Search CRC LEME :

powered by FreeFind

Publication Policy

Open File Report Series

OFRS Index


Regolith Maps

Annual Reports

Articles & Papers


Minerals Briefs

"Focus on Salt"

Other LEME Reports

Order Form

Open File Report 51

Regolith-landform relationships in the Bottle Creek Orientation Study, Western Australia

Churchward, H.M., Butler, I.K. and Smith, R.E.

Regolith-landform Relationships

A framework of regolith stratigraphy and landforms was established for the area surrounding the Bottle Creek Au deposits, some 200 km north west of Kalgoorlie, as a basis for geochemical studies. An early phase of broad reconnaissance provided several sites at which the regolith was examined in detail and the landform-regolith relationships were defined. A regolith-landform map (1:10 000 scale) was produced for the upper Bottle Creek catchment. To test the findings of the detailed investigation, a study of the regolith was carried out over an area of approximately 450 km2 surrounding the detailed study at a reconnaissance scale. A 1:25 000 map of this was produced. Several well-defined regolith types were identified by these studies which relate, directly or indirectly, to a deeply weathered mantle and to its modification by landform processes. These regolith materials were either horizons of a deep profile developed by in situ weathering of basement rock or of transported debris, derived from this profile by erosion. Generally they form extensive surface and subsurface bodies so that the regolith, at any particular location, commonly comprises several strata.

The nature of the regolith stratigraphy is strongly related to the landforms with which they are associated so that a framework of landform regimes provides a useful concept for considering the regolith in this area. Thus, units of relatively stable, deeply weathered tracts were recognized as relics of a once more extensive landsurface that has been fragmented by fluvial action and replaced by erosional and depositional regimes.

At Bottle Creek, regolith types associated with the ancient, deeply weathered landsurface (the residual regimes) are mainly various expressions of the (upper) ferruginous horizon of the laterite profile, along with the mottled zone, the pallid saprolite and the saprock. Several transported regolith types, of colluvial and alluvial origin, were recognised in both erosional and depositional regimes.

Ferruginous Horizon(s)

The ferruginous horizon comprises various types of lateritic residuum but the more common are duricrusts having abundant lateritic pisoliths, generally with yellow-brown skins, set in a brown to red-brown clayey matrix. At depth, this material merges with the more ductile clays of the mottled zone. In addition, large irregular to lensoid bodies occur in the upper part of the regolith. The rock-like nature of these masses contrasts with the surrounding brittle, pisolitic, lateritic residuum referred to above. Some of these masses are Fe-rich, having a dusky red matrix and dark brown to black nodules that can be magnetic. Such materials have been placed in the broad class of Fe-rich duricrusts. Another rock-like mass is diffusely mottled brown to pale brown and some have vermiform voids. These can be variously classified as ferruginous saprolite, vermiform duricrust or fragmentary duricrust. Other Fe-rich bodies in the upper regolith are goethite-rich pods which generally have box-work textures. At Bottle Creek, these gossan-like bodies occur close to a carbonaceous, previously pyritic shale in the underlying parent greenstone sequence.

Residual Regimes

The residual regimes at Bottle Creek form gently undulating tracts that are extensive on the divide between the Raeside and Ballard drainages. The principal type location for the regolith stratigraphy of this regime is at the Emu test pit, within this undulating tract. Most crests in this terrain are slightly stripped with consequent exposure of an array of ferruginous materials from the upper parts of the regolith. These are predominantly ferruginous saprolite but there are also some pisoliths and pieces of Fe-rich duricrust, as well as clay and sand released by weathering. Such materials contribute to the colluvial mantle that extend down-slope from the crest, covering the pisolitic, lateritic residuum; the latter forming the more extensive substratum of the residual regime. On the mid- and upper slopes, colluvium is less than 1 m thick; whilst beneath the lower slopes and local drainage floors, the colluvium is as much as 4 m thick. The lag composition also varies with topographic position; coarse fragments of ferruginous saprolite dominate the crests. Some yellow-brown cutan-coated pisoliths also occur here and are generally indicative of some subcropping pisolitic, lateritic residuum. On surfaces, down-slope from these crests, lags of dark brown to black granules are dominant; there is little quartz or lithic material. The soils are acid and have developed in a fine, sandy loam colluvium which has granules of similar composition to the lag. Hardpans appear at a depth of 1 m and continue for depths of from 3-8 m.

Erosional Regimes

The landforms and the regolith types in the erosional regimes present a more complex picture reflecting active geomorphic processes. Deeper units of the weathered mantle, as well as country rock, are exposed. This regolith is dominated by a shallow, generally calcareous soil, and a lag of lithic fragments; there are outcrops of vein quartz and goethitic Fe-segregations. Gentle slopes occur as pediments below low breakaways. These slopes are mantled by acid red earths, developed in a pedisediment, and have a lag dominated by coarse, ferruginous saprolite, lithic fragments and quartz. Erosion is active in such areas.

Depositional Regimes

The most extensive depositional tracts are mantled by a friable clay, being an alluvium of sheet flood origin; acid red earths have developed on this material. The lags are dark brown to black granules of mixed origin, with medium sized (2-4 cm) lithic and ferruginous saprolite fragments, and quartz clasts, as a minor, though characteristic component. This alluvium overlies pallid saprolite and saprock at a depth of 1-1.5 m but it may also be found mantling pockets of pisolitic lateritic residuum or coarse deposits in palaeochannels. Some of the depositional tracts are being further modified by erosion, resulting in land-surfaces having regolith types comparable with those in other erosional regimes.

Regolith Evolution and a Framework for Geochemical Dispersion

The topographic relationships and regolith stratigraphies revealed by this study indicate a polyphase, multi-process history. Many of the regolith types resulting from this complex array of processes, have a distinctive pattern. The residual regimes at Bottle Creek are dominated by a regolith that is the result of intense in situ weathering, some of the uppermost regolith has been deposited by local colluviation. These areas have had a relatively stable geomorphic history. In contrast, depositional regimes here represent areas that have received fluvial detritus from much further afield and this material varies from highly weathered to relatively fresh and is generally of diverse lithological origin. Prior to deposition, these areas can have been subjected to widespread, though incomplete, stripping of the more weathered regolith types. The regolith in erosional regimes is, in detail, complex with exposure of a variety of variably weathered lithologies. Understanding this general geomorphic framework assists our appreciation of geochemical dispersion and thus it provides a basis for the developing sampling strategies for this weathered terrain.

Last updated: Thursday, January 06, 2000 08:57 AM


Cooperative Research Centres Australia

About Us | News & Events | Research
Publications | Education | Staff Only | Links

Contact Us | Disclaimer | Sitemap
© CRC LEME 2004

CRC LEME is established and supported under the Australian Government's Cooperative Research Centres Program. The CRC Program is an Australian Government initiative which brings together research groups with common interests.

CRC LEME Core Parties