The weathering of norite at Black Hill,South Australia

Black Hill is a boulder‐strewn residual of norite standing 45 m above the Murray Plains about 80 km northeast of Adelaide. Between the boulders, the crystalline rock has weathered to a dark‐brown terra rossa‐rendzina soil with calcite, illite, kaolin, and hematite as the principal secondary minerals. At one site on the smooth lower slopes of the outcrop, the material above the norite consists of partially weathered granular fragments with considerable dolomite and some calcite in nodular form. Below the surface, the rock has been weathered along joints to produce the clay mineral nontronite, and between each corestone and this plastic clay there is a zone of laminated but essentially unaltered rock 10–25 cm thick. Pieces of amorphous silica occur sporadically in some joints and on the surface.

The dark‐brown soil appears to be related to the present environment but the nontronite in the joint weathering, the dolomite, and the amorphous silica are all consistent with the norite having been inundated, possibly during the Miocene marine transgression.

Calculations based on the retention of elements such as aluminium, potassium, titanium, and iron have been used to predict quantitatively the amount of certain minerals such as quartz, calcite, and dolomite introduced to the weathering profiles.     

新生代构造抬升对地表化学风化和全球气候变化的影响

全球新生代构造抬升 ,特别是南亚喜马拉雅青藏高原和南美安底斯山脉和Altiplano高原在新生代的抬升对地表化学风化和全球气候变化产生了重要影响。它对地表化学风化的影响主要表现为引起造山带地区化学风化能力的提高 ;而它对全球气候变化的影响则主要表现在两个方面 ,一是直接的物理影响 ,即通过对大气和海洋循环的影响来对大气变化产生作用 ;一是通过对地表硅酸盐岩石的化学风化造成大气CO2 变化和全球温度的改变 ,从而对气候变化产生间接的生物化学效应。目前看来 ,新生代构造抬升造成的大气CO2 浓度减少是造成全球新生代气候变冷的重要原因。这已得到了近 10年来计算机大气环流模型 (GCMs)数值模拟和野外实验研究的支持 ,但在关于地表化学风化的主要控制因素 ,以及海洋Sr同位素是否可作为反映地表化学风化速率变化的替代性标志和气候变化反馈机制等方面 ,还需要作进一步研究。     

Exposure ages from relict lateral moraines overridden by the Fennoscandian ice sheet

Lateral moraines constructed along west to east sloping outlet glaciers from mountain centred, pre-last glacial maximum (LGM) ice fields of limited extent remain largely preserved in the northern Swedish landscape despite overriding by continental ice sheets, most recently during the last glacial. From field evidence, including geomorphological relationships and a detailed weathering profile including a buried soil, we have identified seven such lateral moraines that were overridden by the expansion and growth of the Fennoscandian ice sheet. Cosmogenic ¹⁰Be and ²⁶Al exposure ages of 19 boulders from the crests of these moraines, combined with the field evidence, are correlated to episodes of moraine stabilisation, Pleistocene surface weathering, and glacial overriding. The last deglaciation event dominates the exposure ages, with ¹⁰Be and ²⁶Al data derived from 15 moraine boulders indicating regional deglaciation 9600 ± 200 yr ago. This is the most robust numerical age for the final deglaciation of the Fennoscandian ice sheet. The older apparent exposure ages of the remaining boulders (14,600-26,400 yr) can be explained by cosmogenic nuclide inheritance from previous exposure of the moraine crests during the last glacial cycle. Their potential exposure history, based on local glacial chronologies, indicates that the current moraine morphologies formed at the latest during marine oxygen isotope stage 5. Although numerous deglaciation ages were obtained, this study demonstrates that numerical ages need to be treated with caution and assessed in light of the geomorphological evidence indicating moraines are not necessarily formed by the event that dominates the cosmogenic nuclide data.     

