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1.
Research about the impact of prehistoric human subsistence strategies upon prey populations of shellfish is based on the assumption that trends in shell size are directly related to collection strategies. A shell midden on San Juan Island, Washington, where archaeological research is sponsored by the University of Washington and hosted by the National Park Service, provides large samples of shells from separate archaeological deposits. the shells from each deposit produce size-frequency distributions that vary in shape (modality). Because several factors (the original prey population, the method of collection and transport, the environment of deposition, and post-depositional processes) affect the size-frequency distribution differentially, the shell measurements do not provide the direct archaeological information expected.  相似文献   

2.
A distinctive Shell and Sand Sheet found beneath the marine ponds of Anegada, British Virgin Islands, was formed by a post-1650 AD overwash event, but its origin (tsunami or hurricane) was unclear. This study assesses the taphonomic characters of the shell and large clast material (>2?mm) to determine its provenance and origin. Pond-wide stratigraphic units (Shelly Mud, Shell and Sand Sheet, Mud Cap) were analyzed (12 samples) at four sites in Bumber Well and Red Pond along with eight samples from the Shell and Sand Sheet in a 2-km transect of Bumber Well. Mollusks in the pond muds include Anomalocardia spp. and cerithids with no allochthonous shells from the offshore reef-flat. Results show that the shells and clasts (>2?mm) are derived from the erosion and winnowing of the underlying Shelly Mud of the former marine pond, forming a distinctive sheet-like deposit with Homotrema sand. The Shell and Sand Sheet contains articulated Anomalocardia bivalves and moderate numbers of angular fragments (approximately 35%) that are likely from crab predation. Radiocarbon dates of articulated Anomalocardia specimens from the Shell and Sand Sheet range widely (approximately 4000?years), with shell condition (pristine to variably preserved) showing no correlation with age. The articulated condition of the bivalves with the wide-ranging dates suggests erosion and winnowing of the underlying Shelly Mud but minimal transport of the bivalves. The Shell and Sand Sheet has taphonomic characteristics indicative of a widespread tsunami overwash (sheet-like extent and articulated specimens) but lacks allochthonous reef-flat shells. Reef-flat shell material may not have penetrated the pond, as a tsunami would have to cross the reef-flat and overtop high dunes (2.2?m) hindering transport of larger shell material but allowing the Homotrema sand to penetrate. Processes including hurricane overwash, pond wave action, or tidal channel opening and closure are not favoured interpretations as they would not produce extensive sheet-like deposits. Taphonomic analysis is hampered by the limited (400?C500?years BP) depositional history from Anegada??s ponds and the lack of comparative data from other Caribbean locations.  相似文献   

3.
Two beds containing relatively thick and highly concentrated oyster shell banks occurring in the densely fossiliferous Wadi Al Abraq outcrop (Upper Eocene Maadi Formation, Cairo-Sukhna Road, Egypt). Such shell beds are almost monospecific. The upper level consists mainly of Ostrea clotbeyi Bellardi, topped by a considerable concentration of Carolia placunoides Cantraine, while in the lower level occurs Ostrea multicostata Deshayes. In both beds, the upper contact with the overlying marl bed (about 2 m thick) is sharp and sometimes erosive. Similarly, the contact with the underlying shale (1.5 m thick) is sharp and erosional. The shell beds are composed mainly of loosely packed oyster shells and their fragments (usually with a high proportion of disarticulated specimens) set in the sandy limestone matrix. The collected oysters show bioerosion and skeletobiont assemblages. Bioerosion trace fossils comprise ten ichnospecies while skeletozoans comprise five taxa. In general, shells of O. clotbeyi show a higher incidence of bioerosion than O. multicostata. The shells' chaotic orientation and their moderate fragmentation indicate that the shell bed formation was associated with high energy events. On the other hand, the low frequency of articulated specimens suggests that the shell beds are parautochthonous oyster banks' remnants. The taphonomic features of the studied assemblage indicate deposition in a shallow-water, wave-dominated environment.  相似文献   

