The aquatic and semiaquatic biota in Miocene amber from the Campo LA Granja mine (Chiapas,Mexico): Paleoenvironmental implications

Amber from the Campo La Granja mine in Chiapas, Mexico, is distinct from other sources of amber in Chiapas. Campo La Granja amber has distinct layers created by successive flows of resin with thin layers of sand on most surfaces. Aquatic and semi-aquatic arthropods are commonly found. Together these pieces of evidence suggest an estuarine environment similar to modern mangrove communities. The aquatic crustaceans are the most intriguing aspect of the biota. A large number of ostracods have been found in the amber—many with their carapaces open, suggesting that they were alive and submerged in water at the time of entombment. The only known examples of brachyuran crabs preserved in amber are found in the Campo La Granja amber. Amphipods, copepods, isopods, and tanaids are also members of the crustacean fauna preserved in amber.     

An explanation for the varves of the Castile evaporites (Upper Permian), Texas and New Mexico, USA

Abstract Extraordinary sequences of conspicuous, pervasive and laterally persistent varves characterize the Castile evaporites. They occur as singlets (calcite laminae), couplets (calcite laminae interstratified with anhydrite laminae), thick couplets (calcite laminae interstratified with thin anhydrite beds) and triplets (calcite and anhydrite laminae interstratified with thin halite beds). The varves accumulated in a deep (initially ≈ 550 m), persistently stratified, saline lake surrounded by an extinct reef. The lake had formed when the reef grew across a channel between an embayment and the ocean. Although located virtually on the palaeo-equator, the lake experienced negligible meteoric influx and extreme seasonality. During the season of high relative humidity, more marine groundwater entered the lake through the permeable reef barrier than exited as reflux and, secondarily, as evaporation. Consequently, the lake level rose by up to several metres to sea level. The ‘refreshening’ decreased salinity and replenished dissolved CO₂– the critical nutrient limiting growth of indigenous phytoplankton. Algae proliferated, pH increased and CaCO₃ precipitated. It mixed with organic matter to form a thin, dark lamina. During the season of low relative humidity, tens of cubic kilometres of water evaporated from and, secondarily, leaked out through the surrounding reef. More water exited than entered, brine level fell below sea level, and salinity of the upper brine layer increased. Gypsum usually precipitated and rained onto the basin floor forming a couplet; infrequently, halite also precipitated forming a triplet. Every few thousand years, for <50 to several hundred years, the lake became unstratified during the dry season, and wind-induced overturn allowed a layer of gypsum crystals up to ≈ 2 cm high to precipitate on the basin floor. Each layer, now thin beds of anhydrite nodules and anhydrite pseudomorphs after gypsum, and an underlying lamina of CaCO₃ and admixed organic matter formed a thick couplet. The different varve types recur with a period of 1800–3000 years reflecting climatic changes on a millennial time scale.     

湖北恩施硒富集区中二叠世孤峰组露头层序地层研究

通过实测地层剖面的精细研究,结合微量元素地球化学分析,探讨了恩施地区富硒地层中二叠世孤峰组层序界面识别、层序划分与对比及其对硒元素空间分布的影响。研究区中二叠世孤峰组可总结为4种基本层序类型,分别为炭硅质页岩-炭硅质岩、炭硅质页岩-炭硅质岩夹灰岩透镜体、炭硅质岩-含炭泥晶灰岩和炭硅质岩-含炭细晶白云岩;识别出3个层序界面,其中孤峰组与茅口组接触界线处为Ⅲ型层序界面,孤峰组中部的岩性结构转换面为Ⅱ型层序界面,龙潭组与孤峰组接触界线处为Ⅰ型层序界面;划分了2个三级层序(SQ1、SQ2)。通过层序对比分析,随着海侵作用的持续发展,区域上层序内部结构及基本层序类型分异明显,沉积古地理格局表现为南浅北深的特征;通过层序地层与硒异常耦合关系分析,发现了孤峰组内部Ⅱ型层序界面附近具高硒异常特征,确立了新的找矿标志。     

The Cenomanian short snout enchodontid fishes (Aulopifomes,Enchodontidae) from Sierra Madre Formation,Chiapas, southeastern Mexico

In this paper the short snout Cenomanian enchodontids from the El Chango quarry, Chiapas, southeastern Mexico, are reviewed. Unicachichthys multidentata nov. gen. and sp. is named and identified as a new member of the Enchodontidae because it has the predorsal scute series that is the distinctive character of such family. This new genus resembles other short snout enchodontids, as Enchodus and Eurypholis; however, Unicachichthys differs from these and other members of the family because it shows characters ever observed, including a multitoothed dermopalatine, the presence of a basal sclerotic bone, and the serrations in the posterior edge of the preopercle vertical limb. The inclusion of Unicachichthys in two phylogenetic analyses, based on studies previously performed, suggests that this is a primitive representative of the family Enchodontidae. Additionally specimens of two different species of Enchodus from the El Chango quarry are also described; these specimens are so scarce and fragmentary that it is not possible to determine their specific taxonomic identity. The record of these Mexican fossils enrich the knowledge about the diversity of Cenomanian enchodontids in the North American domain of the Tethys Sea, which now contains new taxa that should be considered in future efforts to recognize the phylogenetic and biogeographic processes experienced by this fish group.     

