Cenozoic Eucla Basin and associated palaeovalleys,southern Australia — Climatic and tectonic influences on landscape evolution,sedimentation and heavy mineral accumulation

The Eucla Basin including the vast Nullarbor Plain lies on the margins of the Yilgarn, Musgrave and Gawler cratons in southern Australia and owes its distinctive landscape to a unique set of interactions between eustatic, climatic and tectonic processes over the last ~ 50 Ma. Understanding of the history of the basin and the palaeovalleys that drained from the surrounding cratons are important because they contain major mineral deposits, and the sediments derived from them contain remobilised gold, uranium, and heavy minerals. In particular, a remarkably preserved palaeoshoreline sequence along the north-eastern margin of the Eucla Basin is highly prospective for heavy mineral placer deposits. The record of marine, marginal marine, estuarine, fluvial and lacustrine environments, as constrained mainly by an extensive borehole dataset, reflects major depositional events during the Palaeocene–Early Eocene, Middle–Late Eocene, Oligocene–Early Miocene, Middle Miocene–Early Pliocene and Pliocene–Quaternary. These events reflect the key role of eustatic sea-level variation which, during highstands, inundated the craton margins, flooding palaeovalleys to up to 400 km inboard of the present coastline. However, a systematic eastward migration of the depocentre across the Eucla Basin during the Neogene, together with apparent flow reversals in a number of palaeovalley systems draining the Gawler Craton, suggest that the Eucla Basin has also been subject to differential vertical movements, expressed as a west-side up, east-side down tilting of ~ 100–200 m. This differential movement forms part of a broader north-down–southwest-up dynamic topographic tilting of the Australian continent associated with relatively fast (6–7 cm/yr) northward plate motion since fast spreading commenced in the Southern Ocean at ~ 43 Ma. We suggest that the evolving dynamic topography field has played a key role in facilitating development of placer deposits, largely through multistage, eastward reworking of near-shore sequences during highstand transgressive cycles on a progressively tilting platform under the influence of persistent westerly weather systems.     

白垩纪红层冰筏沉积的古气候及古地理意义

冰筏沉积是负载沉积物的冰块进入海洋或湖泊中，冰块融化沉积物附落所形成的，它们可形成于高海拔具有明显气候垂直分带的低纬度地区或者是高纬度地区。我们在研究松辽盆地白垩纪古气候时，首次在泉头组层中发现冰筏沉积。根据泉头组孢粉组合的研究透明，它们除了具有热带－亚热植物的孢粉以外，还有少量喜冷分子混生现象。     

Phanerozoic general circulation model results and quantitative climate data for Australia

Quantitative data bearing on Phanerozoic climates of Australia are summarised and comparisons are made with the predictions of atmospheric general circulation models over 12 time slices from the Silurian to the Miocene. Summaries of the model derived mean temperature and precipitation conditions are given individually for each of four regions (southeast, northeast, southwest and northwest). Model results provide the means of estimating climate conditions in many cases, but there is fair agreement between the two approaches in most cases in which data are available. On model results, the southeast experienced winter freezing conditions from the Permian through until the middle Cretaceous and was consistently the coolest of the four regions, in part because of its proximity to the pole, but also because it was the site of elevated terrain throughout most of the Phanerozoic. It also tended to be among the wettest parts of Australia over this time and was subject to frequent winter storms. The northeast, in contrast, was often the warmest part of the continent, although strong seasonality and freezing winter temperatures were common during the Mesozoic and Palaeozoic. Strong seasonality (>30°C contrast between winter and summer) also apparently characterised most of the record for the southwest region, except for the Tertiary, part of the Jurassic, and the Early Palaeozoic. Together with the southeast, this region experienced the most intense effects of winter, especially in the Triassic. Strong seasonality also affected the northwest region through much of the Palaeozoic and Mesozoic and, interestingly, the highest summer temperatures on the continent are predicted for the southwest for most of the time slices studied. The models used today are imperfect but, nonetheless, offer a means of prediction and will perhaps stimulate the gathering of quantitative data in a continent where information is hard to come by and for which not much has yet been presented.     

松辽盆地白垩纪冰筏沉积的发现及其地质意义

冰筏沉积是负载沉积物的冰块在春夏之交筏运到海洋或湖泊中，冰块融化沉积物坠落所形成的。它们可见于高海拔具有明显气候垂直分带的低纬度地区或者是高纬度地区。在研究松辽盆地白垩纪古气候时，首次在泉头组红层中发现冰筏沉积。根据泉头组孢粉组合的研究表明，它们除了具有热带—亚热带植物的孢粉以外，还有少量喜冷分子混生现象。经研究这些矛盾现象，反映泉头组时期盆地外围地形高差大，形成明显的气候垂直分带现象，这是造成这些矛盾现象的主要原因。冰筏沉积的发现对松辽盆地古气候古地理的恢复皆具有十分重要的意义     

First known Cretaceous glaciation: Livingston Tillite Member of the Cadna‐owie Formation,South Australia

A 2 m‐thick diamictite occurs near the base of the Cretaceous Eromanga Basin succession at Trinity Well, at the northern extremity of the Flinders Ranges in South Australia. The diamictite consists of a matrix of silt‐ and clay‐size particles and a framework of sand and coarser materials up to small boulder size. Scanning electron microscope study reveals the presence of numerous quartz grains displaying extreme angularity and surface textures attributed to glacial crushing. Sandy sediments considered as fluvioglacial in origin and a locally developed facies displaying flow structures attributed to solifluction processes constitute the basal 3–5 m of the sequence. In places these directly underlie the diamictite and rest with angularity on Neoproterozoic Adelaidean strata. Conformably above the diamictite at the type locality ‘Recorder Hill’ is a sequence approximately 15 m thick of fine sand and silt units containing lonestones up to ～70 cm diameter and hummocky cross‐stratification. These sediments have been assigned to the Cadna‐owie Formation and are dated on palynology as Berriasian to Valanginian. The occurrence of diamictite containing glacially affected quartz grains contributes to our interpretation that the southern margin of the Eromanga Basin, and at least the adjacent part of the northern Flinders Ranges, were affected by glaciation in the Early Cretaceous. The associated dropstone and solifluction facies and nearby glendonite pseudomorphs after ikaite are further evidence of at least intermittent cold climates at this time.     