论雷暴风化作用

在位于冀东地区海拔１８４０多米的都山顶峰，笔者首次发现了雷暴风化作用，并目睹了雷暴风化作用的进行。在都山顶部，发现了雷暴风化砾石堆，分析了雷暴风化作用的物理依据，估算出一次雷暴作用所释放能量的０．０７％－０．００７％就可使１ｍ＾３的花岗岩断裂。阐述了雷暴风化作用的时空分布规律，指出雷暴风化作用主要发生在低纬度近山地地区。气候比较温暖且降水量较多的地史时期是雷暴风化作用较发育阶段。对于中国东部中低山     

Origin of sheet structure, 1. Morphology and boundary conditions

Sheet structure, or large-scale exfoliation, is the division of a rock mass into lenses, plates or “sheets” approximately parallel to the earth's surface. Sheet fractures, which separate the plates, are characterized by surface markings resembling those formed during the brittle fracture of metals, glass and ceramics and those on joints in rock. Lineations, or hackle marks, parallel to the direction of fracture propagation, are common. Branching occurs during propagation so that the fractures appear as many echelon segments in profile. The opposing surfaces of sheet fractures observed in quarry walls and natural exposures are typically in contact. Damage, if any, to rock adjacent to sheet fractures is generally limited to a zone less than one centimeter wide. Sheets tend to parallel preferred orientations of microcracks in many rock masses. Most sheets that are exposed today are of prehistoric origin, but in some rock masses, such as Chelmsford granite in Massachusetts, new sheets continue to form.

Sheet structure forms in environments of high differential stress, dominated by large-scale compression parallel to an exposed rock surface. In parts of the Chelmsford granite the magnitude of this compression is greater than 30 MPa in the zone of sheeting. High differential stresses near the ground surface can result from several natural agents, including contemporary tectonic forces, vertical unloading of a rock mass that formed at depth under high triaxial compression, and suppression of expansion that would otherwise result from temperature increases or chemical alteration of the rock. Exfoliation of boulders during forest fires and spheroidal weathering of boulders appears to occur under similar states of stress, although the agents responsible for the stresses differ.     

Weathering rates over 40,000 years based on changes in rock properties of porous rhyolite

Weathering rates based on temporal changes in a variety of rock properties were examined for four dated lava domes made of porous rhyolite. The lava domes were erupted at 1.1, 2.6, 20 and 40 ka BP. The time from each eruptive event to the present provide elapsed weathering times. The lava domes have many cooling joints formed immediately after eruption. Joint-bounded blocks have a high porosity, resulting in deep weathering. The blocks have almost uniform characteristics from top to bottom in one outcrop. The mineralogical, chemical, physical and mechanical properties of the four rhyolites were analysed. These results show that the chemical properties and physical properties, including specific surface area, change slowly in the early stage of weathering (0–20,000 years) and quickly in the later stage (20,000–40,000 years), while mechanical strength and bulk density or porosity of the rhyolites change rapidly in the early stage and slowly in the later stage. Mechanical strength reduces by 70–90% relative to fresh rock in 40,000 years, although chemical and physical properties change by only 1023̄0 %. It is concluded that different weathering properties exhibit different rates and patterns of change over 40,000 years, and that mechanical strength is the property most susceptible to weathering.     

岩体化学风化的非连续性及其科学意义

岩体化学风化在空间上具有高度的非连续性，这种非连续性广泛存在于从宏观、细观到微观的所有尺度。宏观结构面是化学风化最主要的发生场所；风化岩体内，新鲜岩块被沿结构面内法线方向发育的腐蚀带包围，呈斑点状分散于腐岩中。微缝等细观损伤普遍存在于各类岩石中；化学风化从岩块内不同空间位置的水力有效空隙向三维空间扩展，决定了细观尺度上化学风化的非连续性。矿物溶解是在晶体中具有过剩表面能的缺陷位置优先发生的，因而具有显著的微观非连续性。由于非连续特性，化学风化可增大水岩界面，提升矿物溶解反应的规模及速率。通过对既有损伤的扩展及在损伤空间堆积残余物，化学风化具有分离—裂化岩体、岩块及造岩矿物的重要作用，这种作用可使以新鲜岩石为主的岩体大规模脱离母岩，而堆积于坡脚的岩石块体在化学风化的继续作用下，可裂解为更小的岩屑或矿物碎屑，为向水体搬运创造条件，从而极大地促进斜坡夷平及地貌重塑进程。     

