OR/15/045 Challenges for interpreting borehole logs
|Kendall, R S. 2015. Conceptual cross-sections of superficial deposits in Cardiff . British Geological Survey Internal Report, OR/15/045.|
One of the biggest difficulties envisaged with discerning the various superficial deposits beneath Cardiff is that many of them, with the exception of the Tidal Flat Deposits are essentially stony lithologies. The challenge of distinguishing between the various stony deposits is compounded by the fact that fluvioglacial deposits and modern alluvium are essentially reworked tills. Many of the borehole records will not contain enough information to confidently assign the lithologies described to a deposit type.
It may be possible to gain confidence on deciding what the lithologies represent by considering their distribution using the geological map (BGS, 1989). The distribution of the deposits on the map was in part defined by distinctive landforms and larger outcrop sections where features such as bedding may have been available in temporary sections at the time of the mapping. The conceptual cross-sections will also give an indication of the relationship of a deposit to others that could be reasonably expected to occur adjacent to it.
Although the conceptual cross-sections provide an indication of the relationship of one deposit to another, there is considerable uncertainty regarding the amount of down-cutting and erosion which may have occurred before the deposition on the next deposit. The Quaternary history of the area is one dominated by erosion and rapid/localised deposition: ice-sheets cutting into bedrock and depositing tills; outwash rivers eroding away till and bedrock and depositing gravels; rivers cutting into the gravels and tills and depositing their alluvium. From the perspective of predicting the presence or absence of each deposit, the uncertainty lies in how deeply each erosive event cut into the previous—the previous deposit may be partly or completely removed by the subsequent erosional event.
Another area of uncertainty when interpreting borehole logs lies in the variability of the deposits. For example, the till is described as gravelly but also contains lenses of sand and gravel and lenses of clays. There is a challenge to distinguish between a lens of different material and what might be a more laterally-extensive deposit from a point source of information, such as a borehole. Where it is known that this variability exists, lenses of different lithologies are shown on the conceptual cross-sections to help with interpretation.
Buried valleys (Anderson and Blundell, 1965) present another feature which may cause ambiguity in interpreting logs. Buried valleys occur widely in areas of Britain affected by lowland glaciation and are deep scours formed by focussed meltwater erosion beneath a glacier. In other parts of Britain, these valleys can be upto 80 metres deep and contain a chaotic admixture of Till and sand and gravel. If the log does not reach rockhead, identifying the presence or absence of a buried valley can be difficult to determine. Likewise, determining the genetic context of basal sands and gravels can also prove to be challenging.
- BRITISH GEOLOGICAL SURVEY. 1989. Cardiff, England and Wales sheet 263, drift edition. 1:50 000. (Keyworth, Nottingham: British Geological Survey.)
- ANDERSON, J G C and BLUNDELL, C R K. 1965. The Sub-Drift Rock-Surface and Buried Valleys of the Cardiff District. Proceedings of the Geological Association, Vol 76, Part 4.