Geothermal Gradients, Organic Metamorphism, and Mineral Diagenesis in Tertiary Rocks, Santa Ynez Mountains, California: ABSTRACT

Abstract

Organic metamorphism, as expressed by vitrinite reflectance, may be used to correlate mineral diagenesis with the thermal history of a sedimentary basin. Levels of organic metamorphism were compared with diagenetic minerals in three Paleogene sandstone and shale sections of the Santa Ynez Mountains and were used to determine paleotemperatures by means of the established time-temperature-reflectance relation. Calculated paleogeothermal gradients range from 31.0 to 40.3°C/km in the eastern end of the basin to 47.2 to 51.0°C/km in the west. Changes in the geothermal gradient in the Santa Ynez Mountains through the Tertiary were modeled using present surface temperatures and heat generation at the earth's center as boundary conditions. For instantaneous and continuous deposition, the governing equation is a linear parabolic partial differential equation. To model continuous deposition, a small dispersion coefficient term is used. The equations were solved by finite difference numerical techniques involving matrix solution by the Thomas algorithm. In instantaneous deposition, thermal equilibration is rapid in terms of geologic time, with temperatures restored to their presedimentation levels after 500,000 years. For continuous deposition, sedimentation rates of the Paleogene strata are slow enough to allow for nearly compl te thermal equilibration during deposition. The first appearance of laumontite in the eastern and central parts of the basin occurs at vitrinite reflectivities of 1.1 and 1.3% Ro (200°C) respectively, while laumontite first occurs at vitrinite reflectance of 0.5% Ro (110°C) in the western end of the basin. Variation in the ratio of fluid pressure to total pressure may be responsible for the observed distribution of laumontite. The transition from a mixed-layer illite/smectite illite-chlorite asemblage to an illite-chlorite assemblage in the interbedded shales occurs at vitrinite reflectance of 2.4% Ro (235°C) in the eastern part of the basin. End_of_Article - Last_Page 937------------