Palamakumbura, R, and Auton, C. 2019. New exposed section in the Ardersier Silts Formation; November 20th-21st, 2017. British Geological Survey Internal Report, OR/19/016.
Introduction to field results
The exposed trench section started from the position of the head apparatus installed on the pipeline at the top of the degraded Lateglacial cliff where the Ardersier Silts Formation were exposed. It continued downslope into the outcrop of the Holocene raised marine deposits. The exposed trench section, which ran approximately east to west was logged from the top towards the base of the slope. The logging was confined to the outcrop of the Ardersier Formation and overlying Late Devensian raised marine deposits. The field results are presented as a series of localities, [Field Observation Points (FOP’s)] that were generally numbered sequentially from the top of the trench to the bottom. The only exception to this was Localities 13–15 which were number inversely down the section (see Figure 3, lower section log).
Field Observation Points 1–6 were assigned to previously described localities in the Arderiser Silts Formation in the Ardersier area, which are summarised to provide a brief stratigraphical context for the results from this newly exposed section (see Surrounding sites of interest).
The new section is described in three parts, an upper (localities 7–8), middle (localities 9–11) and lower part (localities 12–16). Each of these parts are distinguished based of different primary and secondary sedimentary and tectonic features.
File:OR19016fig3.jpgFigure 3 Sediment logs of the upper, middle and lower parts of the exposed trench based on new field observations.
Surrounding sites of interest
Field observation points 1, 2, 4 and 6 are of sites of particular interest to this work and discussed below. In contrast, field observation points 3 and 5 are not relevant here and are therefore not included. The locations of these field observation points is shown on Figure 2.
Field Observation Point 1
Collected on: 15/11/2017 15:11:01 At Coordinates: X: 279390 - Y: 856160 Location Description: Jamieson's pit Summary Label: Jamieson's pit
This locality corresponds with the original section described by Jamieson (1874)[1] and subsequently known as ‘Jamieson’s Pit’ where marine molluscs, ostracods and foraminifera where recorded associated with beds of silt and clay. The shells are of cold water aspect, but not necessarily High Arctic or particularly characteristic of ice-proximal conditions. The section is capped by a diamicton (the Baddock Till).
Field Observation Point 2
Collected on: 15/11/2017 16:33:11 At Coordinates: X: 278490 - Y: 856470 Location Description: Kirkton section Summary Label: Kirkton section
The ‘Kirkton Section’ locality corresponds with the section excavated in 1990, into the bluff below a Late Devensian raised shoreline at 21.6 m above OD. The 1990 section exposed flat-lying, moderately to thickly bedded, rhythmically laminated sands, silts and clays, that showed little or no evidence of glacitectonic disturbance or hydrofracturing.
Field Observation Point 4
Collected on: 15/11/2017 16:37:42 At Coordinates: X: 278210 - Y: 855650 Location Description: Hillhead Summary Label: Hillhead section
The ‘Hillhead section’ locality corresponds with the natural exposure at the top of the main Postglacial cliff line. It exposes sand with ripple and cross-bedding structures that are cut by low-angle silt-lined shears. The Baddock Till overlies the Hillhead Sand Member.
Field Observation Point 6
Collected on: 15/11/2017 16:50:12 At Coordinates: X: 278030 - Y: 855980 Location Description: Contorted silts section Summary Label: Contorted silts
The ‘Contorted Silts Section’ locality corresponds with the natural exposure close to the top of the Main Postglacial cliffline that exposes thinly interbedded fine-grained sands and clayey silts that exhibit abundant soft sediment deformation (ball and pillow) structures. The beds are deformed by large scale (1–5 m wavelength) folds, flat lying thrusts and with small scale high-angle normal faults.
The upper section part of the 2017 section
The upper part of section is described in localities 7 and 8. Significantly, these localities include the upper boundary of the Ardersier Silts Formation with the overlying Devensian raised marine sediments.
Field Observation Point 7
Collected on: 21/11/2017 10:08:17 At Coordinates: X: 277929.508 - Y: 856239.477 Location Description: Top part of trench exposing Late Devensian raised beach and contact with possible Hillhead Sands Member Summary Label: Raised beach deposit
The first locality is from the upper most part of the trench section and contains three distinct sedimentary units (Figure 4). The entire section is capped by a 20 cm thick dark brown soil. The upper most unit exposed is a 30 cm-thick gravel deposit, composed of well-rounded to sub rounded cobble-sized clasts varying in size from 1 to 12 cm. The deposit is clast supported with a coarse-grained sand matrix. The unit has poorly developed upward fining and horizontal stratification. The gravel is interpreted as a Late Devensian raised beach deposit. The unit unconformably overlies the central unit.