金沙江下虎跳滑石板滑坡破坏机理及稳定性分析

1996年10月28日发生在金沙江下虎跳峡口的滑石板滑坡,因堵塞金沙江、摧毁公路而引起高度重视。野外调查发现,滑石板滑坡是典型结构面控制的顺层基岩滑坡,层面和两组近于垂直层面的节理将边坡厚层灰岩切割成不同规模的菱形块体,长期的风化作用、重力、降雨、人工开挖公路等因素作用下,这些菱形块体从顺层稳定状态演化为稳定性较差的悬挂体,受河谷边坡斜向割切影响,表现为梯级叠次悬挂,最外边的悬挂体稳定性最差,最容易发生快速滑动。因此,该滑坡每次发生破坏滑动的体积不会很大,但是具有多次重复发生的特点,值得高度注意。     

博格达山北缘前陆褶皱冲断带构造特征及其动力学意义

博格达山北麓褶皱逆冲带是现代地质学研究的一个热点,但是大部分的研究都集中于古生代和新生代的构造变形,对于中生代多期构造变形少有关注。本文通过野外地质观测和卫星照片解析,结合前人地震剖面,识别出中生代以来博格达山北麓褶皱逆冲带内3期不同的构造变形: 1)是由于印度-欧亚板块碰撞远程效应所引起的新生代逆冲推覆构造,表现为一系列逆冲断层及相关的褶皱; 2)是侏罗纪末-白垩纪初博格达山遭受挤压变形,在山前形成宽缓的褶皱; 3)是博格达三叠纪末-侏罗纪初的构造变形,可能对应于博格达山的初始隆起,这期构造变形表现为博格达山北麓上二叠统的强烈褶皱变形。这些结果表明博格达山的隆升具有多期次的特点,主要受亚洲板块南缘不同时期小陆块碰撞的控制。     

Reevaluation of multiparameter relative dating techniques and their application to the glacial sequence along the eastern escarpment of the Sierra Nevada,California

Four valleys, recently studied by other workers, were examined along the eastern Sierra Nevada to refine relative-dating techniques. A variety of weathering parameters and soil properties fail to delineate more than two major post-Sherwin Pleistocene glaciations. We correlate these two glaciations with the Tahoe and Tioga Glaciations. Type Mono Basin Till, usually considered to be pre-Tahoe, exhibits the following weathering similarities with Tahoe Till, if both are under sagebrush: (1) grusification of subsurface granitic boulders; (2) degree of pitting, mineral relief, and rind development on surface granitic boulders; and (3) very slight clay increase in the B horizon. Type Casa Diablo Till also has weathering characteristics similar to Tahoe Till, except a slightly more developed Bt horizon is present. Hence, dates on basalt of 0.126 ± 0.025 and 0.062 ± 0.013 my Casa Diablo Till also has weathering characteristics similar to Tahoe Till, except a slightly more developed Bt horizon is present. Hence, dates on basalt of 0.126 ± 0.025 and 0.062 ± 0.013 my (Bailey et al. 1976), which bracket type Casa Diablo, may provide age control on the Tahoe glaciation. In addition, we are unable to demonstrate that the Tenaya is a separate glaciation. In three of the four valleys studied our weathering data for Tenaya Till are equivalent with those for Tioga Till, but with those for Tahoe Till in the fourth valley. We were not satisfied with our ability to differentiate the Casa Diablo, Mono Basin, and Tenaya as separate glaciations even though data were collected in the type areas for two of these deposits. Reasons for suggesting a change back to a two-fold Tahoe-Tioga glacial sequence, rather than the present five-fold sequence, are that we have measured a greater number of parameters than has been done previously, soils were submitted to detailed laboratory analyses, and surface weathering features were studied under consistent present vegetation cover to avoid possible problems induced by ancient forest fires. Nevertheless our relative-dating scheme does not rule out the possibility of a more detailed glacial sequence.     

Geological,geochemical and mineralogical features of some bauxite deposits from Nurra (Western Sardinia,Italy): insights on conditions of formation and parental affinity

Bauxite deposits are widespread in NW Sardinia. They formed during the middle Cretaceous, in consequence of a period of emergence of the Mesozoic carbonate shelf. In the Nurra area the geometries derived by the Middle Cretaceous tectonic phases controlled the ore typologies. Two bauxite profiles, laying on different bedrocks, were sampled. The bauxitization proceeded from the surface downward, with the accumulation of Al₂O₃ and residual ‘immobile’ elements (Al, Ti, HFSE), and corresponding mobility and loss of SiO₂ and Fe₂O₃. Epigenetic kaolinite formed close to faults and joints, probably as a result of silicification, introduced by low temperature hydrothermal solutions. Rare earth elements, especially LREE, are concentrated in Fe-rich bauxite horizons, probably due to scavenging by goethite. REE-enrichment is not observed in the boehmite-rich horizons. Very high REE contents are observed in a Fe-depleted horizon due to the occurrence of REE accessory minerals, probably of the bastnäsite group. Conservative indices, including TiO₂/Al₂O₃ and Ti/Cr ratios, and Eu anomalies (Eu/Eu*), suggest that the deposits formed by weathering of sediments derived from mafic rocks of the Hercynian basement. This, in turn, implies that the basement was exposed during middle Cretaceous.     

