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J D A Clarke and A L Reisz

October 2004


Integrating Airborne Electromagnetic (AEM) and borehole data from the St George study region has revealed several new aspects of the architecture of the fluvial sediments that cover much of the area and the underlying weathered bedrock. This report contends that:

  • The Cretaceous bedrock of the region was deeply weathered prior to incision of a
    palaeodrainage network, comprising the Dirranbandi and Moonie Palaeovalleys and their
    tributaries. The saprolite is variably and discontinuously ferruginised, with major ferruginous zones apparently coinciding with conductive zones on AEM sections and Layered earth Inversion (LEI) depth slices. Overprinting silicification locally obliterates the conductive signature.
  • The deepest palaeovalleys were incised to fresh rock below the base of weathering, up to 200m below the present land surface. This is based on the interpretation of AEM sections which show the truncation of weathered bedrock against the sides of the palaeovalleys, confirming the interpreted drill hole information. There is a suggestion of structural control, possibly sag along deep-seated faults, to the rectilinear nature of the palaeodrainage network.
  • The fluvial succession first filled the incised palaeovalleys and then onlapped the shoulders of the palaeovalleys, forming a laterally unconfined aggrading plain. Bedrock hills in the study area are isolated remnants, rising above the level of the encroaching sedimentary plain. Like the contemporary rivers of the region, deposition occurred from mud dominated anastomosing rivers with sinuous to meandering channels. Braided streams are a subordinate facies during the aggradational plain phase.
  • Deposition of the cover sediments mostly likely commenced in the Pliocene, based on
    palynological evidence and has continued to the present without major breaks in the
    stratigraphy. The sedimentary architecture has, however, changed several times though time, the most significant such event coinciding with the change from confined sedimentation in the palaeovalleys to unconfined sedimentation on the aggrading plain, and due to the internal sedimentary dynamics of that plain. There is no evidence of ongoing tectonic influence or changes in external base level to the sedimentary succession.
  • There are three discrete layers evident in the AEM LEI depth slices. They consist of: the 40-160 m aggregate layer (incised valley fill). The 15-40 m aggregate layer (the aggrading plain fill). The 0-15 m aggregate layer (with the distinctive signature of the modern surface).

The documented fluvial architecture of the Lower Balonne Airborne Geophysics Project (LBAGP) has numerous implications for the hydrology of the region. In particular the documented architecture affects the aquifer structure, connectivity, and recharge potential. The implications are:

  • A predominance of anastomosing over braided or meandering architectures means that sand bodies are more likely to aggrade vertically than accrete horizontally. As a result the lateral extent of sand bodies is low. Because axial extent remains high this introduces a strong anisotropy to the aquifer geometry.
  • The sands of the palaeovalley fill (40-160 m aggregate layer) host the confined subartesianm lower aquifer of previous workers. These sands are likely to be recharged by horizontal flow upstream of the LBAGP.
  • Sands of the aggrading plain succession (15-40 m aggregate layer) host the unconfined upper aquifer. Especially important to the aquifer’s storage potential are localised braided stream facies between Dirranbandi and St George. The upper aquifer is likely to be recharged by thick anastomosing sands associated with contemporary and recently abandoned channels, and where the braided stream facies reach the surface north of the Noondoo Rises.

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