The central unit is a light brown, fine to medium-grained sand with planar lamination and some cross lamination picked out by with occasional dark brown-coloured layers (Figure 5). The unit is in total 60 cm thick, and does not seem to vary in thickness across the exposed section. A basal pebble lag is observed discontinuously along the basal unconformity. The lag comprises well rounded cobbles of 3–6 cm in size and with no evident internal sedimentary organisation. It is inferred that this unit is likely to be the Hill Head Sand Member.
The lowest unit (Figure 6) is a pale grey to yellow-coloured, medium to fine-grained unlithified sand with ripple cross-bedding. Steep angled lamination with some signs of shearing. The unit generally dips steeply northward. This dip is the result of local small scale faulting. Fine and medium-grained sand layers are also observed.
The unconformity separating the lower and middle units is an undulating surface, varying in height by approximately 3–5 cm (Figure 6c). A faint dark to light brown layering is observed along the unconformity, which is result of iron staining of the grains of the middle unit along this surface. The layering is parallel to the surface of the unconformity and cross-cuts the parallel lamination of the middle unit.
Photos
File:OR19016fig4.jpgFigure 4(a) Overview of upper part of section, covering possible Hillhead Member Sands (pale grey) and overlying raised beach deposits and (b) Contact between upper (Late Devensian) raised beach gravel and underlying (central) laminated and cross-laminated sands.
File:OR19016fig5.jpgFigure 5 Unconformity between possible Hillhead Member (Ardersier Silts Formation) and overlying raised beach sands. Fine to medium-grained cross-bedding can be seen in the Hillhead Member.
File:OR19016fig6.jpgFigure 6(a) Lamination in the (central) Late Devensian raised beach sands unit; (b) Raised Beach gravel capping the entire unit; and (c) Cobble lag along unconformity between Hillhead Member and the overlying raised beach sands.
Field Observation Point 8
Collected on: 21/11/2017 10:55:00 At Coordinates: X: 277928.673 - Y: 856239.577 Location Description: Start of clay and gravel injected layers in sand Summary Label: Upper injected clay
Locality 8 is the next locality along the trench and located approximately 30 cm below the unconformity surface observed at Locality 7. This locality comprises a clay vein and brecciated clasts composed of the surrounding sand that are observed within clay (Figure 7). The sand breccia clasts are angular and variable in shape and size from square to oblong in shape and from 5–30 cm in size. No obvious orientation or sorting was noted of the clasts. Generally, the breccia is matrix-supported. Some original sedimentary structures are visible in the breccia clasts, including lamination and cross-bedding. The surrounding clay is a pale pink in colour and forms an approximately 0.5 m thick by 1.5 m wide unit. The clay unit is massive with no obvious sedimentary structures.
The sand unit is fine to medium-grained and generally pale white in colour with occasional light brown layers. The sedimentary structures within the sand are generally sub-horizontal.
Photos
File:OR19016fig7.jpgFigure 7(a) Overview of top of Ardersier Silts section, showing clay and gravel injections and brecciated sand blocks; (b) Small-scale brecciation in the top part of the Ardersier Silts (Hillhead Member); and (c–d) Small scale brecciation in the top part of the Ardersier Silts (Hillhead Member).
Middle section part of section
Field Observation Point 9
Collected on: 21/11/2017 11:15:51 At Coordinates: X: 277926.601 - Y: 856239.945 Location Description: Thin (1–3 cm) clay injections in tilted sand unit, in the upper part of section Summary Label: Upper clay injection
This locality is situated further eastwards along the trench and down section at 2.2 m depth, representing a slightly lower part of the pale white-colour unlithified sand seen at Locality 8. There are least three units identified in the sand, distinguished by different sedimentary structures and cross-cutting relationships (Figure 8). Firstly, the upper sand truncates the cross-bedded middle unit. Generally, the sand grains are sub-angular, well-sorted and comprised of quartz, feldspar and occasional heavy minerals. Small reverse faults of mostly 2–5 cm displacement, with occasional 20 cm displacements, are observed to cross-cut the entire exposed section. Secondly, a 1 m thick unit of light brown to grey, fine to medium-grained sand with planar cross-bedding. Finally, the upper most part of the sand is a pale white to grey in colour, fine-grained and comprises planar lamination and some small planar cross-bedding (Figure 9). The lower unit of pale white, fine-grained sand with planar lamination.
