Thermal Impacts of Salt on Sandstones and Mudstones in the Lusitanian Basin, Portugal

摘要

In order to assess the impact of salt structures on paleotemperatures in adjacent sedimentary rocks, a field based study which quantified thermally driven diagenetic products as a function of distance from salt was undertaken. Several salt diapirs are now exposed at the surface of the Earth within the Lusitanian Basin and offered a unique opportunity to document paleothermal anomalies by studying thermally controlled diagenesis in adjacent, exposed Upper Jurassic rocks. Four transects were chosen within Upper Jurassic rocks that contained both sandstones and mudstones. Quartz cementation and albitization of feldspars, both of which are thermally driven processes, were quantified in sandstones. Vitrinite reflectance, a commonly used indicator of maximum paleotemperature, was documented within mudstone samples. Together these diagenetic products were used to identify patterns in the size and magnitude of paleo thermal anomalies around salt structures.Relatively minor quantities of quartz cement and albite were found within all the transects, confirming that the rocks were shallowly buried and never reached critical temperatures high enough for significant quartz cementation or albitization of feldspars. Vitrinite reflectance for three of the transects showed elevated values near diapirs, which decreased in value as distance from salt increased. However, one transect displayed suppressed temperatures near the salt structure, which indicated a difference in the burial history of the diapir or adjacent sediments at that location. Corresponding temperatures converted from vitrinite reflectance documented positive anomalies up to 40 °C and a negative anomaly approximately 20 °C. Previous numerical models have based the size and magnitude of thermal anomalies around salt on the width, height, and depth of burial of salt structures. However, no consistent relationship was found with regards to diapir width and size of thermal anomalies. The variable sizes and magnitudes of the thermal anomalies in Upper Jurassic rocks in the Central Lusitanian Basin are interpreted as being primarily a function of diapir burial depth at the time the rocks were at their greatest burial depth.