4.
The Picos II Paleontological Site (PIIPS), located in the city of Piranhas, in Alagoas State, is a fossil deposit with a pond-like geomorphological feature, a kind of peculiar deposit for the Quaternary of Northeast Brazil. A detailed taphonomic study is presented about the PIIPS assemblage, from material collection to the laboratory analysis. The conservation state of the collected fossils in PIIPS is different from that found in other tank deposits, showing a high degree of specimen preservation. Two taxa of pleistocenic mammals have been identified: Eremotherium laurillardi and Notiomastodon platensis, and fragments of an avian bone, whose taxonomic identification was not possible. The taphonomic analysis indicates that the deposited material experienced a short transportation in high-energy event, with a short period of subaerial exposure before the final burial, where at least the final disarticulation occurred in situ, which explains the high conservation level of the material. The sedimentological analysis made in each layer of the deposit corroborates the information related to the transport of the material, which were obtained in taphonomic analysis. Regarding these aspects, PIIPS reflects nearly non-biased biocenotic patterns. The well-preserved material found in PIIPS demonstrates the great potential of deposits with pond-like geomorphological features, with sidelines that are less steep than traditional tanks, providing smoother transport and accommodation for skeletal elements, which is a distinguishing factor in preserving fossils of vertebrates.  相似文献   

5.
Size sorting of shells and valves of transported death assemblages of Mytilus edulis is shown to have taken place, and greatly affected size-frequency distributions of shell accumulations deposited in a beach environment. Selective transport and deposition of different size grades gives histograms which may be strongly skewed or multimodal, and include numerous small shells. The usually predicted bell-shaped distributions for sorted assemblages are not always produced. Little evidence of growth and mortality rates can be deduced with confidence from the transported assemblages.  相似文献   

6.
In the Saoura, the brachiopod shell beds, so-called niveau coralligène, correspond to a major shell deposit dated to the Late Emsian. Brachiopods and crinoids dominate the benthic assemblage that contains also corals, bryozoans, trilobites, goniatites, and orthocones. This major level has a large geographic distribution and it is characterized by a wide brachiopod diversity due to time-averaging, taphonomic feedback and alternate bottom conditions changing from soft to shelly and firm. This kind of brachiopod association is linked to a transgressive onlap system. At regional extent, we can correlate this major shell bed to similar shell deposits from the Ahnet-Mouydir, Tindouf, and Zemmour areas. It indicates an important transgressive event underlined by change in the sedimentation from detritic deposits to carbonate sediments. To cite this article: A. Ouali Mehadji et al., C. R. Geoscience 336 (2004).  相似文献   

7.
Upper Cretaceous vertebrate accumulations from the Adamantina Formation are known due to their high taxonomic diversity. On the other hand, taphonomic analyses still are rare, limiting the understanding of processes related to the biostratinomic and fossildiagenetic histories of this lithostratigraphic unit. In 2005, fossils were collected from an outcrop located at Jales municipality, state of São Paulo, southeastern Brazil. From this outcrop, a well-preserved Baurusuchus was recovered, which displays a peculiar set of taphonomic signatures. This paper identifies and interprets taphonomic features of a specimen of Baurusuchus (Crocodyliformes, Baurusuchidae; UFRJ DG 418-R) from the Adamantina Formation (Upper Cretaceous of the Bauru Basin), in Jales municipality, state of São Paulo. Brief taphonomic comparisons with other specimens previously studied (crocodiles and dinosaurs) and a lithofaciological analysis of the outcrop were undertaken in order to enhance the knowledge of the stratigraphy and paleoenvironment and improve the time resolution for the Adamantina Formation in the region of Jales. Furthermore, paleoecological data were interpreted based on the taphonomic analysis.The fossil is composed of an articulated segment of vertebral column, ribs, part of the pelvic girdle and gastralia. There is no hydraulic equivalence between both the specimen of Baurusuchus and the size of quartz grain predominant in the fossiliferous layer, suggesting death in situ or short transport as a “water carcass”. Teeth marks identified on the pubes were assigned to a small/juvenile baurusuchid crocodyliform or a theropod dinosaur. The repositioning of some elements (ribs and dorsal osteoderms) is suggestive of mummification. Desiccation marks were observed and attributed to the stage 1 of weathering. These features suggest subaerial exposure of the carcass prior to burial, however, probably after the mummification. On the other hand, the subaerial exposure was short, because the individual was not fully disarticulated. Furthermore, the degrees of articulation and preservation of the specimen nullify the hypothesis of reworking.Lithofaciological and taphonomic analyses suggest a well-drained floodplain as the burial environment and an arid or semi-arid climate in the region of Jales outcrop. In addition, the taphonomic signatures seem to indicate a time resolution about 100–104 years for the layer where the crocodyliforms were collected, but it seems to have, within this time limit, temporal-mixing among terrestrial crocodiles and dinosaurs collected from the same layer, suggesting time-averaging in this outcrop.  相似文献   