Two sub-horizontal to sub-vertical clay veins are observed in the upper part of the middle section (Figure 3 and Figure 8). The larger of the two clay veins is sub-horizontal in the upper most part of the sand, and then steeply dips down to truncate the entire exposed sand section (Figure 10). The clay vein is c. 20 cm in thickness when sub-horizontal, but reduces in thickness to 5 cm thick where it becomes sub-vertical. The sub-horizontal part of the clay cuts acutely across the parallel lamination preserved in the sand, and then follows the path of a reverse fault, which has approximately 20 cm of displacement. The clay is a pinkish red in colour with discontinuous reddish brown staining. The orientation of the reddish brown staining is variable, but it is generally parallel to the orientation of the clay vein and predominantly observed along the edges of the vein. A number of brown-coloured fluid staining pathways are noted. These start in the clay vein and continue into the sand unit, before fading out after 30–50 cm. The staining pathways have highly variable orientations, but tend to both follow and also cross-cut sedimentary structures (Figure 11).
The second clay vein is 1–2 cm thick and approximately 20 cm long, dipping at c. 45° (Figure 12). The clay is a reddish grey colour and is apparently structureless at the macro-scale. Samples were taken from the two clay veins at this locality, with the aim of describing any possible microstructures associated with them, in order to understand the processes controlling their emplacement.
Photos
File:OR19016fig8.jpgFigure 8 Overview of section with SW dipping clay layers (injections?) in laminated and occasionally layered sand.File:OR19016fig9.jpgFigure 9(a) Oxidation along lamination and stepping up between lamination; (b) Planar to tabular cross-bedding with alteration along bedding. Alteration commonly truncated at base of cosets; some high-angle features cut cross lamination; (c) Fine and coarse-grained sand layers in the cross-bedded units; and (d) Fine and coarse-grained sand layers in the cross-bedded units (possible traces of silt drapes on ripples in finer sand unit).File:OR19016fig10.jpgFigure 10 Clay layer changing orientation and thickness from bedding parallel (thick, shallow dipping) to thin, steeply dipping. In addition, oxidation veins appear to come off the clay layer.File:OR19016fig11.jpgFigure 11 Thin clay/silt layer cross-cutting lamination structures in sand with oxidation bands coming off the clay.File:OR19016fig12.jpgFigure 12 Photo of samples 1 and 2 box in the section.
Samples
Original Sample Number
Sample Type
Sample Comment
CAA_1
Superficial Deposit Sample
45° clay injection which is 25 cm long and 1 cm wide. The clay truncates medium to coarse sand. Sample is at 2.2 m depth.
CAA_2
Superficial Deposit Sample
45° sub vertical clay injection with brown veining coming off in multiple roughly oblique directions. Sample is from 2.7 m depth.
Structural observations
Feature
Azimuth
Dip
2nd Attrib
3rd Attrib
Grouping
Comment
Strata_Inclined_2
270
50
dip of clay seam of sample 2
Field Observation Point 10
Collected on: 21/11/2017 13:17:26 At Coordinates: X: 277922.19 - Y: 856239.945 Location Description: Brecciated blocks of sand surrounding a clay layer Summary Label: Clay layer breccia
Field observation point 10 is at a breccia horizon within the sand, which is at 4.6 m depth in the section. The breccia predominantly comprises 5–10 cm sub-angular blocks of sand, some rounded blocks are also present (Figure 13). Variation in the orientation of sedimentary structures within the blocks suggest that they are locally rotated. The breccia is associated with a sub-horizontal approximately 2 cm-thick clay vein. It comprises a reddish brown clay with light brown edges and occasional centimetre-sized pockets of silt. As was the case with the previous clay veins in the section, there are a number of fluid oxidation pathways, which propagate from the veins. The fluid veins follow a stepped path away from the clay seams. They follow bedding structures for several centimetres and then step to the next structure.