利用黄土中的碳同位素半定量地重建古温度

在相对稳定的气候阶段,土壤中的CO2与其中的碳酸盐之间处于同位素平衡状态,这意味着二者的碳同位素分馏值可作为地质温度计估算古温度。对中国洛川黄土剖面的10组黄土-古土壤序列L1-1-S8同时做了碳酸盐和有机质的碳同位素分析。由于这些地层单元可以与深海氧同位素记录的2-21阶段对比,其中L1-1-L2可与南极Vostok冰心的大气CO2浓度记录对比,由此通过相关分析确定了各地层形成时的平均大气CO2浓度及其δ^183C值,再根据Cerling模型,用有机质δ^13C值代替土壤呼吸CO2的δ^13C,算出土壤CO2的δ^13值。将该值与次生碳酸盐的δ^13C值代入Deines分馏方程可算出各地层单元的古温度。该温度代表了平均深度约30cm处的土壤温度,相当于当时的夏季大气温度。所有数据与黄土-古土壤地层学记录的气候变化吻合,定量地反映了近800ka来黄土高原古气候的变化。     

Characteristics and evolution of the Tertiary palaeovalleys in the northwest Gawler Craton,South Australia

Integrated geoscientific datasets have contributed to an understanding of the Tertiary palaeovalleys once draining the Gawler Craton. Systematic investigations of both the shape and depth of the channels are based on interpretations from field exposures, a compendium of geological and drilling data, computer modelling of ancient landscapes, topographic and evaluated digital elevation models, remote sensing imagery, magnetics, seismic, gravity, airborne and transient electromagnetics, and radiometrics. Physical property contrasts that exist between the channel sediments and the underlying bedrock, for example, can be differentiated by geophysical methods to locate the incised‐valley thalweg. Evidence from sedimentology is combined with evidence from other geological and geophysical characteristics to arrive at a general reconstruction of palaeovalley architecture and history. The palaeovalleys were originally incised into the weathered pre‐Tertiary landscape of mostly weathered basement, and Tertiary fluvial, lacustrine, estuarine and even marine sediments accumulated during the Eocene and Miocene. Marine influence extended at least 100 km up the palaeovalleys during at least three major transgressions in the Eocene and Miocene intervals. Major sedimentary phases occurred in the Paleocene to Early Eocene, Middle to Late Eocene, Oligocene to Early Miocene, and Middle Miocene to Early Pliocene times.     

IX. The St. Vincent Basin

Late Palaeozoic glaciation in Australia, discovered over a century ago, is now known to have covered a large part of the continent. In South Australia, tillite and outwash debris lie upon clearly striated pavements within glacial valleys, and show that ice sheets with valley tongues moved northward from sources now occupied by deep ocean south of the continent. These glaciers reached into the Cooper, Arckaringa, and Pedirka Basins at the end of the Carboniferous and laid down patches of till in the Early Permian, now preserved largely in the subsurface. In Tasmania, an ice sheet waxed in the latest Carboniferous from sources to the west of the island, and deposited till and “drop‐stones” into fossiliferous marine strata until well into the Late Permian. In Victoria, the ice cap laid down till on a striated floor, and here and there sequences of outwash, including boulder pavements. In New South Wales, continental glaciation expanded eastward to the sea early in the Permian, and left a record intercalated with volcanics and coal beds into the Late Permian. Bordering the Tamworth Trough of northern New South Wales, and occurring also in the highlands of New England, alpine glaciers left a record in the form of striated stones and dropstones, in very thick sequences of fluviatile, lacustrine, and marine clastic sediments. The mountains existed in Middle and early Late Carboniferous times, and were largely worn down to gentle relief when continental glaciers expanded northward in the Early Permian. A non‐glacial interval at the end of the Carboniferous therefore probably occurred in New South Wales. In Queensland, alpine glaciers occupied mountains at the western rim of the Bowen Basin at the end of the Carboniferous. Large blocks carried by icebergs from glaciers of unknown locations were dropped into Lower and Upper Permian strata of the Bowen Basin as well. In Western Australia Early Permian ice centres were located on the Yilgarn Block, east of the Perth Basin, on the Pilbara Block southwest of the Canning Basin, and on the Kimberley Block. Evidence for this glaciation consists mostly of ice‐rafted debris and fluvial‐glacial and glacial‐marine strata that reached as far north as the Bonaparte Gulf Basin.

The rapid growth northward of continental glaciers in Australia near the end of the Carboniferous corresponds with a rapid shift of palaeolatitude as judged from Irving's palaeomagnetic studies. The ice sheet grew quickly upon upland areas when Gondwanaland moved to a near polar position and the unfrozen Palaeo‐Pacific lay near at hand to provide an abundant source of moisture.     

由黄土中碳同位素来判定末次间冰期以来的古气候

本文通过在洛川黄土地层中采样和测定,绘制出了黄土地层δ~(13)C变化曲线。将之与美、欧、深海、极冰等多种气候曲线做对比,结果表明它是一种极敏感的代用性气候指标,而且它对湿度反应最为灵敏。配合过去所研究过的用全铁含量来反映温度变化的作法,使我们在把δ~(13)C曲线转换成直接气候指标方面更向前趋近了一步。