximately 15 km-long and 2–3 km wide corridor just before the mouth of the estuary. Due to this narrowing of the Firth of the Forth the Late Devensian raised marine deposits are found proximal to the modern river. From borehole records the deposits predominantly comprise silty and sandy clay and gravel that are deposited directly on till.

The topographic narrowing of the Firth of the Forth at the estuary mouth has a strong control of the extent of preserved post-glacial marine deposits. A form of the current topography is likely to have existed during the deposition of post-glacial marine deposits, which has been subsequently isostatically uplifted. The sediments preserved in this area appear to represent a subtidal glaciomarine environment due to their similarity with Errol Clay Formation deposits described at the Gallowflat Claypit (Peacock, 2003[8]). It is interpreted that the post-glacial sea-level is represented by the limit of the inland extent of the Devensian marine deposits around the Firth of the Forth. Post-glacial marine deposits that infilled the main Firth of the Forth estuary area and has partially been eroded away during later erosive and depositional estuarine phases. Small embayments within the narrow topographic corridor coincide with lows in the topography and are infilled with predominantly clay and probably represent semi-enclosed marine lagoons.

From Bo’ness to Stirling the extent of the Firth of the Forth post-glacial deposits widens with Late Devensian marine deposits observed up to 10 km inland of the modern river. The raised marine deposits are observed on a surface at 30–40 m A.O.D. The distribution of the raised marine deposits in this area is not simply parallel to the current Firth of the Forth, but is highly variable and comprises deposits that vary from 300 m to nearly 4 km wide. Borehole records from the mapped raised marine surface show that deposits on the northern side of the Firth of the Forth again comprises sandy clay with occasional gravel. However on the southern side the deposits comprise major proportions of sand and gravel with occasional sandy clay, with a more developed stratification and inter fingering of the various lithologies. The Plean cross section through the raised marine deposits on the southern side of the Firth of the Forth, show a predominantly clay-rich deposit with minor amounts of gravel that is approximately 4 m thick. This is overlain by interbedded sand, gravel and clay of approximately 6–7 m thickness. Generally, the deposits on the southern side of the Forth of Firth are distinctly different to the more typical Errol Clay Formation-type deposits on the northern side of the Firth of Forth, as the southern side deposits have more sand and gravel and represent a higher-energy depositional environments, such as local fluvial to deltaic settings.

The topographic widening of the Firth of the Forth appears to have had a strong control of the depositional environment in this area. The southern side of the Firth of the Forth is interpreted as a local embayment, which was a proglacial fluvial or estuarine environment immediately after the deposition of subtidal glaciomarine Errol Clay Formation sediments. It is likely that the estuarine deposits were more extensive across the embayment and the current distribution of the deposits is a consequence of Holocene fluvial erosion. This area represents a major post-glacial fluvial to deltaic environment that ran from Bo’ness to Kincardine. Notably, there is are post- glacial deltaic deposits found on the northern side of the Firth of the Forth or in the Ochil Hills over-deepening inlet. These deposit are likely equivalent to coarse-grained sand deposits overlying the Errol Clay marine deposits that were described in the Tay-Earn Estuary and are termed the Culfargie Beds (Paterson, Armstrong, & Browne, 1981[9]). The Culfargie beds were interpreted as a prograding deltaic setting that is observed locally across the estuary (Paterson, Armstrong, & Browne, 1981[9]).

Upstream of Stirling and in the proximal part of the River Forth the Late Devensian raised marine deposits are described as a stiff, reddish clay with rare shells that is 2 m thick. At the edges of the River Forth catchment, at 40 m A.O.D., the mapped raised marine deposits from borehole records comprise a dark red-brown silty sand with pebbles of fine-grained sandstone and siltstone. These more coarser-grained sediments are likely to represent a more energetic setting such as a local fluvial to deltaic environment.

The sediments beyond the topographic gap of the Campsie Fells and Ochil Hills and before Loch Lomond and Trossachs national park hills represents a distinct depositional component of the Firth of the Forth. Unlike further downstream in the Firth of the Forth, the sediments that represent the post-glacial marine environment are preserved across the entire estuary as a relatively thin veneer of beach deposits along the edge of mapped marine limit. The deposits in this area reflect the post-glacial marine incursion resulting in the deposition of subtidal glaciomarine sediments that was followed by sea-level retreat which is reflected by the beach deposits preserved along the edges of the River Forth catchment.