Photos
File:OR19016fig13.jpgFigure 13 Brecciation of sand unit along clay layer (vein/intrusion?). Lamination structures are visible and rotated. The dug out clay/iron pan used for dip measurements is shown in (c).
Field Observation Point 11
Collected on: 21/11/2017 13:45:36 At Coordinates: X: 277920.337 - Y: 856240.192 Location Description: Heavy veining along lamination and potential faulting Summary Label: Veining and faulting
The next field observation point 11 is located immediately below the clay vein and breccia of observation point 10 in the logged section. The observation point represents an approximately 2 m-thick part of the section comprising fine to medium-grained, pale grey to maroon-coloured sand (Figure 14). Similar sedimentary structures to those seen at observation point 10 are present. These include planar lamination and low angle planar cross-bedding. In addition, a number of reverse faults are also observed, with centimetres of offset visible and with variable steep dips of between 45°–70°. This part of the section is also rich in oxidising fluid pathways, marked by brown staining. These follow both the sedimentary structures and the faults (Figure 15 and Figure 16). The fluid pathways are light to dark brown in colour and vary in thickness from 2–4 cm. At the contact between a reverse fault and the planar lamination the oxidising veins often form an oxidised unit that is 5–10 cm thick.
Photos
File:OR19016fig14.jpgFigure 14 Oxidation pathways along planar bedding, stepping up between bedding pathways and interacting with inferred faults.File:OR19016fig15.jpgFigure 15 Oxidation along bedding in cross-bedded sands.File:OR19016fig16.jpgFigure 16 Laminated sand with oxidation pathways cross-cutting bedding.File:OR19016fig17.jpgFigure 17 Oxidation along fault with diffusion along bedding.
Lower section part of section
Field Observation Point 12
Collected on: 21/11/2017 14:02:47 At Coordinates: X: 277918.43 - Y: 856240.182 Location Description: Thick vertical clay vein within sand Summary Label: Thick clay vein
Field observation point 12 marks the start of the lower section (see Figure 3), which is at approximately 6 m-depth in the overall exposure. The main part of this observation point is the 20–40 cm thick sub-vertical clay vein that cuts across the entire trench (Figure 18). The vein is thinnest (20 cm) at the base of the exposure and thickens upwards to 40 cm. At the start of the thickest part of the vein there is a small veinlet (c. 3 cm thick) on its lower (left) side. The main clay vein is pale grey to light brown in colour, with a faint sub-vertical discontinuous layering (parallel to the vein orientation). Parts of the edges of the main vein have a dark brown diffuse oxidation boundary grading into the surrounding sand. This is particularly notable on the upper (right hand) of the vein. Within the clay vein there are discontinuous layers and lenses of fine-grained sand and silt. Occasional roots are observed penetrating the clay vein from the sloping ground surface above.
The clay vein cuts through medium-grained, grey-coloured sand, with faint planar bedding and oxidising fluid pathways along the bedding planes. The sand is sub-angular to angular and well-sorted. There are also a number of smaller (1–5 cm wide), grey-coloured clay veins in the sand. These clay veins also have thin-discontinuous layers of silt and fine-grained sand within the clay, parallel to the orientation of each vein. Fluid oxidation pathways are a light-brown colour and run straight through the sand without following any of the faint sedimentary structures.
Photos
File:OR19016fig18.jpgFigure 18(a) Overview photograph of thick clay injection into sands; (b) Oxidation layering and diffusive oxidation boundary off the thick clay injection; (c) Close up of oxidation layering and diffusive oxidation boundary off the thick clay injection; (d) Small clay veins coming off the main unit, with diffusive oxidising upper boundary; (e) Oxidation layering in clay; (f) Diffusive oxidation boundary along clay vein and main clay injection unit; (g) Sandy lenses within the central part of the large clay injection; (h) Diffusive oxidation along the boundary of the clay into the sand; (i) Diffusive oxidation along the boundary of the clay into the sand; and (j) Vertical clay/silt vein with graded lamination cutting across laminated sand.
