Paul Bishop

Drainage rearrangement by river capture, beheading and diversion:

Drainage rearrangement, involving stream piracy (capture), drainage diversion and/or beheading, may be significant for sediment budgets (including sediment provenance) and biotic distributions, as well as for its more usually considered role in landscape evolution. The processes involved in drainage rearrangement are not as self-evident as its abundant literature indicates. This is especially the case with the commonly invoked stream capture. The key process in stream capture, namely, drainage head retreat, is difficult to envisage as a normal part of drainage net evolution, especially in the light of recent findings on drainage hollow evolution. Stream capture may therefore be a relatively rare event in drainage net evolution. This, and uncertainties with interpretations of supposed elbows of capture, mean that stream capture should not be routinely invoked in interpretations of long-term drainage evolution. Further uncertainties associated with the maintenance of drainage lines during the erosion of significant crustal sections, especially in faulted and folded terrains, diminish the likelihood of many supposed examples of stream capture. It is more likely that examples of drainage rearrangement attributed to stream capture were generated by drainage diversion, but even this may involve special conditions.

Southeast Australian late Mesozoic and Cenozoic denudation rates: A test for late Tertiary increases in continental denudation

Throughout the Tertiary, average rates of erosion of southeast Australia have been uniformly low (about 10 −3 mm/yr of surface lowering), indicating that this part of the Australian continent did not contribute to a rapid increase in ocean-basin sedimentation over the past 15 or 20 m.y. Somewhat higher Mesozoic rates may be the result of minor tectonism, but explanation for the generally low Cenozoic and Mesozoic erosion rates may be sought in the forest cover and general tectonic stability of southeast Australia over this time period. Australia9s gradually increasing aridity/climatic seasonally over the past 20 m.y. has apparently not resulted in higher average rates of erosion of the continent, except perhaps in the very latest Tertiary and Quaternary.

Horizontal stability of the Australian continental drainage divide in south central New South Wales during the Cainozoic

Some recent models of the history of the east Australian continental margin suggest that the continental drainage divide has migrated to the west during the Cainozoic, whereas other models suggest a stable divide. All recent models of the history of the highlands (as distinct from the margin) imply a stable divide. Detailed data from Cainozoic channels in the proximal and distal reaches of the Lachlan and Wollondilly Rivers indicate divide stability in their proximal reaches for much of the Cainozoic, and at least throughout the Neogene in their distal reaches. Models of Tasman Sea formation, therefore, must incorporate a relatively stable Australian hinterland.

Limits to the age of the Lapstone monocline, N.S.W.—a palaeomagnetic study

Abstract The age of formation of the Lapstone Monocline has been long considered to be late Pliocene/early Pleistocene (the Kosciusko Uplift) but it is now generally thought to be much older. Palaeomagnetic data from haematite‐rich beds within the Hawkesbury Sandstone on and about the monocline indicate that it formed before the oldest haematite was introduced to these beds. The age of this oldest haematite is 15 ± 7 Ma. On the basis of these data, the age of the monocline is unlikely to be less than 8 Ma, probably exceeds 15 Ma, and could be older than 22 Ma.

Stability or change: a review of ideas on ancient drainage in Eastern New South Wales

Summary Reconstruction of ancient drainage requires the determination of (i) ancient flow direction(s) and (ii) the age(s) when such palaeoflow directions were operative. One approach to these problems is ‘geological’ and generally uses lavas which have flowed down ancient river valleys to reconstruct former drainage. A second, ‘morphological’ approach is based on W. M. Daviss Cycle of Erosion and uses the morphology of river valleys to determine both the age and former flow directions of rivers. When applied to eastern N.S.W., the geological approach is usually conservative, concluding that ancient drainage generally parallels the modern, while the morphological concludes that there has been, major disruption to the river systems of eastern N.S.W. in the relatively recent past, with concomitant westward migration of the Divide.

Silicified wood of Early Miocene Nothofagus, Acacia and Myrtaceae (aff. Eucalyptus B) from the upper Lachlan valley, New South Wales

Three specimens of silicified wood, two transported and one probably in situ, have been found in association with Early Miocene basaltic lavas in the upper Lachlan valley, N.S.W. On the basis of their preserved structure, the three specimens have been identified as belonging to Nothofagus, Acacia, and the family Myrtaceae, respectively. The specimen of Nothofagus (one of the transported specimens) constitutes the first identification of a macrofossil of this genus from this interval in southeastern mainland Australia, while its conjunction with the myrtaceous specimens (one probably in situ) may indicate vegetation zonation according to elevation, as proposed on pollen evidence from Kiandra. Such zonation is not demanded by the upper Lachlan evidence, however, but if it is favoured, reconstruction of the Early Miocene geography of the upper Lachlan constrains the maximum elevation above the locality of the myrtaceous fossils to be 350 m.

Absence of caesium-137 from recent sediments in eastern Australia — Indications of catchment processes?

Summary Sediment and soil containing the isotope caesium-137 ( 137 Cs) are generally understood to have been sub-aerially exposed within the last 35 to 40 years. The horizon of first appearance of 137 Cs in sedimentary deposits in Australia is equated with the mid-1950s, the time of first appearance of the isotope in Australia. Some sediments, however, which are known to have been deposited in alluvial cutoffs since the mid-1950s do not contain the isotope. This is interpreted as resulting from the high magnitude of the events which entrained and deposited the sediment. The sediment was eroded from sub-surface sites, and deposited and buried rapidly, thereby preventing the adsorption of the isotope. The data indicate that 137 Cs should be used very cautiously as a dating tool in settings where a good knowledge of the fluvial and sedimentological events accompanying the emplacement of the sediments is not available.

Early Miocene flow‐foot breccia from the Upper Lachlan Valley, New South Wales: Characteristics and significance

Abstract Flow‐foot breccia in the Upper Lachlan valley, NSW, probably formed when Early Miocene lavas dammed the Lachlan River and lava flow continued into the resultant lake. The distribution of the breccia and its non‐brecciated lateral equivalent indicates flow in the Early Miocene Lachlan River in the same direction as at present. Other evidence for the lava‐dammed lake includes palaeocurrent data consistent with flow into the lake, a small outcrop of fine‐grained, laminated sediments which would have been under the lake waters and, near the flow‐foot breccia, abundant silicified wood which probably resulted from drowning of the local vegetation, progradation of the flow‐foot breccia around the trees and their subsequent silicification.