Regolith-landform mapping: principles and application in WA Richard Langford

What is regolith? Material, whether cemented or unconsolidated, that overlies unweathered bedrock Formed by weathering and/or other geomorphic processes including erosion, transport, sedimentation, and chemical deposition Obscures geological structures and lithologies Blankets, to depths of hundreds of metres, potential mineral deposits Hosts a variety of derivative mineral deposits ranging from placers to chemical precipitates

Why map regolith? Record information Provide derivative maps type, variation regolith-landscape maps Provide derivative maps engineering geology, regolith geochemistry, etc Visualize regolith data aid understanding of relationships Regolith research mostly based on mapping

Landscape-based mapping Soil mapping Morphological mapping Geomorphological mapping Regolith-landform mapping Numerical land systems mapping Land capability mapping Land resource studies Land evaluation frameworks Engineering geological mapping

Soil mapping Soil classification versus mapping units Classification does not indicate how soils are associated in the landscape Purity of mapping units Soils closely related to position in the landscape — catena/toposequence concept Prediction based on position in the landscape Allows mapping from aerial photos using landforms as basis for subdivision Scale independent — essential link is relationship between soil types and landforms

Catena concept 1 Particular slope forms are associated with particular soil sequences “ a unit of mapping convenience . . . a grouping of soils which while they fall wide apart in a natural system of classification on account of fundamental and morphological differences, are yet linked in their occurrence by conditions of topography and are repeated in the same relationships to each other wherever the same conditions are met with” (G. Milne)

Catena concept 2 Differences between soils of a catena are generally related to differences in their position in the landscape and their drainage characteristics Important factors include slope steepness, and slope processes such as surface wash, solution, and mass movements Underlying rock is the parent material only in upland soils Rest of catena developed in transported material

Morphological mapping Based on the recognition of breaks and changes of slope, rates of change of slope, and slope form lines Indicates form and steepness of landscapes plan and profile convexity and concavity slope categories Essential building block for all geological mapping basis of field mapping training in BGS

Geomorphological mapping Uses morphological map as its foundation Adds information related to landscape processes Fieldwork needed to determine regolith-landform relationships Examples: RED scheme of Anand & Smith, based on assumptions about the former landscape soil maps using catenas for soil-landscape land system maps, similar to regolith-landscape maps

Regolith-landform mapping Basis for both soil and land systems maps Unit boundaries based on landforms Map polygons described in terms of regolith types and landforms Models of landscape evolution constrain the predictions about regolith-landform relationships Regional models will not predict the location of specific regolith materials, but allow prediction of regolith-landform associations (land systems) Local models will predict the location of specific regolith materials

Land system maps 1 Recurring pattern of topography, soils and vegetation Underlying geology, landforms, soil, hydrology and vegetation Used to predict land properties in unknown areas Similar result to regolith-landscape maps AGSO (GA) regolith-terrain maps recurring landscape elements and their associated underlying regolith packages landform area characterized by similar landform and regolith attributes

Land system maps 2 Map units are not the same as bedrock units Method is not new, but unfamiliar Method is scale-independent Models of landscape evolution are useful for predicting materials, vegetation, rock, and landform characteristics Has proved effective for rapid terrain classification of many areas Based on the principle that changes in the landform characteristics can be expected to affect the soil and land cover characteristics of an area Adopted soil catena/toposequence concept

Regolith stratigraphy Bedrock mapping techniques inappropriate Inappropriate to give formal rock unit names Law of superposition does not apply Nature of regolith reflects the evolution of the landform on which it occurs Correlation of land surfaces is difficult Cannot naturally be placed in time stratigraphy Lithostratigraphic units for transported regolith only Placing in situ regolith in a stratigraphy is illogical

Process-Material-Landform Assess each unit Compromise Dominance Basis of map code Landform Material

Examples Howatharra King of the West and White Flag Lakes Kalgoorlie use of landform in mapping mapping rock and regolith together simple codes for a simple situation King of the West and White Flag Lakes problems with spectral properties direct observation of landform and material codes from space or on the ground? Kalgoorlie portraying a simple interpretation can the codes be made to work?

Howatharra landform & geology 1 Double scarp Newmarracarra Limestone Regionally 175-200 m AHD

Howatharra landform & geology 2

Howatharra landform & geology 3 Importance of landform mappable features eg breaks in slope topography eg talus slopes and footslopes Importance of weathering fresh or ferruginized limestone and shale Rock or regolith? process - fluvial and marine (rock) or weathering? material - limestone, weathered rock, residual sand and gravel, colluvium landform - plateau and escarpment Jd, Jc, Rf, Rz, Ct, Cf

King of the West & White Flag Lakes Decorrelation stretch 457 RGB terrain units compositional variability see Tapley & Gozzard 1992 Ratios 5/7 4/7 4/2 RGB mapping landforms 5/7 clay & vegetation 4/7 clay vs iron 4/2 ferrous vs ferric iron Direct observation process material landform

King of the West & White Flag Lakes

King of the West & White Flag Lakes King of the West Lake process - fluvial or valley fill material - fluvial landform - remnant terrace At or Vt White Flag Lake process - distal colluvial material - fluvial or colluvial landform - fan Cf

Inverted topography 1

Inverted topography 2

Inverted movie (please wait 20s)

Inverted topography 3 Is the regolith now rock? A or R? process - fluvial and residual material - fluvial and valley fill sand and gravel over completely weathered bedrock or residual soil landform - hillcrest A or R? a landform code of A is misleading ignoring the fluvial origin is misleading which parameter gets the guernsey? Appropriate map codes demonstrate understanding and complement, not supplement, the description

Landform as a surrogate Pros faster and more efficient than fieldwork continuous coverage represents spatial relationships well can be applied in other areas Cons need field validation of correlation good for thin cover, but not subsurface variations genesis or age not related to landform

Summary 1 At regional scales, regolith-landform maps are similar to land system maps At local scales, regolith-landform maps are more like soils maps At both scales landforms provide the basis for mapping Not enough to describe landforms Essential to understand geomorphological principles behind regolith and landform development

Summary 2 Regolith-landform mapping Provides a basis for predicting the distribution of regolith materials Allows comparison between areas Provides the foundation for a series of thematic maps useful in land use planning, such as land system maps Establishes exploration methods for an area