Structural observations
Feature
Azimuth
Dip
2nd Attrib
3rd Attrib
Grouping
Comment
Strata_Inclined_2
246
58
dipping clay vein
Strata_Inclined_2
248
73
dipping clay vein
Field Observation Point 13
Collected on: 22/11/2017 09:29:23 At Coordinates: X: 277919.091 - Y: 856240.909 Location Description: Sand and clay deformation with ripple structures Summary Label: sand and clay deformed
Field observation point 13 is lower in the trench and nearer to the base of the sequence. It is interpreted as a large multi-phase hydrofracture system of approximately 1.5 m thick with an infill of clay, silt and fine to coarse-grained sand (Figure 19).
Some sedimentary structures are preserved in the sand layers, including planar lamination and ripples (Figure 20). However, the coarser-grained sand layers have no preserved internal sedimentary structures. The sand can be interpreted as beds that are broken up into 20–100 cm-sized blocks, but appear to be essentially in situ, but they are surrounded by clay. The blocks generally dip towards the west. Alternatively, this part of the sequence could be interpreted as large blocks of clay surrounded by brecciated or fluidised sand. This seems less likely as the sand contains depositional structures such as ripples and planar lamination that are less likely to be formed during injection of fluidised sand. Across the trench section are patches of dark to light reddish brown iron oxide coatings of sand grains (Figure 22). These are isolated patches rather than that characteristic of the discrete fluid pathways seen elsewhere.
Micromorphology Sample No. 4 (Figure 21) was taken from a part of the section comprising sub-vertical interbedded clay and silt layers, to investigate microstructures related to emplacement of the hydrofracture sediment.
Photos
File:OR19016fig19.jpgFigure 19 Overview of section of blocks of sand and silt, with some preserved sedimentary structures, in clay, in the lower part of the section.
File:OR19016fig20.jpgFigure 20 A blocks of sand with cross-bedding preserved dipping down to the SW.
Sample taken at the contact between the clay and graded sands within major hydrofracture infill
Field Observation Point 14
Collected on: 22/11/2017 09:46:31 At Coordinates: X: 277917.305 - Y: 856241.174000001 Location Description: Brecciated sand surrounded by clay Summary Label: Brecciated sand
Field observation point 14 is located above field observation point 13 and represents an areas of brecciated sand with blocks from 2–20 cm in size (Figure 23). The blocks are surrounded by light grey to brown-coloured clay. There are a number of vertical and horizontal oxidation fluid pathways that cut across the sand blocks and follow the edges of clay patches.
Micromorphology Sample 3 (Figure 24) is taken from a horizontal clay vein that cuts through the sand with a vertical oxidation fluid pathway and some poorly-preserved small ripples.
Photos
File:OR19016fig23.jpgFigure 23 Vertical oxidation veins cutting through cross-bedded silt and clay; close up photograph of one of the blocks of sand, showing the well-sorted nature of the sand; and moderately-rounded 5 cm block of fine-grained sand surrounded by clay and a contact between coarse to fine-grained sand and clay.File:OR19016fig24.jpgFigure 24 Photograph of the sample 3 tin inserted into the section.
Samples
Original Sample Number
Sample Type
Sample Comment
CAA_3
Superficial Deposit Sample
Sample through blocky sand, horizontal veining and clay
Field Observation Point 15
Collected on: 22/11/2017 10:22:06 At Coordinates: X: 277917.147 - Y: 856240.380000001 Location Description: intensive clay veining in sand Summary Label: intensive clay veins
Field observation point 15 is situated further east along section from field observation point 12. The section comprises grey to light brown, fine to coarse-grained sand with interconnected clay veins (Figure 25 and Figure 26). The clay veins are 1–30 cm thick and up to 1.5 m long, and dip towards the west at approximately 45°. Although the veins are predominantly comprised of clay, they also contain occasional 1–5 cm thick layers of silt and fine-grained sand. The intensive clay veining has resulted in the brecciation of the sand, with rounded to angular blocks of sand surrounded by clay.
The top edges of several clay veins form iron oxidation boundaries with the surrounding sand. Several of these boundaries are diffuse over several centimetres. The iron oxide grain coatings are a dark black-brown to light brown in colour.
Occasional scattered well-rounded, 1–4 cm-sized pebbles are observed within the sand.
