OR/19/016 Discussion

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.

The newly described section of Quaternary sediments, outside Ardersier Village, provides new insights into the depositional environment and temporal changes of the Moray Firth ice-stream during the Devensian deglaciation.

Three depositional units, separated by discontinuities, are distinguished in the newly described section. Firstly, an upper gravel unit of less than 30 cm erosionally overlies a sand unit of 60 cm thick. The final lowest unit makes up the majority of the section and is likely over 8 m thick. The base of this lower most unit was not exposed in the excavated trench. Throughout the lower unit are number of glacitectonic features including brecciation, clay veins and local thrust faults. Each of the depositional and structural features are summarised and discussed below.

Depositional setting

The upper most unit comprises a poorly sorted gravel with well-rounded to sub-rounded clasts, that vary in size from 1–12 cm. The deposit is clast supported with a coarse-grained sand matrix. Overlying the deposit is a dark brown-coloured soil. This unit likely represents a raised-beach setting associated with the Main Postglacial Cliffline, which is similar to that described at the top of the Kirkton section (Merritt et al., 1995).

Unconformably underlying the raised beach deposit is a dark brown-coloured sand with planar lamination and low-amplitude tabular cross-bedding. The internal sedimentary architecture of this unit is similar to that described from the Hillhead Sand Member at the Hillhead Section (Merritt et al., 1995), which was interpreted as a high-energy shallow marine setting.

The majority of the sequence is the interbedded sequence of fine-grained sands and silts with sedimentary structures that include planar and cross lamination and structureless massive sand. The planar cross-bedding is bi-directional, towards the north-west and towards the south-east. Overall, the unit is composed of a rhythmically stacked sequence of bedded silt and fine-grained sand. The succession is interpreted as a shallow-marine setting which was effected by tidal currents. This entire unit is interpreted as part of the Ardersier Silts Formation, as it is similar in many respects to the sequence from the Jamieson’s Pit section (as described in Fletcher et al., 1996). The observed sedimentary structures fit with the Ardersier Silts Formation being interpreted as a glacial subaqueous deposit. The offshore core provides an insight into the distal low energy part of the system, representing a low energy anoxic setting.

Overall, the entire section represents three distinct depositional units, including: 1) the oldest part of the sequence is a distal low energy marine environment of the Ardersier Silts Formation; 2) unconformably overlying the lower-most unit is a shallow-marine deposit of the Hillhead Sand Member; and finally 3) unconformably overlying the entire section is raised-beach deposit associated with the Postglacial Cliffline.

Structural discussion

The main body of the section, interpreted as the Ardersier Silts Formation, contains a number glaciotectonic features such as clay veining, brecciation and faulting. The deformation does not continue into the overlying Hillhead Member and beach deposits, suggesting that the deformation was prior to the deposition of these units. Three major types of deformation are observed, firstly clay veining of varying scales from 1–40 cm thick and varying from sub-vertical to sub-horizontal. The clay veins are observed at various points throughout the section. Due to limited exposure of the section within the trench the full length of each vein could not be traced. In general, the clay veins were structureless except on occasion along the boundaries where some poorly developed layering, of silt and clay, parallel to the boundary of the vein was observed.

Brecciation of the inclusions of silt and sand into angular poorly-sorted clasts within a clay vein was observed at several points in the upper (FOP 8) and lower (FOP 16) parts of the section. The scale of brecciation greatly varies from within veins of 10 cm thickness up to clay-vein features that were 40 cm thick.

Finally, small-scale thrust faults were observed within the upper and central parts of the section with offsets of approximately 5 cm (FOP 10). The faulting was most clearly observed within the central part of the section where no clay veining or brecciation were present. Major iron staining pathways were observed in this central section, which followed fault planes and sedimentary bedding structures. Smaller scale deformation structures, such as brecciated intraclasts and ball and pillow and flame structures, are also observed offshore within the lower energy clays.

The hydrofracture system-type features are typically found in glacial sediment that have been subsequently overridden by an ice stream (Lee and Phillips, 2008; Phillips and Hughes, 2014). Ice-stream related deformation of the Ardersier Silts Formation is also interpreted for the nearby-contorted sediments section (Firth, 1989; Gordon and Merritt, 1993). However, there is one major difference, which is in the style of deformation, which in the newly described section is brittle compared to ductile in the contorted Ardersier Silts section. This variation in the style of the deformation is not uncommon and is seen in other hybrid hydrofracture networks with multiple phases of deformation, such as in Norfolk, England (Lee and Phillips, 2008).

Regional comparison

The newly studied section predominantly comprises sediments of the Ardersier Silts Formation. The section is c. 8 m thick and comprises sedimentary structures that represent a coastal to shallow-marine setting, which is equivalent to the Ardersier Silts Formation sediments observed locally at the ‘consorted silts’ locality and at Jamieson’s Pit (Merritt et al., 2018). The well-preserved hydrofracture network, although different to that observed at the contorted silts section, is also likely formed as a consequence of ice-push and possible glacial over-riding by the Moray-Firth ice-stream during deglaciation.