Main perth shoreline

The ‘Main Perth Shoreline’ is described in the Perth area at approximately 21 m A.O.D. (Smith, Sissons, & Cullingford, 1969) representing a balance between the isostatic rebound and eustatic sea-level rise that are in balance for a relatively short-lived time period after glacial retreat (Browne, Gould, & Akhurst, 2015[10]). A platform of the same height is not recognised in the Firth of Forth area, however potentially there is a post-glacial shoreline that varies from 40 m A.O.D. in the western most parts of the estuary to 30 m A.O.D. in the east. Further mapping would be needed to define this feature.

Post glacial estuarine infill

In the immediate shallow subsurface surrounding the Firth of the Forth the mapping describes raised tidal and intertidal flat deposits. Regional studies of the sediment in the shallow subsurface have been few and mainly focused on the deposits around Grangemouth and Bothkennar (Barras & Paul, 1999[11]). The thickness and composition of these sediments is now investigated across a wider region with the available borehole records. There is a significant variation in thickness of the deposits from 20–100 m. The thickness of the estuarine infill appears to be dependent on the thickness of till and existing subsurface topography that has strongly controlled accommodation space. In the narrow corridor from the Forth Road Bridge to Grangemouth the estuarine infill deposits are up to nearly 60 m in thickness and predominantly comprise clay with occasional sand and gravel channels. The narrow corridor (2–3 km wide) is infilled with post-glacial estuarine sediments. Thin (1–2 m thick) sand and gravel channels are interpreted in the upper half of the section. There is little variation in the composition and thickness of sediments across the Firth of Forth channel.

In the Grangemouth to Stirling area the Firth of the Forth broadens in association with dynamic changes in the composition and thickness of the post-glacial estuarine infill. Immediately east of Bo’ness the Firth of the Forth broadens from 3–4 km to nearly 10 km wide, which is associated with the thickest post-glacial estuarine deposits in the entire Firth of the Forth. The deposits comprise predominantly clay and silt with occasional sand and gravel channels. In the deepest, part of the estuary around Grangemouth, the sediment infills an asymmetric valley, with deepest part of the valley below the modern River Forth. On the southern side of the valley the infill is predominantly clay with discontinuous inter-fingering beds of silt and occasional sand and gravel. On the northern part of the valley the silt becomes dominant in the lower part of the section but scarce in the upper part. In addition, large and more significantly channels of sand and gravel are noted at the position of the modern River Forth.

The preserved till deposits in the Grangemouth area form a thin veneer on the original bedrock topography, which is either the result of glacial depositional or post-glacial erosional processes. There is a large amount of accommodation space above the till, which is infilled by post-glacial estuarine sediments. In contrast, the till deposits across the Skinflats section infills the pre-existing topography leaving a relatively planar surface for the post-glacial estuarine deposits to be deposited on. The thickness of the estuarine sediments appears to be strongly controlled by the pre-existing top-till palaeo-surface, which is in turn likely strongly controlled by pre-LGM and LGM glacial processes.

In the upper part of the central section near Stirling there is significant change in the estuarine sediment infill composition and thickness. This area is the intersection of Ochil Hills over-deepening and the narrowing of the estuarine system at Stirling. The area can be divided into three parts: firstly a southern part that underlies the City of Stirling; secondly a central part that underlies the River Forth; and thirdly a northern part that is at the foot of Ochil Hills. In the most southern part there is no till preserved, the central part comprises till deposits of approximately 16 m, while the northern area the Ochil Hills over-deepening has till deposits of nearly 40 m thickness. The estuarine infill in the southern area comprises a nearly 40 m-thick sequence of predominantly clay with occasional sand and silt channels. In the central areas beneath the modern River Forth the estuarine infill comprises 3 m-thick deposits of gravel sitting directly on till. The northern area comprises a 50 m-thick deposit of clay with regular channels of sand and gravel. It is likely that the variation in lithologies across the different areas can be attributed to different processes of deposition across the estuary. Firstly, the southern and central areas can be linked with the main Firth of the Forth estuarine sequence, with the central section correlated with the current River Forth channels, which seems to be represented by sand and gravel channels from Stirling to the Forth Road Bridge. The southern area comprises clay with inter-fingering silt and sand that is similar to areas further east up to Grangemouth, which is just south of the current River Forth. Finally, the northern area is the western most part of the Ochil Hills over-deepening feature reflecting an overflow area of the Firth of the Forth.