8.
迤纳厂矿床是康滇地区典型的铁-铜-稀土矿床之一.因为其特殊的矿物组合(磁铁矿与黄铜矿共生)以及富含稀土矿物,这类矿床一直是矿床学家研究的热点.然而由于其围岩遭受不同程度的变质和蚀变作用,这类矿床的成矿时代与矿床成因一直存在争议.本文通过对迤纳厂组中层状凝灰岩和火山角砾岩所含锆石进行LA-ICP-MS U-Pb定年来揭示迤纳厂组的最大沉积年龄.大多数锆石具有明显的震荡环带和较高的Th/U比值(＞0.4)表明它们均是岩浆锆石,近200粒碎屑锆石的207 pb/206 Pb年龄大致可以分为四组:1.75 ～ 1.88Ga,1.90 ～2.00Ga,2.02～2.20Ga和2.30 ～2.40Ga,且最老年龄在3.0Ga左右,而最年轻年龄在1750Ma左右.这一定年结果反映了迤纳厂组的沉积上限大约为1.7Ga,并且在康滇地区可能还有更老的基底存在.通过对矿石矿物黄铜矿的Re-Os同位素定年测试,直接限定了矿床的成矿时代.6个黄铜矿样品的Re-Os同位素等时线年龄为1690±99Ma(MSWD =9.0),模式年龄的加权平均值为1685±37Ma(MSWD=3.0),表明该矿床形成于距今约1.7Ga.另外,主要类型矿石具显著的正铕异常和轻稀土富集的特点,与现代海底热液极其相似.年代学研究显示矿床的形成时代和地层的沉积时代大致相同,而REE揭示的流体特征反映其成矿作用与海底热液活动有关.这些结果暗示了该矿床为海底火山喷发-同生沉积形成.近来的较多研究证实,康滇地区存在1.7Ga左右较大范围的岩浆活动和较多同期铁-铜矿床,表明该期岩浆活动可能是制约该区铁-铜矿床形成的关键因素,且岩浆活动可能与约1.7Ga的Columbia超大陆的裂解事件有关.  相似文献   

9.
Pleistocene vertebrates from Itaboraí Basin have not been taphonomically studied prior to this work, limiting the understanding of the deposition and preservation of the only Pleistocene vertebrate accumulation known for the state of Rio de Janeiro. In this work, the taphonomic signatures of the Pleistocene vertebrate assemblage of Itaboraí are identified and interpreted in order to increase the knowledge about the formation of this fossil association and the paleoecology of the region of Rio de Janeiro during the late Pleistocene. Our analysis shows that the thanatocoenosis was exposed to the biostratinomic processes during a small time span; that it is parautochthonous; and experienced short transport distances by normal fluvial streams and floods. Subsequently, the fossiliferous horizon was quickly covered by the superjacent soil. Yet, the skeletal elements were fractured and deformed during the sedimentary compaction. The differential preservation of megamammal bones is associated to the bone resistance against those destructive processes and to the specific anatomical features. Comparison between Itaboraí and other Brazilian Pleistocene vertebrate accumulations shows that the Itaboraí fossil accumulation was less affected by taphonomic processes, although it is also a time-averaged fossil concentration. Finally, some of the taphonomic features indicate an arid paleoclimate.  相似文献   

