Most of my professional career as a diamond exploration geologist was traipsing over the country sampling soils and drainages for kimberlitic indicator minerals, and successfully too I might add. Some preliminaries though: The geological profession can be categorised into two major camps, hard rock and soft rock. Hard rockers deal with volcanics, metamorphic and igneous rocks while soft-rockers deal with sedimentation, geomorphology, and hydrocarbons. Neither camp works on the other’s rocks. And there’s a small number of geologists who look for diamond deposits, and as kimberlite pipes can be found on cratons and sedimentary basins, our geological experience is a lot wider than our hard and soft rock peers.
So as a diamond geologist I tend to observe things my peers don’t, such as unusual occurrences of chernozem or black soil termite mounds on granitic outcrop in the middle of a paddock observed near Halls Creek in the Kimberley Region of Western Australia. This occurrence was one of those WTF ????? moments.
Some close ups follow:
The problem is that in this area there are no chernozems or black soils, and the question then is how? This mystery was compounded when I was working for South Star diamonds during 2005 and following up some previous De Beers results west of Cue on Meka Station in the Northern Goldfields region where I encountered a similar “black-soil” termite mound sitting on granitoids and other felsic rocks.
Mention these observations to one’s peers and the reaction is one of BS and dismissive mirth, leaving you with the feeling you had uttered a profanity. Profanity because such things, in terms of mainstream, settled geology, cannot exist. Well they do. Geomorphologists will probably go back to the pub and try to forget this termite provocation by intellectually numbing themselves with copious spirits and liquors.
Except we diamond explorers to make sense of the heavy mineral distribution on the land surface need to understand the erosional history of the areas we are working in.
As for the black-soil or chernozem termite mound remnants, the only plausible explanation is that there was a very recent widespread erosional event that stripped most of the regolith away leaving isolated outliers of bedrock with, I presume, isolated more resistant remnants of termite constructions.
When did this erosional event occur? According to geomorphologist Hugh Pringle who I encountered, with his PhD student, at Wooleen Station in the Murchison Region of WA, the erosional event occurred shortly after the introduction of sheep to the region by the British settlers, or some 150 years ago. This seems to be borne out by a remark Australian ABC documentary personality Jack Absolom recounted by aboriginal explanations for the various salt lakes dotted in southern Western Australia as being caused by the introduction of sheep, in one of his weekly editions on TV.
I should mention that in the Murchison District De Beers noticed all of the kimberlites they located in that region had zero indicator minerals on top of them. Pringle explained that this was caused by the once-only erosional effect caused by the introduction of sheep which resulted in a short downslope movement of the residual soil into the drainages, leaving denuded outcrops with small pedestals of bedrock as left-overs.
It seems there are a few keys missing from the Lyellian toolbox of the present and the past, especially the very recent past.
Update: There were no sheep near Halls Creek.