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1.
ULRICH HEIMHOFER DANIEL ARIZTEGUI MARC LENNIGER STEPHEN P. HESSELBO DAVID M. MARTILL ARISTOTELES M. RIOS‐NETTO 《Sedimentology》2010,57(2):677-694
The laminated limestones of the Early Cretaceous Crato Formation of the Araripe Basin (North‐eastern Brazil) are world‐famous for their exceptionally well‐preserved and taxonomically diverse fossil fauna and flora. Whereas the fossil biota has received considerable attention, only a few studies have focused on the sedimentary characteristics and palaeoenvironmental conditions which prevailed during formation of the Crato Fossil Lagerstätte. The Nova Olinda Member represents the lowermost and thickest unit (up to 10 m) of the Crato Formation and is characterized by a pronounced rhythmically bedded, pale to dark lamination. To obtain information on palaeoenvironmental conditions, sample slabs derived from three local stratigraphic sections within the Araripe Basin were studied using high‐resolution multiproxy techniques including detailed logging, petrography, μ‐XRF scanning and stable isotope geochemistry. Integration of lithological and petrographic evidence indicates that the bulk of the Nova Olinda limestone formed via authigenic precipitation of calcite from within the upper water column, most probably induced and/or mediated by phytoplankton and picoplankton activity. A significant contribution from a benthonic, carbonate‐secreting microbial mat community is not supported by these results. Deposition took place under anoxic and, at least during certain episodes, hypersaline bottom water conditions, as evidenced by the virtually undisturbed lamination pattern, the absence of a benthonic fauna and by the occurrence of halite pseudomorphs. Input of allochthonous, catchment‐derived siliciclastics to the basin during times of laminite formation was strongly reduced. The δ18O values of authigenic carbonate precipitates (between ?7·1 and ?5·1‰) point to a 18O‐poor meteoric water source and support a continental freshwater setting for the Nova Olinda Member. The δ13C values, which are comparatively rich in 13C (between ?0·1 and +1·9‰), are interpreted to reflect reduced throughflow of water in a restricted basin, promoting equilibration with atmospheric CO2, probably in concert with stagnant conditions and low input of soil‐derived carbon. Integration of lithological and isotopic evidence indicates a shift from closed to semi‐closed conditions towards a more open lake system during the onset of laminite deposition in the Crato Formation. 相似文献
2.
Rubidium‐strontium and strontium isotope data for eight whole‐rock samples of granite varieties from the Encounter Bay area, South Australia, yield an isochron age of 487 ± 37 m.y. Two specimens of albitised granite, formed as a result of late‐stage metasomatic alteration of original megacrystic granite, conform to this isochron. These data support a genetic relation between granites and late‐stage metasomatic alteration as suspected from field, petrographical and geochemical studies. Eight samples from contiguous Kanmantoo Group metasedimentary rocks have an isochron age of 487 ± 60 m.y. Thus this metamorphic event is coincident with emplacement of the Encounter Bay Granite. The initial Sr87Sr86 ratio for the Encounter Bay Granite (0.719) is significantly higher than initial ratios for the Palmer (0.709) and Anabama (0.705) Granites from the same region and can be attributed to either remobilisation or incorporation of strontium from older crustal material in the intrusion. The apparent initial Sr87/Sr86 ratio for the Kanmantoo Group metasedimentary rocks (0.722) can not be distinguished from that for the Encounter Bay Granite within the analytical uncertainties. Compatability of ages and high initial Sr87Sr86 ratios suggest that the granites formed by remobilisation of associated crustal rock. 相似文献
3.