10.
The Kings Creek catchment, southeastern Queensland, contains a variety of Pleistocene – Holocene depositional settings. Fluvial depositional accumulation processes in the catchment reflect both high-energy channel and low-energy episodic overbank deposition. The lithofacies and depositional environments of locality QML796 were examined in detail to aid interpretation of taphonomic accumulation patterns of large and small taxa in the deposit. The basal fossiliferous unit was deposited in a meandering channel and passes upward into overbank deposits that include ephemeral interfluve channels and splays. The most striking taphonomic observations on vertebrates at the locality include: (i) low representation of post-cranial elements; (ii) high degree of bone breakage; (iii) variable abrasion with most identifiable bone elements having a low to moderate degree of abrasion; (iv) low rates of bone weathering; (v) a low degree of carnivore bone modification; and (vi) a low degree of articulated or associated specimens. Collectively, these data suggest that the material was transported into the deposit from the surrounding proximal floodplain and that the assemblages reflect substantial hydraulic sorting. However, despite that, sequential faunal horizons show a stepwise decrease in taxonomic diversity that cannot be explained by sampling or taphonomic bias. The decreasing diversity includes loss of some, but not all, megafauna and is consistent with a progressive local loss of megafauna in the catchment over an extended interval of time. Data are consistent with a climate change model for megafauna extinction but not with nearly simultaneous extinction of megafauna as required by the human-induced blitzkrieg extinction hypothesis.  相似文献   

11.
Biodegradation of shell material is widespread in the cool-water skeletal carbonate deposits on Scott shelf, northwestern Vancouver Island, and is especially evident in the large aragonitic bivalves, Glycymeris and Humilaria, major primary sediment contributors. Ten types of endolithic microborings have been identified in the shells, including representatives of green algae (e.g., Ostreobium quekettii), blue-green algae (e.g., Plectonema terebrans, ?Scytonema sp.), fungi, bacteria and clionid sponges, as well as macroborings of phoronids, polychaetes and naticid gatropods. Microcrystalline carbonate is not precipitated in vacated bores. Boring physically weakens the shells, rendering them more prone to mechanical abrasion during sediment transport and bioturbation, and to biological abrasion by grazing benthos. Tumbling experiments demonstrate that the rate of carbonate mud production is much greater for bored as compared to fresh bivalve shells, and that mud production rates decrease with tumbling time because most endolithic microborings are confined to the periphery of grains. Boring also increases significantly the porosity and surface area of skeletal grains, and destroys their organic matrix, making them susceptible to maceration and dissolution on cool-water shelves. Fostered by the generally low rates of carbonate production and accumulation, many aragonitic bivalve shells on Scott shelf have become thoroughly degraded through a combination of endolithic microboring, maceration and dissolution within about 1000 years in ambient sea water. In geologic terms, such selective taphonomic loss of skeletal material may be considerable in ancient temperate-shelf limestones and should be evaluated when interpreting their paleoecology and paleoenvironments.  相似文献   

12.
四川龙门山区(前山)早泥盆世地层中,特别是在白柳坪、甘溪、谢家湾三组地层中,含有相当数量的介壳层。它包括呈两向伸展的介壳毯和三向伸展的介壳层(包括透镜体),据在地层中产出的层位,自身的特点及其与围岩的关系等方面综合分析,表明介壳毯形成于正常波基面向海一侧,介壳岩则是位于波基面以下,正常风暴浪基面附近向海一侧的部位;前者主要为簸扬作用和沉积速率快慢相间的幕式沉积过程的结果,后者则和风暴作用有关,属近积风暴岩,介壳岩的研究不仅对正确划分白柳坪、甘溪、谢家湾三组提供了充分的岩性依据,而且也为正确解释古环境及盆地沉积史提供大量可靠依据。  相似文献   