青藏高原新生代地壳变形对同碰撞岩浆侵位的制约

同碰撞岩浆作用是青藏高原新生代构造-岩浆活动的一种重要形式，各种类型岩浆岩广泛分布于高原内不同的构造单元中，其中有的来自地幔，不过规模都不大，它们是如何穿过异常厚的地壳侵位到近地表或喷出地表，这还是个未解之谜。根据构造分析，这些岩浆岩均侵位于新生代向斜构造中，例如侵入于高原东南边缘的楚雄复向斜、兰坪．思茅复向斜和老君山向斜的碱性岩，侵入于高原南缘的北喜马拉雅拉轨岗日向斜的片麻状花岗岩以及喷出于高原北部巴颜喀拉和雁石坪复向斜的安山-玄武岩。文中通过一个力学模式，说明这些岩浆岩的侵位受控于地壳应力场特征，即：向斜构造的下部承受的是张应力，地壳发生减薄和张裂，下地壳或上地幔熔融物质以此为通道发生向上的侵位。不过，因向斜顶部地壳承受的是挤压应力，地壳发生挤压缩短，所以只有少量岩浆能侵位到近地表并发生变形。与此相反，背斜构造的下部地壳产生的是挤压应力，阻止了下伏地壳内岩浆的侵入。因此，岩浆的侵位一般不会沿背斜发生。这一力学机制解释了为什么青藏高原规模很小的同碰撞岩浆能穿过异常厚的地壳沿一系列向斜构造侵位到近地表或喷出地表的原因。     

Rounded cobbles that have not travelled far: incorporation of corestones from saprolites in the South Mountain area of southern Nova Scotia, Canada

Saprolitic palaeosurfaces occur at several localities on the granitoid rocks of the South Mountain Batholith of Nova Scotia. There are three ages of saprolites within the study area: pre-Pleistocene, pre-Triassic and pre-Carboniferous. Within these ‘in-place’ weathered horizons, there are remnant ellipsoidal blocks of unweathered granitoid referred to as corestones. These corestones are isolated rounded pods of relatively unweathered material surrounded by rotted granitoid saprolitic material. The weathered material which surrounds these corestones is poorly consolidated and easily eroded. The erosion of these horizons produces a lag deposit that contains many rounded corestones which can be incorporated into subsequent sedimentary units. The rounded boulders, cobbles and pebbles of granite within many of the Pleistocene glacial deposits in southern Nova Scotia are probably related to the incorporation of these saprolite related structures, given the locally derived (within 400 m of the source) nature of the tills. The presence of saprolites at unconformities of various ages on the South Mountain granitoid rocks suggests that incorporation of saprolitic material probably occurred along a number of palaeosurfaces in the past. The recognition of this process has implications for the interpretation of rounded granite-clast conglomerates and quartz-rich sandstones of various ages within the stratigraphic record of eastern Canada. Similar palaeosurfaces elsewhere in the world also have related saprolite derived sedimentary rocks associated with them. In summary, well-rounded spherical pebbles, cobbles and boulders of granitoid material incorporated in sedimentary strata need not have travelled far from source nor are they necessarily recycled from older conglomerates.     

