Wednesday, July 5, 2017

View from Alice's Tardis

(20170706-24) :-

Gravity, and the relentless imperative of flatness.


Western Australia's experience of The Great Regression = the time when the inland seas and lakes ran of the land into the developing ocean basins leaving in their wake - The Agnew Pub, .. which according to legend has been there forever.

C&P to Google Earth search bar < -27.952608°, 120.393448° >

(Agnew, Western Australia) [Picture = Agnew pub.]

Compared are two erosional surfaces that developed over Archaean granites. Note the very pronounced incision that can result from very slight topographical differences.   One surface is flat-as-a-tack, and only slightly lateritic (duricrust; darker brown in the upper central part of the figure) developed as a veneer on eroded, partly intrusive granite that formed a basement to inselbergs of tightly folded 'greenstones' [marginally submarine lavas, chert, iron formation and shaly and arenaceous sediments.] which form low hills out of the frame.

 The other surface is the more recent drainage cutting into the underlying lighter-coloured saprolite. The land drops about thirty metres over the step of the breakaway and a further thirty metres in seven kilometres towards the lower boundary of the figure .  The Yellow Pin marks some highly degraded sand dunes of indeterminate age on the older surface (possibly even partly older than the younger drainage radiating off the high ground?).The breakaway offers a good profile through the saprolite.

Not quite sure though what to make of the topographical contrast as regards climate variability.  Laterite forms in tropical and subtropical climates implying high rainfall.  Very selective chemical weathering is reflected by stony and spongy laterites being almost exclusively developed only over the more iron-magnesium rich greenstones which in the general area rise little more than fifty metres over the granite plain and are virtually absent over the granite duricrust.  The flatness of this granite surface is remarkable and is distinctive in having virtually no drainage.

About 10-15m below this laterite surface and slightly out of the frame is another, characterised by very broad open drainage (Link).  Two lateritic surfaces therefore.  Three, if that over the greenstones is counted, and all of them being cannibalised by the younger erosion of the present active profile.

Both surfaces (counting the two laterite surfaces as one) reflect a climate that was much wetter than today.  Despite today's rains being quite heavy on occasion and very quickly forming a sheet of water over the flatter parts of the landscape when it does rain, I personally doubt whether they could have caused the erosion apparent, which would appear to me to be much more likely due to the waning phases of that sub-tropical climate represented by the laterite  - when Australia (and Antarctica) (and Africa) were located much further north (closer to India)  [An old landscape indeed.]

 By its continent-wide distribution and extreme flatness, the lateritic duricrust is a surface of virtually zero erosion potential, .. as close to 'beach' and sea-level as it almost possible to be.  Not the sea of today however, but the anastomosing epicontinental 'seas' and lakes that covered the land before the breakthrough of the mantle of the Southern Ocean, remnants of which remain in the southward-draining, ennervated lake /river system of today.  The lighter-coloured, more recent aggressive fluvial incisison represents further reduction in response to epeirogenic (continent-wide) 'uplift' of the land outwards from the core [drop in sea-level].

(The Agnew pub?   Just a blast from the past, ... Visit it before any planned vacation to Mars.

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