13.
Uranium Deposit 302 is a large hydrothermal uranium deposit occurring in fault zones within a granite. Pitchblende is the only primary uranium mineral found in the ore. The associated minerals include quartz, fluorite, hematite and pyrite. This paper focusses on the mechanism of transport and deposition of uranium in hydrothermal oreforming solutions responsible for the deposit. It is concluded that uranium might be not only oxidized in a relatively reducing environment, but also reduced in a relatively oxidizing environment, depending on the speciation of uranium in the solutions. The formation of pitchblende is closely related to uranium reduction at the stage of uranium mineralization.  相似文献   

14.
Holocene evolution and human occupation of the Sixteen Mile Beach barrier dunes on the southwest coast of South Africa between Yzerfontein and Saldanha Bay are inferred from the radiocarbon ages of calcareous dune sand, limpet shell (Patella spp.) manuports and gull-dropped white mussel shells (Donax serra). A series of coast-parallel dunes have prograded seaward in response to an overall marine regression since the mid-Holocene with dated shell from relict foredunes indicating periods of shoreline progradation that correspond to drops in sea level at around 5900, 4500 and 2400 calibrated years before the present (cal yr B.P.). However, the active foredune, extensively covered by a layer of gull-dropped shell, has migrated 500 m inland by the recycling of eroded dune sand in response to an approximate 1 m sea level rise over the last 700 yr. Manuported limpet shells from relict blowouts on landward vegetated dunes indicate human occupation of coastal dune sites at 6200 and 6000 cal yr B.P. and help to fill the mid-Holocene gap in the regional archaeological record. Coastal midden shells associated with small hearth sites exposed in blowouts on the active foredune are contemporaneous (1600-500 cal yr B.P.) with large midden sites on the western margin of Langebaan Lagoon and suggest an increase in marine resource utilisation associated with the arrival of pastoralism in the Western Cape.  相似文献   

15.
Inhabited by humans for over 12,000 calendar years, California's Channel Islands contain thousands of archaeological sites, ranging from dense shell middens and villages to small lithic scatters and camps. Similar to many islands around the world, the Channel Islands have a dearth of burrowing animals and limited historical development leading to generally good preservation of archaeological constituents and relatively high stratigraphic integrity. Despite these favorable preservation conditions, numerous natural and cultural processes have impacted the island's archaeological record. Channel Islands archaeologists, however, have given relatively limited attention to the effects of taphonomic and formation processes. The authors provide an overview of taphonomic and formation processes affecting Channel Islands archaeology, illustrating the importance of regional taphonomic syntheses in the management, preservation, and interpretation of archaeological sites. These data also demonstrate the significance of detailing formation processes in islands and other areas where burrowing rodents and other disturbances are thought to be absent or limited. © 2006 Wiley Periodicals, Inc.  相似文献   

16.
采用浅地层剖面测量、沉积物柱样测年和海图水深计算对比相结合的方法,探讨位于长江入海泥沙向南输送通道上的朱家尖岛海域的泥质沉积速率的历史变化,并通过水文泥沙测量和沉积物取样分析寻找长江入海泥沙向南输送的佐证。结果表明:朱家尖岛海域全新世海平面上升以来泥质沉积厚度为5~20m,相当于沉积速率0.06~0.25cm/yr;1840~1960年区域平均沉积速率为1.4cm/yr,近60年沉积速率仅为此前120年沉积速率的一半左右。据此推断:全新世后期以来该海域泥质沉积速率先后经历了一个长期(千年尺度)缓慢的增大阶段和一个短期(年代尺度)迅速的减小阶段,与长江流域人类活动和河口湾充填作用共同造成的入海泥沙通量的总体变化趋势相吻合。  相似文献   

17.
The Chahe copper deposit occurs in the Early Proterozoic metamorphic series in the area of Chahe and its chalcopyrite yielded a Pb−Pb isochron age of 951±36 Ma, providing evidence for copper mineralization at the early stage of the Jinning movement. The ore-forming material came from terrestrial clastic sediments and marine volcanic eruption and the ores were deposited in a relatively open beach environment. The Jinning movement led to folding and metamorphism of country strata, as well as to the rebomilization and transport of copper, resulting in ore deposition in structurally weak locations. This deposit is a volcano-sedimentary metamorphic deposit.  相似文献   

