Sediments deposited by glacial meltwaters (for example, ice-contact delta deposits) form permeable packages in the subsurface that can act as reservoirs for both water and hydrocarbons. They are also an important source of aggregate for the construction industry. As reservoirs they are challenging to characterize in terms of their structure, flow and storage properties due to their complex depositional history. In this study, ice-contact deltas of Salpausselkä I and II end moraines in Southern Finland are studied using a combination of geomorphological mapping, sedimentological studies and near surface geophysical methods. Sedimentary logs from isolated outcrops were correlated to ground penetrating radar (GPR) profiles to unravel the internal structure and depositional history of these ice-contact deltas. Subsequently, electrical resistivity tomography (ERT) and gravity data were analysed to estimate the depth to bedrock and to model porosity distribution within the sediments. Results of the study suggest that the delta deposits have a broad range of porosities (10 to 42%) with lowest values found in the bottomset beds. The most variable porosities are in the subaqueous ice-contact–fan zone, and consistently high porosities occur in delta foreset/topset facies. Detailed sedimentary logging linked to the GPR data shows heterogeneities such as mud drapes on foresets and kettle holes which are below the resolution of ERT and gravity methods but significantly affect reservoir properties of the deltas. Moreover, oscillation of the ice-margin may have introduced larger heterogeneities (for example, buried ice marginal ridges, or eskers) into the sedimentary sequence which are atypical for other Gilbert-type deltas. Finally, subglacially sculpted, highly variable bedrock topography exerts a major control on sediment distribution within the delta making reservoir volume and quality less predictable. This work has implications for present-day freshwater aquifers and low enthalpy geothermal energy in southern Finland and other deglaciated regions, as well as hydrocarbon exploration of analogous deposits in the subsurface from Pleistocene and pre-Pleistocene glaciogenic sequences.
We would like to thank Mariusz Mis ́kowicz, Markus Valkama and Aleksi Tuunainen for their help and company during the fieldwork. Hard work becomes much more enjoyable when there are good people around to share the burden. We are also grateful for the help and support pro- vided by the team at the Geological Survey of Finland Espoo Office without whom this work would have not been possible. Authors are very grateful for the constructive comments and sug- gestions offered by two anonymous reviewers which allowed us to greatly improve the manu- script. Least but not last, we would like to thank the editorial board of Sedimentology, Dr Victoria Valdez, Dr Ian Kane and Elaine Richardson for their prompt and efficient handling of the manu- script in times when none of the above should be taken for granted.
Natural Environment Research Council. Grant Number: NEM00578X/1The data that support the findings of this study are available from the corresponding author upon reasonable request.
- Younger Dryas
- glaciogenic sediments
- ground penetrating radar
- ice-contact deltas
- reservoir properties