In central Wisconsin, Cambrian strata of the Elk Mound Group record deposition on open‐coast, wave‐dominated tidal flats. Mature, medium‐grained quartz arenite is dominated by parallel‐bedding with upper‐flow regime parallel‐lamination, deposited during high‐energy storms that also produced three‐dimensional bedforms on the flats. Abundant wave ripples were produced as storms waned or during fair weather, in water depths ≤2 m. Indicators of variably shallow water (washout structures and stranded cnidarian medusae) and subaerial exposure (adhesion marks, rain‐drop impressions and desiccation cracks, including cracked medusae) are abundant. Parallel‐bedded facies preserve a Cruziana ichnofacies, similar to other Cambrian tidal‐flat deposits. Flats were dissected by small, mainly straight channels, the floors of which were grazed intensely by molluscs. Most channels were ephemeral but some developed low levées, point bars and cut‐banks, probably reflecting stabilization by abundant microbial mats and biofilms. Channels were filled with trough cross‐bedding that is interpreted to have been produced mainly during storm runoff. The strata resemble deposits of open‐coast, wave‐dominated tidal flats on the east coast of India and west coast of Korea. Ancient wave‐dominated and open‐coast tidal flats documented to date appear to have been limited to mud‐rich strata with ‘classic’ tidal indicators such as flaser bedding and tidal bundles. The Cambrian (Miaolingian to early Furongian) Elk Mound Group demonstrates that sandy, wave‐dominated tidal flats also can be recognized in the stratigraphic record. 相似文献
4.
Ischadites lindstroemi Hinde 1884 is described from the Ordovician Daylesford Formation (lower part of the Bowan Park Group) of central western New South Wales. The species characterises a distinctive unit in the formation, occurring in bands throughout the unit. From a study of the associated lithofacies and their stratigraphic distribution in the limestones, an interpretation of the environment in which the species lived is presented. The species is considered to have inhabited a shallow‐water, lime‐mud environment, which was off‐shore and marginal to calcarenite shoals. 相似文献
5.
A. C. Moore 《Australian Journal of Earth Sciences》2013,60(3):243-258
The orthopyroxenes of Gosse Pile, a layered ultramafic sequence within the Giles Complex of central Australia, show unusual exsolution features such as the development of ‘hour‐glass’ shapes by clinopyroxene lamellae, and also the exsolution of spinel and rutile. Both orthopyroxene and clinopyroxene have high R2O3 contents and their distribution coefficients for Mg and Fe are high (average KD = 0.88). Some, if not all, of these features are considered to indicate that the Gosse Pile rocks crystallized under high pressure. This interpretation also explains the predominance of orthopyroxene over olivine as an early cumulus phase. Estimated conditions during crystallization are 10–14 kilobars and between 1,200‐l,400°C. 相似文献
6.
Abstract The Lower Cambrian Chengjiang Lagerstätte provides a good window to explore the origin and radiation of early bilaterians. Here we describe a netted sclerite‐bearing worm Tabelliscolex hexagonus gen. et sp. nov., and tentatively assign it to palaeoscolecidans of priapulids. The cuticle of the animal is covered with two kinds of platy sclerites which are constructed by hexagonally arranged tubercles. Similar structures of the sclerites can be seen on some Cambrian palaeoscolecidans and lobopods, so, this new species is critical for understanding the relationships between lobopods and palaeoscolecidans. 相似文献
7.
《地学前缘(英文版)》2022,13(6):101430
Over one hundred arthropod fossil species have been described from the famous Chengjiang Lagerstätte (South China, Cambrian Stage 3, ca. 518 Ma) including a diverse assemblage of radiodonts–a group containing Anomalocaris and its relatives. These iconic stem-group euarthropods include some of the largest animals of the time, and some are known from hundreds of specimens. A longstanding conundrum has been the rarity or absence of hurdiids from Cambrian Series 2 Lagerstätten like Chengjiang. This is because radiodonts are generally common in such deposits and the oldest radiodont ever discovered is a hurdiid. Furthermore, this family displays the widest geographic and temporal ranges of all radiodont families, and the highest diversity. Here we document the first hurdiid frontal appendages from Chengjiang, which display unique features within the family and may provide insights for understanding the character evolution of hurdiid appendages. The palaeoenvironmental distribution of hurdiids suggests that the rarity of hurdiids in Chengjiang may be due to a preference for deeper water environments, and the later success of this family from the Wuliuan onwards may relate to their ability to tolerate cooler water temperatures than other radiodont families. The palaeogeographical, palaeoenvironmental, and stratigraphical patterns observed in hurdiids maybe caused in part by the limited distributions of Konservat-Lagerstätten in the Cambrian as well. 相似文献
8.
