OFR 151.pdf - CRC LEME

However, the data are emphatic that rainfall gradients across the basin were similar in
direction but not in strength to those of the present-day.
Climates in the western Murray were suboptimal for the extensive development of
Nothofagus (temperate) or Araucaria (subtropical) rainforest during the Oligocene although
conditions remained seasonally wetter (humid) and warmer (mesotherm range) than at
present. Trends in rare thermophilous taxa suggest mean air temperatures reached maximum
values during the late Early to Middle Miocene, whilst diminishing Araucaria and
Lagarostrobos values imply that climates became effectively drier about the same time.
Marine flooding appears to have allowed grass communities to form around the margins of
the basin at a time when grasses were uncommon or absent in drier, interfluve habitats.
Conditions in the central Murray Basin initially were more equable in terms of rainfall
(perhumid) and mean temperatures (upper mesotherm) but became more strongly seasonal
during the Miocene. Helping maintain high humidity were the marine transgression of the
basin during the Early Oligocene and Late Oligocene-Middle Miocene, and rivers draining
the Southeastern Highlands. The replacement of Nothofagus (Brassospora) dominated
rainforest (Oligocene) by communities successively dominated by Araucariaceae (~Early
Miocene) then Casuarinaceae and Myrtaceae (~Middle Miocene) is reliable evidence that
conditions became effectively drier and increasingly seasonal during a period when mean
temperatures were increasing elsewhere in southern Australia.
The reconstruction of Oligo-Miocene climates in the eastern Murray Basin is complicated by
the influx of miospores derived from plants growing on the south-west slopes of the
Southeastern Highlands. For example Lophosoria and Nothofagus (Lophozonia) spp. are
consistently recorded in Oligo-Miocene assemblages in this sector but are rare elsewhere in
the basin. The data confirm that conditions were wet (perhumid) relative to the western
Murray Basin and probably cooler (upper microtherm-lower mesotherm) than the central
Murray Basin. Rainfall became effectively reduced (possibly more seasonal) during the
Middle Miocene. Warm temperatures and high sea level are likely to have increased
orographic cloudiness, and therefore increased humidity and lower mean temperatures, at
higher elevations on the Southeastern Highlands (cf. Martin 1973, 1993).
3. Gippsland Basin
Upper Nothofagidites asperus Zone microfloras representing the Eocene-Oligocene transition
lack many of the uncommon to rare taxa found in microfloras representing the Late Eocene
and Early-Late Oligocene lowland vegetation. Although the role of climatic change is blurred
by a major fall in global sea levels, very high relative abundances of Nothofagus
(Brassospora) spp. confirm that conditions remained wet to very wet (perhumid) throughout
the year. Accordingly, the most ecologically convincing explanation for the observed floristic
impoverishment is that mean temperatures decreased catastrophically during the Eocene-
Oligocene transition. If correct, then Upper Nothofagidites asperus Zone microfloras are
contemporary with the Lemonthyme Glaciation of northwestern Tasmania and development
of the Circumantarctic Current. Subsequent developments such as the re-appearance of
species with warm temperate-subtropical NLRs in the onshore Gippsland Basin, point to
gradual warming during the Early Oligocene-Early Miocene (Proteacidites tuberculatus
Zone). Maximum temperatures (mesotherm range) occurred late in the Early Miocene (early
Canthiumidites bellus Zone time). Conditions remained uniformly very wet (perhumid) but
rainfall may have become more seasonal during the Middle Miocene, based on the decline in
Nothofagus (Brassospora) spp. relative to sclerophyll taxa such as Myrtaceae and Proteaceae.
Runoff remained adequate to support Lagarostrobos swamp forests and herb-dominated
wetlands into Late Miocene time.
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