Finally the proximal parts of the Firth of the Forth upstream of the Stirling topographic gap comprises an occasional peat deposit and then a 10 m-thick sequence of clay, which overlies Late Devensian raised marine deposits. The mapped extent of the estuarine sequence in this area is up to 3.5 km wide and extending up to the Menteith Hills, which is approximately 30 km west of Stirling. The deposits in this area represent the most inland part of the estuary and reflect a very different type of deposit than is observed at Stirling and further east. The preservation of peat at the base of the estuarine sequence reflects the subaerial exposure of the marine sediments prior to flooding and the deposition of estuarine sediments. The overlying estuarine sediments comprise only clay with silt laminae. There is a shallow over-deepening on the southern margin that is infilled with silt and sand. This upper part of the Firth of the Forth is interpreted as representing a low-energy proximal part of the estuary.

Bothkennar gravel

The representation of the Loch Lomond glaciation in the Firth of the Forth is currently only documented by the Bothkennar Gravel Formation. Peacock (1998)[12] described a ‘buried gravel layer’ that is continuous from Grangemouth to Stirling at up to 6 m depth that has been interpreted to be ice-rafted deposits from ice bergs in the Firth of the Forth. From boreholes Peacock (1990) describes the gravels as containing angular to rounded clasts, of variable sorting and some striations, consistent with ice-rafted deposition according to Gilbert (1990)[13]. The borehole records studied show discontinuous buried gravel deposits particularly between Grangemouth and the Forth Road Bridge. The deposits vary from 2 to 4 m depth and are found interbedded within the upper part of the post-glacial estuarine clay and silt sediments. From the available borehole data, a buried gravel layer can only occasionally be seen in the narrow corridor at the mouth of the Firth of Forth. This is interpreted as representing the accumulation of ice raft debris in the corridor where it is trapped and results in ice-raft deposits.

Holocene raised marine deposits and beach deposits

Holocene-raised marine deposits are mapped in the narrow topographic corridor at the mouth of the Firth of the Forth. Borehole records in this area describe a relatively continuous approximately 1 m-thick bed of shell-rich silt, which is continuous across the Firth of the Forth and onshore up to 20 m A.O.D. and 500 m inland. Overlying, the Holocene-aged raised marine deposits is a mapped Holocene-aged raised beach deposit. Borehole records from these mapped areas indicate that the raised marine deposit predominantly comprise clay, whereas the raised beach deposit is composed of sand. The Holocene-aged raised marine and beach deposits are not mapped west of Bo’ness.

Main post-glacial shoreline

The ‘Main post-glacial shoreline’ is described as being from 14.8–6.1 m A.O.D., from west to east in the Firth of the Forth (Cullingford, 1991[14]). The age of the shoreline has been dated as 6800 years BP using 14C (Cullingford, 1991[14]).

References

  1. BOULTON, G, PEACOCK, J, and SUTHERLAND, D. 1991. Quaternary. Geology of Scotland, Vol. 3, 503–543.
  2. 2.0 2.1 SHENNAN, I, BRADLEY, S, MILNE, G, BROOKS, A, BASSETT, S, and HAMILTON, S. 2006. Relative sea‐level changes, glacial isostatic modelling and ice‐sheet reconstructions from the British Isles since the Last Glacial Maximum. Journal of Quaternary Science, Vol. 21, 585–599.
  3. LAMBECK, K. 1991. Glacial rebound and sea‐level change in the British Isles. Terra Nova, Vol. 3, 379–389.
  4. PELTIER, W, and FAIRBANKS, R G. 2006. Global glacial ice volume and Last Glacial Maximum duration from an extended Barbados sea level record. Quaternary Science Reviews, Vol. 25, 3322–3337.
  5. LEE, J R, BUSSCHERS, F S, and SEJRUP, H P. 2012. Pre-Weichselian Quaternary glaciations of the British Isles, The Netherlands, Norway and adjacent marine areas south of 68 N: implications for long-term ice sheet development in northern Europe. Quaternary Science Reviews, Vol. 44, 213–228.
  6. PORTER, S C. 1989. Some geological implications of average Quaternary glacial conditions. Quaternary Research, Vol. 32, 245–261.
  7. KIRBY, R P. 1966. The glacial geomorphology of the Esk Basin, Midlothian. Unpublished PhD thesis, University of Edinburgh.
  8. 8.0 8.1 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.
  9. 9.0 9.1 PATERSON, I B, ARMSTRONG, M, and BROWNE, M. 1981. Quaternary estuarine deposits in the Tay-Earn area, Scotland. HMSO, CF81/07 (London).
  10. 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.)
  11. 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.
  12. PEACOCK, J. 1998. The Bothkennar Gravel Formation (‘buried gravel layer’) of the Forth Estuary. Scottish Journal of Geology, Vol. 34, 1–5.
  13. GILBERT, R. 1990. Rafting in glacimarine environments. Geological Society, London, Special Publications, Vol. 53, 105–120.
  14. 14.0 14.1 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.