Divining Burial and Thermal Histories from Indicator Data: Application & Limitations

Abstract

Any property of a sediment which depends on temperature or stress can potentially provide a record of the burial and thermal history that the sediment has experienced. Such indicators include the present day temperatures, fission-track age and length data, vitrinite reflectance, spore colour, ³⁹Ar / ⁴⁰Ar ages from feldspars or diagenetic illites, smectite/illite ratios, biomarker transformations and stress dependent properties such as porosity and sonic velocity.

Almost all known thermal indicators are measurements of irreversible transformations. Thus up until the point at which the transformation is complete these indicators primarily record the maximum temperature. Fission-tracks, however, are a reversible indicator that provides information (over a restricted temperature range), about the heating/cooling rate, maximum temperature reached and timing of thermal events e.g. cooling (uplift). They can potentially give useful information about the uplift history and in favourable circumstances both the burial and uplift history.

A suite of indicators can be used in conjunction with a thermal/stress modelling program to refine the burial and thermal history. Clearly, the more thermal indicators which are included in the analysis, the greater the constraints that can be placed on the thermal history. However, at some time during the burial history even the most resistant indicator may have been reset, and the temperature history prior to this time is thus completely unconstrained. This point can be as recent as the present day for cases where the current temperature is the historical maximum. The situation is even more complex when multiple phases of uplift/burial/thermal events have occurred. Thus reverse thermal models are limited in their ability to see into the past. In order to model the temperature history by forward thermal modelling assumptions have to be made about the unconstrained portions of the burial history i.e. the amount of section missing at unconformities, and the variables controlling the heat flow.

The process of fitting the calibration data is generally carried out on a well-by-well basis adjusting the model parameters to fit the constraining data. However, such a scheme commonly leads to inconsistencies when evaluating the thermal history of a larger area because insufficient attention is paid to the likely range of variation (e.g. the length-scale over which heat flow can vary).

A large amount of apatite fission-track data is available for the Northwest of England (Lake District, East Irish Sea Basin and Cheshire Basin). In this area the post-Triassic burial history is very poorly constrained due to erosion and non-deposition. Explanations for the patterns seen in the AFT data include: a Tertiary heat flow pulse, hot fluids circulating through the basin, and several km’s of uplift. The use of multiple thermal indicators can be used to show that some interpretations based only on AFT data can be misleading and that there is no unique solution using thermal modeling alone.

In conclusion, interpretations based solely on AFT data cannot supply a unique answer to burial history problems. The use of multiple thermal indicator data improves resolution. Howard, the inherent limitations on the use of indicator data and the lack of unique solutions from forward modeling ensures that it is difficult or impossible to distinguish between some hypotheses based on indicator data alone. Ultimately the additional data necessary to refine the thermal history can only be found from (regional) geological data, and fundamental geodynamic constraints.