Aspects of the sedimentology, diagenesis and palaeomagnetism of the Capitan shelf margin (Upper Permian), Guadalupe mountains, West Texas/New Mexico, U.S.A.

Abstract

Petrographic, palaeomagnetic and outcrop studies of limestones and dolostones from the Capitan reef complex, Guadalupe Mountains, west Texas and New Mexico, have provided time constraints on diagenetic events, and demonstrated the crucial role of calcium sulphate in the history of these strata. This thesis is divided into six chapters: chapter I gives an introduction to the scope of the study and the geology of the study area; chapters 2,3,4 and 5 give the results and Interpretations of sedimentological, diagenetic, magnetic fabric and palaeomagnetic/magnotic mineralogy studies respectively. Chapter 6 presents a discussion of the interplay of these results and interpretations, and conclusions. Emplacement of anhydrite in the strata of the Capitan shelf margin, took place at an early stage, post-dating syn-depositional marine cementation by aragonite and high-magnesian calcite, and minor syn-sedimentary dolomitisation. Desiccation of the Delaware Basin In late-Upper Guadaluplan times exposed the Capitan shelf margin. Consequently, evaporatively-concentrated brines migrated from the shelf Into the Delaware Basin via the strata of the Capitan shelf margin. Anhydrite cement occluded all porosity and fractures in the strata of the Capitan shelf margin which remained after marine cementation, and this cementation was accompanied by fabric-selective dolomitisation. Continued movement of brines during early burial, due in part to compactional dewatering of the Castile Formation, caused the growth of replacive anhydrite and concomitant pervasive, fabric-destructive dolomitisation, particularly in the lower foreslope facies. Bacterial sulphate reduction accompanied the input of meteoric water into the Capitan shelf margin during periods of uplift and erosion, and caused the calcitisation of some anhydrite and the growth of minor diagenetic magnetite, pyrrhotite and other sulphide minerals. These magnetic minerals have been palaeomagnetically dated and indicate Jurassic and late-Cretaceous to Tertiary ages. These ages are consistent with the periods of uplift and erosion Indicated by the burial history of these strata. Rapid uplift and faulting, associated with the development of the basin-and-range province of the southwestern USA, caused the wholesale dissolution of anhydrite, and established a meteoric aquifer In the Capitan shelf margin. Significant volumes of calcite spars were precipitated into the pore space and fractures which remained after the dissolution of anhydrite. Oxidation of most sulphides to haemalite and the development of now magnetic phases Le., goethite and haematite, occurred during this stage. These magnetic minerals have been dated palaeomagnetically, and indicate times of formation of 80 Ma to present-day. These ages are consistent with the period of rapid uplift and erosion which formed the Guadalupe Mountains during late-Cretaceous times to the present day.