A 1.5 m thick coquinite discovered in the Upper Jurassic La Casita Formation of the Sierra El Jabalà near Gomez Farías, Coahuila, northeastern Mexico qualifies as a concentration Lagerstâtte owing to its richness in marine vertebrates. Ichthyosaurs, pliosaurs and crocodilians were described to some detail, but other taxa remained unstudied and the precise biostratigraphical age, as well as paleoecological conditions that led to the formation of the fossil deposit, are not known in detail. Here we describe ammonites, aptychi, bivalves and radiolarians, which allow for a stratigraphic assignation of the deposit to the uppermost Kimmeridgian Beckeri Zone. The unit under consideration accumulated in a hemipelagic mud bottom environment during a period of time characterized by low oxygen conditions, while a short term benthic colonization phase near the top of the coquinite corresponds to increased oxygen availability. A combination of upwelling, bottom currents, winnowing, offshore winds, storm events, circulatory nutrient traps, low oxygenated bottom waters, and a transgressional regime with reduced net sedimentation was crucial factors for the subsequent concentration of fossils, as well as for marine phosphate generation and phosphorus migration. 相似文献
9.
David M. Martill Nizar IbrahimPaulo M. Brito Lahssen BaiderSamir Zhouri Robert LoveridgeDarren Naish Richard Hing 《Cretaceous Research》2011,32(4):433-446
Laminated, dolomitised marine limestones within a transgressive fluvio-lagoonal-carbonate platform sequence at Gara Sbaa in the Kem Kem region of south eastern Morocco yield well preserved marine fishes and crustaceans. A rarer terrestrial component includes delicate fern fronds, insects and a lizard. Sedimentological structures and stratigraphic context indicate initial shallow waters with in a carbonate lagoon with benthic microbial mats followed by deepening. High precision dating has yet to be accomplished, but a Late Cenomanian or Early Turonian age is indicated on the basis of faunal and sequence-stratigraphic considerations. The fish assemblage has affinities with mid-Cretaceous ichthyofaunas in South America and Lebanon, including taxa in common at generic level. 相似文献
10.
A new ophiuroid species from the Hauptrogenstein Formation (Middle Jurassic, Bajocian) of the Rehhag ridge near Schöntal, northwestern Switzerland, is described based on 34 specimens. The impression of an indeterminate astropectinid sea star has also been found. The fully articulated specimens have been smothered by mobile oolitic dunes or sandwaves. The site can be genetically classified as an obrution deposit and belongs to the widely recognized “Hauptrogenstein type”. The remains are assigned to the genus Geocoma d’Orbigny 1850, best known from the type species, G. carinata (Münster), from the Tithonian Plattenkalk of Zandt. As suggested by Kutscher (1997) this species also includes Ophiocten kelheimense Boehm 1889, now documented by numerous well-preserved specimens from the Plattenkalk of Hienheim. Kutscher assigned the Hienheim specimens to the genus Sinosura Hess 1964 and used both names though G. carinata has priority. Despite rather small differences Sinosura is retained as a valid genus besides Geocoma, and both are placed in the family Aplocomidae Hess 1965. Geocoma and Sinosura share small adpressed arm spines with Aplocoma d’Orbigny 1852. Aplocoma is represented by a number of Triassic and Jurassic species, and the subfamily Aplocominae is proposed herein for the three genera. A second subfamily, Ophiopetrinae, is proposed for the genera Ophiopetra Enay &; Hess 1962 and Ophiohybris Hess 1964, and is characterized by larger, erect arm spines. 相似文献