A quarry in southeastern Botswana exposes a weathering profile into a Mesoproterozoic sill of the Umkondo Large Igneous Province
This photo shows a weathering profile through a 1.1 billion-year-old mafic sill of the Umkondo Large Igneous Province that was exposed in a recently developed quarry in southeastern Botswana. My collaborator Richard Hanson is standing on fresh unweathered dolerite bedrock approximately 6 meters below the surface.
An example of spheroidal weathering where a corestone of fresh dolerite with only a thin oxidation layer is inside a zone of partially weathered rock layers called rindlets (Buss et al., 2008) giving the rock the appearance of an onion.
What can nicely be seen in the photo is that at the top of the profile near the surface are spheroidal boulders. These boulders have formed through spheroidal weathering (Chapman and Greenfield, 1949) wherein exfoliation caused by chemical weathering has lead to disaggregation of the rock that has progressed to make the sphere. The photo on the right shows an example of this weathering where a corestone of relatively fresh rock is surrounded by a zone of concentric, partially weathered rindlets (Buss et al., 2008). This process is indicative of expansion during weathering reactions that built up strain leading to the spheroidal fracturing. Further below the surface where Richard is standing in the main photo, there are corestones where the corners of pre-existing joint planes are being rounded off as a result of incipient spheroidal weathering. I was inspired to have a look at these photos yesterday by Prof. Susan Brantley of Penn State who was at the University of Minnesota giving a talk on observations of weathering and regolith development at Critical Zone Observatories in Puerto Rico and Pennsylvania. In the Luquillo Mountains site of Puerto Rico, quartz diorite is weathering spheroidally. They observe a relationship where the spheroidal boulders are smaller higher in the weathering profile but don’t have the excellent exposure given to us in this Botswana quarry. In the quarry, its clear that pre-existing fracture play a large role in determining the size of the weathering boulders. We do see, however, that the smallest most spherical boulders are just below the surface.
We were in Botswana to sample mafic sills like this one for paleomagnetic analysis. This work requires that we are able to sample fresh unweathered material where iron oxides that crystallized out of ancient magma have not been altered by recent surface oxidation. Spheroidal weathering works in our favor as the spheroidal fractures physically break off the chemically weathered rock leaving a core stone with a quite thin oxidation rind under which is unweathered rock with pristine iron oxide minerals. Yahtzee!
- Chapman, R. W., & Greenfield, M. A. (1949). Spheroidal weathering of igneous rocks. American Journal of Science, 247(6), 407-429. http://dx.doi.org/10.2475/ajs.247.6.407
- Buss, H. L., Sak, P. B., Webb, S. M., & Brantley, S. L. (2008). Weathering of the Rio Blanco quartz diorite, Luquillo Mountains, Puerto Rico: Coupling oxidation, dissolution, and fracturing. Geochimica et Cosmochimica Acta,72(18), 4488-4507. http://dx.doi.org/10.1016/j.gca.2008.06.020
A bifurcating clastic dike comprised of sand that was emplaced into a basalt flow in the Osler Volcanic Group of Ontario, Canada. The coin for scale is a Canadian Penny that has a diameter of 1.9 cm.
As a first-year undergraduate geology student, I remember being a field trip and first observing with great puzzlement a vertical body of sandstone within a lava flow. When my professor David Bice
suggested to other students and I that what we were seeing could be referred to as a “clastic dike” we were certain he was joking. We were used to seeing and thinking about dikes as being related to magmatic activity wherein rising magma forms and fills a fracture resulting in a vertical tabular body of volcanic rock. This same conflict of preconception of what a dike is with the reality that they can be comprised of sediment that we had on that field trip arose in Charles Darwin’s geologic observations while voyaging on the Beagle (Darwin, 1876). It turns out that clastic dikes where sediment cuts across other rock typically in a near vertical seam are relatively common in the geologic record and form by a variety of processes.
The photograph above is a view down onto a clastic dike that intruded into a 1.1 billion-year-old lava flow of the Keweenawan Midcontinent Rift
while the smaller photograph on the right shows a more oblique view of the same dike. The iron oxide mineral hematite within the sand of the dike gives it a reddish hue that results in a striking contrast with the dark grey of the basalt flow. During the time between the eruptions of lava flows in this stratigraphic section of the Osler Volcanic Group, sandstone and conglomerate accumulated. When this lava flow advanced over wet sediment, the watery sediment would have been confined under the flow with the water turning to steam. Under high pressure, a fissure opened up in the lava flow that was infilled with sand—voila, a clastic dike. Turns out Dave Bice wasn’t joking after all.
Jon Husson stands next to a cross-section through a stromatolitic bioherm in the carbonate-dominated Love's Creek Member of the Bitter Springs Formation (central Australia)
This photo shows a cross-sectional view of a stromatolitic bioherm with a diameter of ∼5 meters with Princeton graduate student Jon Husson
for scale. Stratigraphic up is ∼45° up and to the left of the image as can be seen in this annotated version of the photo. Stromatolites are laminated accumulations of carbonate sediment that were produced by growth and metabolism in ancient microbial mats. As cyanobacteria in these mats took in carbon dioxide, local alkalinity increased stimulating the precipitation of carbonate. In addition, extracellular polymeric substances produced by cyanobacteria provides a location where carbonate minerals nucleate and grow while also trapping carbonate sediment that can further contribute to stromatolite growth.
During growth, stromatolites can coalesce into hemisphere-shaped domal deposits termed bioherms that can be many meters across. Such bioherms are commonly comprised of many smaller individual stromatolites such as the these columnar stromatolites with transversely elongate projections pictured in the image to the left (and named following the taxonomic classification of Walter (1972)). The two dollar Australia coin shown in that image for scale has a diameter of 2 cm.