Photos
File:OR19016fig25.jpgFigure 25(a) Overview of section from a thick (30–40 cm) sub-vertical clay vein (left) to a series of thinner (10–20 cm) clay veins inter-fingering the sand (right); and (b) Overview section of the thinner (10–20 cm) sub-vertical to sub-horizontal veins inter-fingering the sand host.File:OR19016fig26.jpgFigure 26(a) Vein of clay with lenses and small blocks of medium-grained sand and with an (iron stained) oxidised upper boundary; (b) Thin clay veins truncating lamination and cross- bedding in the sand. Oxidised upper boundaries of the clay; (c) Layers of silt and sand with well-rounded cobble-sized clasts; (d) Thin veins of clay inter-fingering with sand producing isolated small blocks of sand; (e) Diffusive oxidation boundary along top edge of clay vein; (f) Clay veining surrounding a sand unit with a faint sub-vertical bedding preserved; (g) Contact between sand and silt/clay. Faint sub-horizontal bedding in sand preserved. Sub-vertical layering in silt/clay, defined by variations in colour; (h) Fragmented blocks of sand within a clay vein; (i) Close up photograph of oxidised sand and small fragmented clay layer; (j) Well-rounded cobble-sized clast in sand unit; (k) Close up photograph of the boundary between two veins, with dark manganese-coloured coating of sand; and (l) Faint vertical layering of fine-grained sand and silt within the clay veins.
Field Observation Point 16
Collected on: 22/11/2017 10:52:06 At Coordinates: X: 277913.972 - Y: 856240.221000001 Location Description: intensive clay veining causing brecciation of sand Summary Label: Clay and breccia
The final observation point (FOP 16) is situated at the base of the exposed section and represents a 50 cm wide major hydrofracture network with extensive clay veins in an apparently unstratified sand unit (Figure 27). The base of the exposed vein is at approximately 8 m depth from the uppermost unconformity near the top of the overall sequence. The surrounding sand is fine to medium-grained and a light yellow to grey colour.
The clay vein veining has resulted in the major brecciation of the sand forming sub-rounded blocks that appear to have been deformed within the clay veins. Some of the sand blocks have also been faulted but apparently undeformed. The blocks of sand are highly variable in size from less than 5 cm to nearly 40 cm. Small elongate lenses of fine-grained sand are observed within the clay veins, these are either stretched blocks of sand or original discontinuous layers within the clay.
The clay is dark grey in colour with light brown iron oxide patches along a handful of edges. At the edges of the hydrofracture system there are various iron oxidation fluid pathways associated with the edges of the clay, which meander through the massive structureless surrounding sand. Several of the iron oxidised edges of clay veins comprises two layers, a dark black-brown layer that is less 1 cm thick and a dark to light orange-brown layer that is 1 cm to 5 cm thick and passes diffusely into the sand or clay.
Photos
File:OR19016fig27.jpgFigure 27(a) Overview of section, showing brecciation of sand due to intensive clay veining; (b–c) Fragmented piece of manganese-stained (oxidised) sand; (d) Undulating oxidation boundary along upper clay vein margin and into sand; (e) Blocks of sand surrounded by clay; (f) Oxidation veins that appear to come from clay with highly sporadic pathway through sand; (g) Oxidation boundary on the edge of sand clast; (h) Varying colour of oxidation of clay and sand. Nodules of fine-grained sand to silt within clay veins. Rounding of blocks of sand, from the more distal part of the vein system; (i) Small irregular 'pods' of fine-grained sand to silt enclosed by clay veins; and (j) Rounding of blocks of sand, from the deeper (more distal?) part of the vein system. Close to the base of the logged sequence.
Field Observation Point 17
Collected on: 22/11/2017 11:05:30 At Coordinates: X: 277924.262 - Y: 856239.911 Location Description: clay vein Summary Label: clay vein at 3.6 m depth
File:OR19016fig29.jpgFigure 29 Dug out clay vein surface for taking dip measurement.
Structural observations
Feature
Azimuth
Dip
2nd Attrib
3rd Attrib
Grouping
Comment
Strata_Inclined_2
278
38
dipping clay vein
Field Observation Point 19
Collected on: 22/11/2017 11:39:08 At Coordinates: X: 277916.273 - Y: 856240.38 Location Description: clay vein in breccia Summary Label: clay vein in breccia
Photos
File:OR19016fig30.jpgFigure 30 Dug out clay vein surface for taking dip measurement.
File:OR19016fig31.jpgFigure 31 Dug out clay vein surface for taking dip measurement.
