The impact of Etendeka flood basalts on the diagenesis of the fluvio-aeolian Jurassic Etjo Formation, Namibia

Salomon E, Caracciolo L, Stollhofen H, Bonnell L, Lander RH, Kittel M (2022)


Publication Language: English

Publication Type: Conference contribution, Conference Contribution

Publication year: 2022

Event location: The Geological Society, Burlington House, Piccadilly London

Abstract

The rapid effusion of volcanic rocks of large igneous provinces results in a relatively rapid burial of underlying rocks. In such settings, siliciclastic deposits and potential reservoir rock may on the one hand experience a high degree of mechanical compaction prior to the onset of significant chemical compaction, while on the other hand an increased geothermal gradient or hydrothermal activity may speed up cementation processes.

To assess such impact, we present the results of a study on the diagenesis of the aeolian upper unit of the Jurassic Etjo Formation in Namibia, that was subject to burial related to the emplacement of the voluminous Lower Cretaceous Etendeka flood basalts that reached presumed thicknesses of up to c. 2-5 km, and is today again exposed at the Earth’s surface. We have studied the Etjo Formation at three key localities, namely Waterberg, Mt. Etjo, and Gamsberg, that lie >100 km apart from each other. At all locations, the sandstone is texturally mature and quartzose with very low quantities of feldspars and lithic grains. However, petrographic analysis shows that the Etjo Formation has experienced a very different diagenesis at each location with respect to intergranular volume (IGV) and quartz cementation (Fig. 1). Average IGV and quartz cement values are 23.7 % and 6.5 % (Waterberg), 19.7 % and 15.4 % (Mt. Etjo), and 30.7 % and 24.3 % (Gamsberg). Porosity values, based on buoyancy measurements, are on average 21.0 w.t.% (Waterberg), 8.1 w.t.% (Mt. Etjo), and 5.2 w.t.% (Gamsberg). Also permeability, based on air permeameter measurements, varies between the locations with highest values at Waterberg (15 – 2819 mD), intermediate values at Mt. Etjo (14 – 132 mD), and lowest values at Gamsberg (2 – 82 mD).

We attribute the differences of IGV and quartz cement to spatial thickness variations of the overlying volcanic rock column. Higher compaction and cementation values at Mt. Etjo compared to Waterberg are likely related to the proximity of Mt. Etjo to the presumed center of Etendeka volcanism. Gamsberg is the only location where a first generation of quartz overgrowth formed prior to Etendeka burial, as evidenced by cathodoluminescence imagery, which likely has stabilized the grain framework to prevent further mechanical compaction during this burial. These interpretations are supported by TouchstoneTM numerical modeling for sandstone diagenesis. Permeability appears to be linked to the summed up quantity of quartz cement and eogenetic infiltrated clay.

Our study demonstrates the variability in diagenetic properties that a sandstone formation can attain depending on its location under a massive sheet of flood volcanic rocks.


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How to cite

APA:

Salomon, E., Caracciolo, L., Stollhofen, H., Bonnell, L., Lander, R.H., & Kittel, M. (2022). The impact of Etendeka flood basalts on the diagenesis of the fluvio-aeolian Jurassic Etjo Formation, Namibia. In Proceedings of the The impacts of volcanism on sedimentary basins and their energy resources. The Geological Society, Burlington House, Piccadilly London.

MLA:

Salomon, Eric, et al. "The impact of Etendeka flood basalts on the diagenesis of the fluvio-aeolian Jurassic Etjo Formation, Namibia." Proceedings of the The impacts of volcanism on sedimentary basins and their energy resources, The Geological Society, Burlington House, Piccadilly London 2022.

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