Clayton Grove MSci (Hons) (Int. St.), FGS

How do igneous rocks influence diagenetic history? A case study from the Huab Basin NW Namibia

Supervisors: Richard Brown, Jon Gluyas, Chris Harris (UCT)

My research focuses on the effects of igneous geology on potential hydrocarbon reservoir rocks. I study areas where lava has interacted with aeolian sediments (e.g. The Cretaceous Twyfelfontein Formation dunes Namibia).

These dunes were drowned by lavas of the Etendeka Flood Basalt province, preserving complex field relationships between the sedimentary and igneous environments. Direct diagenetic effects of igneous activity on the sands occur at sediment-lava contacts. Indirect diagenetic effects where dykes and sills compartmentalise the basin, restricting fluid flow, occur throughout the succession. The unique preservation of these igneous effects is a consequence of the combined aridity at the time of drowning and aridity over geological time. Generally sand bodies in contact with lava have their early diagenesis controlled by the igneous geology which leads to porosity reduction by calcite and clay minerals, the relationship is complex and depends on multiple factors.

In dry settings igneous rock emplacement is a controlling factor on the lithification of both non volcanic detrital sediments and limited volcaniclastic sediments. My research suggests that this is through restricted magmatic-derived hydrothermal activity being the over-riding diagenetic fluid in the absence of meteoric or meteoric-derived groundwaters acting to dilute solute charged magmatic water.

In wetter settings such as present day Iceland, free water at the time of emplacement dominates such that the diagenetic signatures of igneous rock emplacement (sub aerial or subsurface) are recorded differently. This is further complicated by the large quantities of volcaniclastic material produced as a consequence of the wet climate.

So far my work has revealed the importance of water (meteoric and magmatic) in a volcanic/sedimentary system in controlling both diagenesis sediment type and quantity. Indeed the morphology of a flood basalt province is determined by the aridity at the time of emplacement and the interplay between igneous rocks and meteoric water. This continuum is formed between end members:

Petrographic Image Analysis

I also design workflows for the petrographic image analysis (PIA) of thin sections. An ImageJ based macro combined with a new 8-bit colour palette has been developed to rapidly measure rock porosity (we have called it jPOR). PIA techniques have also been developed to measure pore throats, grain contacts and pore shape parameters. The ability to rapidly measure these properties enables the swift construction of a large data set without the cost or time associated with traditional methods.