Structural observations
Feature
Azimuth
Dip
2nd Attrib
3rd Attrib
Grouping
Comment
Strata_Inclined_2
262
36
dipping clay vein
Strata_Inclined_2
255
62
dipping clay vein
Clay vein orientations
Dip and azimuth measurements were taken from seven clay veins across the sequence to look for variations in orientation of the veining. When taking a dip and azimuth measurement the overlying sand was dug out and the measurement taken on the planar top surface of the vein (e.g. field observation points 17–19; Figures 28–31). Figure 32 shows the orientations of the clay veins plotted on an equal area stereonet. In general, most of the veins appear to have a similar orientation, steeply dipping towards the west. This is suggestive of clay vein emplacement that is associated with a similar stress field. This veining may be the result of one or multiple events.
File:OR19016fig32.jpgFigure 32 Stereonet plot of the great circles and poles of the several clay veins.
Kessock narrows core
A photograph of a core sample of sediments, showing several of the characteristic attributes of the Ardersier Silts was collected by J D Peacock from one of the offshore site investigation boreholes drilled prior to the construction of the Kessock Bridge, across the Kessock Narrows in the Inner Moray Firth. The site investigation was undertaken in 1969–70. Although the exact borehole from which the sample was collected is now unknown and its depth was not recorded, it was from material that occurred beneath a diamicton, interpreted as a glacial till (J D Peacock and J W Merritt, pers. com.).
File:OR19016fig33.jpgFigure 33 Photograph of core sample of Ardersier Silts, from a Kessock Bridge offshore site investigation borehole, collected and photographed by J D Peacock c. 1969–70.
From detailed study of the photograph, the sample comprises thinly interbedded blue-grey silty clay and rhythmically laminated olive grey sandy silts, which show intricate involuted sedimentary contacts (Figure 33). Some deformation of the margins of the core are evidently a result of the drilling and extraction of the core (clearly visible in the ‘dished’ appearance in the top 5 cm of the sample). However, the involuted nature of the silt-clay interbeds in the middle and lower portions of the core, are the result of soft sediment deformation that is likely associated with the original deposition of the sediment. These features are characteristic of the Ardersier Silts in nearby onshore exposures, such as those at Balnaglack, and at the Kirkton section (Merritt, Auton and Firth, 2017[2]). The olive grey silt units comprise upward-fining rhythmites and there appears to be the presence of possible clay intraclasts, notably within the zone between 5–10 cm from the top of the sample. Incipient ball and pillow and flame structures are associated with small-scale flat-lying rootless folds in the bottom portion of the sample. All of these features are characteristic of the Ardersier Silts exposed in the coastal hinterland between Inverness and Nairn.
File:OR19016fig34.jpgFigure 34 Cross-section of the geology seen in the Kessock Bridge boreholes, showing the possible position of the photographed core sample (modified from Auton, 2006[3]).
Although there is no original record of which borehole core the sample was collected from, examination and interpretation of the borehole and geotechnical logs and accompanying site investigation report for the Kessock Bridge (Auton, 2006[3]), suggests that only one borehole (B5/M1) encountered Ardersier Silts, beneath a diamicton (clay with gravel [blue] in Figure 34) that was interpreted as till. If this is the case, then the photographed core sample was probably collected at between 60–70 m below OD, at a depth of 53–63 m in the core.
The photograph of the core sample is the first image of sediment, identified as being part of the Ardersier Silts Formation, from the offshore area of the Kessock Narrows, and indeed within the subtidal zone of the Inner Moray Firth. It is in the region of 50 years since the core was collected and photographed, but it provides compelling support for the modelled and mapped extent of Late Devensian glaciomarine sediments in the Inverness area.
References
↑JAMIESON, T F. 1874. On the last stage of the glacial period in North Britain. Quarterly Journal of the Geological Society, Vol. 30, 317–338.
↑MERRITT, J, AUTON, C, and FIRTH, C. 2017. Ardersier Peninsula and the Ardersier Silts Formation.
↑ Jump up to: 3.03.1AUTON, C A. 2006. Geological assessment of likely ground conditions for directional drilling for waste water pipeline — North Kessock to Longman, Inverness. British Geological Survey Report G.S. IDA 106816. 19 pp. DOI: 19.13140/RG2.2.21325.69602. Cite error: Invalid <ref> tag; name "Auton 2006" defined multiple times with different content