豫西新生代基性火山岩地球化学特征及源区性质

豫西新生代基性火山岩主要为粗面玄武岩,少量碱玄岩和玄武岩。其地球化学特征表明该区基性火山岩基本代表了幔源原始岩浆的成分,结晶分异作用及地壳物质混染作用并未对本区基性火山岩产生明显影响。初步的研究表明,本区基性火山岩是富集型地幔熔体和亏损型地幔熔体混合的产物。     

Edge‐roundness of boulders of Torridonian Sandstone (northwest Scotland): applications for relative dating and implications for warm and cold climate weathering rates

Kirkbride, M.P. & Bell, C.M. 2009: Edge‐roundness of boulders of Torridonian Sandstone (northwest Scotland): applications for relative dating and implications for warm and cold climate weathering rates. Boreas, 10.1111/j. 1502‐3885.2009.00131.x. ISSN 0300‐9483. The relative ages of late Quaternary morainic and rock avalanche deposits on Late Precambrian Torridonian Sandstone are determined from the characteristic edge‐roundness of constituent boulders. Because weathering of sandstone is manifest as edge‐rounding by granular disintegration, a relative chronology can be derived by measuring the effective radii of curvature of a sample of boulder edges. Thirteen samples totalling 597 individual boulder edges fall into two statistically distinct groups. Moraines of inferred Younger Dryas age (12.9–11.5 kyr BP) are distinguished from moraines of the Wester Ross Re‐advance (≤14.0 kyr BP). One moraine previously assumed to be of Younger Dryas age is reassigned to the older group. The method allows spatial extrapolation of deposit ages from dated sites where lithological and sampling criteria are met. Calculated rates of edge‐rounding imply that granular disintegration was several times more rapid during cold stadial climates than during the Holocene. Used as a proxy for boulder ‘erosion rate’, this indicates that surface loss of grains in glacial climates exceeds that during interglacials by a factor of 2–5, with implications for the calculation of exposure ages from cosmogenic nuclides.     

新生代全球变冷与青藏高原隆升的关系

文中综合分析可以影响新生代全球变冷的四种原因,提出青藏隆升对新生代大气ＣＯ２浓度降低起主导作用,对新生代全球气温的降低起关键控制作用。这种作用是通过青藏高原隆升加剧全球硅酸盐岩和碳酸盐岩的化学风化、有机碳埋藏、植物的光合作用来实现的。而且,青藏高原隆升有可能同洋流改变和行星轨道参数变化于第三纪末至第四纪共同对新生代全球变冷起控制作用。因此,目前首先解决的科学目标应该是：精确刻划青藏高原隆升时代和幅度,并确定青藏高原隆升对新生代全球变冷的贡献,确定一种以青藏高原隆升为主导作用的控制新生代全球变冷的综合模式。     

美国比格内尔山黄土的地球化学特征及其古气候意义

黄土是美国内布拉斯加州地表分布范围最广的沉积物之一,其中末次冰期形成的皮奥瑞亚黄土（Peoria loess,PL） 质量累积速率（Mass Accumulation Rate, MAR） 在比格内尔山（Bignell Hill,BH） 达到世界最高。探究此地区黄土快速沉积 的原因一直是北美中部大平原黄土研究的热点。本文通过对内布拉斯加州中部BH 样品的Nd、Sr 同位素、主微量和稀土元 素的分析,发现内布拉斯加黄土的化学成分相对均一,与上部大陆地壳（UCC） 和中国洛川黄土相比,Fe、Mg 值较低, K、Na 含量相对较高。BH 黄土的化学风化指数（CIA） 值平均为65.7,小于洛川黄土,显示出较弱的化学风化强度。Sr-Nd 同位素结果显示,除了普遍认为的怀特组沉积为黄土的主要源区外,还存在另一个87Sr/86Sr 比值更高, εNd更负的源区,推测 这个源区为北美劳伦冰盖冰川沉积,其对黄土的贡献达30%~45%。BH 黄土剖面的CIA、Na/K 和碳酸盐含量变化表明,在PL 形成时期（约25~18 ka）,化学风化较弱,气候干冷,有助于风尘产生与堆积。     

ON PRINCIPAL RULES IN THE OCCURRENCE OF OIL AND GAS ACCUMULATIONS IN THE WORLD

Anomalous coarse-crystalline garnet amphibolites and possible retrograded eclogites in varying stages of replacement by sodic amphibole + epidote assemblages compose exotic boulders (tectonic blocks) resting unconformably on serpentinite and finergrained, lower-greenschist-grade metabasites and metapelites in the Coastal Range of Chile (41° S Lat.). Major-and trace-elemental compositions of the tectonic blocks are similar to metapillow basalts and metavolcaniclastics within the underlying, in situ, metamorphic complex. Phase compatibilities and mineral compositions within the coarse blueschist/amphibolite suggest both a higher maximum pressure (greater burial depth) and higher P-T retrograde path for the tectonic blocks than for the in situ greenschists, suggesting an inversion of their relative structural position after metamorphism.