18.
The separated valves of bivalve molluscs, certain gastropods, and many brachiopods closely resemble the regular geometrical form known as the shell. The packing concentration of natural accumulations of such organic shells may be placed within bounds with the aid of packing models making use of ordered arrangements of equal geometrical shells, either conical, cylindrical or spherical. The low packing concentrations, comparable with 0·1–0·2, indicated by these models are confirmed by experiments using the shells of four common British species of bivalve or gastropod mollusc. Packings of these shells have a substantial intrinsic strength, and experimentally appear able to support without failing sedimentary overburdens equivalent to loads comparable with 1000 kg mass/m2. The observed and predicted low packing concentrations suggests that natural shell beds can hold relatively very large amounts of pore fluids or mineral cements. The resistance to compaction of the packings means that the high original porosities of natural shell beds have a good chance of being permanently preserved.  相似文献   

19.
Deposits within the floor of the Norwegian Basin were sampled to characterize the deposition from the Storegga Slide, the largest known Holocene‐aged continental margin slope failure complex. A 29 to 67 cm thick veneer of variable‐coloured, finely layered Holocene sediment caps a homogeneous, extremely well‐sorted, poorly consolidated, very fine‐grained, grey‐coloured sediment section that is >20 m thick on the basin floor. This homogeneous unit is interpreted to represent the uppermost deposits generated by a gravity flow associated with the last major Storegga Slide event. Sediments analogous to the inferred source material of the slide deposits were collected from upslope on the Norwegian Margin. Sediments sampled within the basin are distinguishable from the purported source sediments, suggesting that size sorting has significantly altered this material along its flow path. Moreover, the very fine grain size (3·1 ± 0·3 μm) suggests that the >20 m thick homogeneous unit which was sampled settled from suspension after the turbulent flow was over. Although the turbulent phase of the gravity flow that moved material out into the basin may have been brief (days), significantly more time (years) is required for turbid sediments to settle and dewater and for the new sea floor to be colonized with a normal benthonic fauna. Pore water sulphate concentrations within the uppermost 20 m of the event deposit are higher than those normally found in sea water. Apparently the impact of microbial sulphate reduction over the last ca 8·1 cal ka bp since the re‐deposition of these sediments has not been adequate to regenerate a typical sulphate gradient of decreasing concentration with sub‐bottom depth. This observation suggests low rates of microbial reactions, which may be attributed to the refractory carbon composition in these re‐deposited sediments.  相似文献   

20.
在扬子陆块核部黄陵地区发现具球状构造的花岗闪长岩。岩石产于新元古代斑状花岗闪长岩与石英闪长岩之间,出露面积极小。球状花岗闪长岩岩体由主岩、球状体及球间基质三部分组成,球状体呈圆形、椭圆形及不规则状,由球核和同心球壳组成,球体大小多为5~12cm。依据球壳及球核的差异,可大致分为具有多壳层同心结构、具单壳层结构、无明显壳层结构3种类型的球状体。单壳层球壳由角闪石、黑云母等暗色矿物含量高的暗色层组成,具放射状构造;多壳层球壳由含角闪石、黑云母等暗色矿物的暗色层和基本不含或含少量暗色矿物、以斜长石为主的浅色层组成韵律层,具梳状、放射状构造;球核成分可以是同源岩浆组分,也可以是异源捕虏体。单壳层球体,球核、球壳、基质的斜长石牌号依次为36、32、26,酸性程度不断增加,内部呈放射状生长,反映其可能由内向外经岩浆结晶作用形成。初步认为多壳层球体球壳富韵律的变化为富含气水热液的岩浆在动荡的环境下结晶形成,球壳中矿物结晶顺序可能由外向内,核部结晶时间相对更长。  相似文献   

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