Coarse amphibolite-grade metabasites forming the lower portion of a primitive arc system may have been detached from the upper plate and subducted to deeper levels within a zone of distributed shear, resulting in the formation of high-P/T blueschist minerals. A K-Ar age of 304 + 9 Ma on white mica from one of the blueschist blocks is similar to previous ages from the regional metamorphic complex, and suggests that ascent of the tectonic blocks marginal to or within serpentinite diapirs occurred during an early stage of late Paleozoic orogenesis along the Gondwanide margin. Post-orogenic static uplift, sedimentation, and differential erosion then resulted in the present surficial distribution of the blueschist boulders.     

Late Mesozoic-Cenozoic Exhumation of the Northern Hexi Corridor: Constrained by Apatite Fission Track Ages of the Longshoushan

The apatite fission track (AFT) ages and thermal modeling of the Longshoushan and deformation along the northern Hexi Corridor on the northern side of the Qinghai-Tibetan Plateau show that the Longshoushan along the northern corridor had experienced important multi-stage exhumations during the Late Mesozoic and Cenozoic. The AFT ages of 7 samples range from 31.9 Ma to 111.8 Ma. Thermal modeling of the AFT ages of the samples shows that the Longshoushan experienced significant exhumation during the Late Cretaceous to the Early Cenozoic (~130–25 Ma). The Late Cretaceous exhumation of the Longshoushan may have resulted from the continuous compression between the Lhasa and Qiangtang blocks and the flat slab subduction of the Neo-Tethys oceanic plate, which affected wide regions across the Qinghai-Tibetan Plateau. During the Early Cenozoic, the Longshoushan still experienced exhumation, but this process was caused by the Indian-Eurasian collision. Since this time, the Longshoushan was in a stable stage for approximately 20 Ma and experienced erosion. Since ~5 Ma, obvious tectonic deformation occurred along the entire northern Hexi Corridor, which has also been reported from the peripheral regions of the Qinghai-Tibetan Plateau, especially in the Qilianshan and northeastern margin of the plateau. The AFT ages and the Late Cenozoic deformation of the northern Hexi Corridor all indicate that the present northern boundary of the Qinghai-Tibetan Plateau is situated along the northern Hexi Corridor.     

Paleomagnetic age of ferruginous weathering beneath the Hamersley Surface,Pilbara, Western Australia,and the Cenozoic apparent polar wander path

During the Mesozoic and Paleogene, the Precambrian rocks in the Pilbara, Western Australia, underwent erosion and deep weathering that produced an undulating landform now represented by the duricrusted and partly eroded Hamersley Surface. A reddened, ferruginous weathering zone occurs immediately beneath this duricrusted surface. Oriented block samples of ferruginised strata of the Neoarchean–early Paleoproterozoic Hamersley Group exposed within approximately 15 m below the duricrust were collected at 20 sites in roadcuts along the Great Northern Highway between Munjina and Newman and exposures along the adjoining Karijini Drive. Stepwise thermal demagnetisation of cored specimens revealed a stable, high-temperature (680°C) component carried by hematite, with a mean direction (n = 55 specimens) of declination D = 182.0°, inclination I = 52.9° (α₉₅ = 3.6°), indicating a pole position at latitude λ_p = 77.6°S, longitude ?_p = 113.2°E (A₉₅ = 4.3°) and a paleolatitude λ = 33.5 +3.6/–3.3°S. Both normal and reversed polarities are present, indicating that the remanent magnetism was acquired over an interval of at least two polarity chrons (say 10⁵–10⁶ years). Chi-square tests on the determined pole position and three different sets of Cenozoic poles, namely those for dated volcanic rocks in eastern Australia supplemented by poles for Australian Cenozoic weathering horizons, and inferred poles from Pacific Ocean and Indian Ocean hotspot analyses and North American Cenozoic poles rotated to Australian coordinates, yielded a mean age of ca 24 ± 3 Ma, i.e. late Oligocene to early Miocene, interpreted as the time of formation of hematite in the sampled ferruginous zone. The ferruginous weathering occurred under globally warm conditions and was followed during the early to middle Miocene climatic optimum by the deposition of channel iron deposits, which incorporated detrital hematitic material derived from erosion of the ferruginous weathering zone beneath the Hamersley Surface.