Palaeoclimatic inferences from upper Palaeozoic siltstone of the Earp Formation and equivalents, Arizona-New Mexico (USA)

The Late Palaeozoic configuration of Pangaea contributed to a palaeoclimatic extreme that was characterized by both icehouse and monsoonal conditions. This study uses sedimentological, geochemical, and provenance data from silty facies of the Earp and equivalent Supai Formations (Arizona, New Mexico) to shed light on atmospheric circulation and glacial–interglacial climate change in westernmost equatorial Pangaea. Five silt‐rich facies comprise both loessite and marine and fluvially reworked loessite. An initial aeolian origin for the silt is indicated by the remarkably invariant grain size and the laterally continuous, sheet‐like geometry of beds. The silt‐rich facies occur in repetitive facies associations (1–20 m scale) that form mixed continental‐marine (loess, marine‐reworked loess), shallow‐marine, and continental (loess, palaeosol) ‘sequences’. Facies repetitions of both mixed continental‐marine and shallow‐marine sequences reflect a linked glacioeustatic–glacioclimatic control, whereas the continental (loess–palaeosol) couplets reflect a primary glacial–interglacial climatic cyclicity linked to glacioeustasy. Stratigraphic interpretations suggest that aeolian silt flux maximized during glacial to incipient interglacial stages (lowstand to early transgression), and decreased significantly or ceased during interglacials (highstand to early falling stage). Detrital‐zircon geochronological data indicate a transition from dominantly north‐easterly winds during the Middle Pennsylvanian to north‐westerly and south‐easterly winds by the Early Permian, which trend is inferred to reflect the onset of monsoonal circulation in western Pangaea. Relative grain‐size data support the detrital‐zircon data, and exhibit a significant decrease from the Sedona arch/Central Arizona shelf (north) to the Pedregosa basin (south) sections. Whole‐rock geochemical data suggest a relatively unweathered source for the silt in the north, and detrital‐zircon data indicate significant silt was derived from the local basement. These large piles of silt(stone) preserve valuable information for reconstructing both long‐term evolution in atmospheric circulation and short‐term fluctuations in glacial–interglacial climate. Many such indicators for long have been applied to ‘recent’ (Plio‐Pleistocene) loess, but are equally applicable to ‘deep‐time’ strata.     

Palaeomagnetic results from the Chiapanecan Volcanic Arc,Chiapas, Southern Mexico: geomagnetic and geodynamic significance

This article presents the first palaeomagnetic results from 13 independent cooling units in the Chiapanecan Volcanic Arc (ChVA). Six sites were directly dated by Ar–Ar or K–Ar methods: their dates range from 2.14 to 0.23 Ma. We isolated the characteristic palaeodirections for all 13 lavas. Eleven non-transitional directions yield a mean direction with inclination, I?=?30.7°, declination, D?=?4.1°, and precision parameters k?=?63 and α₉₅ = 5.8°. The corresponding mean palaeopole position is Plat = 83.3°, Plong = 203.8°, K?=?227, A ₉₅ = 5.1°. The mean inclination is in good agreement with the expected value for the last 5 million years, as derived from the synthetic North American polar wander path [Besse and Courtillot 2002 Besse, J. and Courtillot, V. 2002. Apparent and true polar wander and the geometry of the magnetic field in the last 200 million years. Journal of Geophysical Research, 107(B11) doi:10.1029/2000JB000050[Crossref], [Web of Science ®] , [Google Scholar], Apparent and true polar wander and the geometry of the magnetic field in the last 200 million years: Journal of Geophysical Research, v. 107, no. B11, p. 2300], but a measured rotation of the palaeodeclination of about 8° with respect to the expected direction suggests the possibility of a clockwise rotation of the studied ChVA units. We have estimated the characteristics of palaeosecular variation through study of the scatter of virtual geomagnetic poles, obtaining a palaeosecular variation parameter S _b = 14.5° with upper limit S _U = 19.6° and lower limit S _L = 11.7°, in reasonable agreement with the fit of model G [McFadden et al., 1988 Quidelleur, X., Carlut, J., Gillot, P.Y. and Soler, V. 2002. Evolution of the geomagnetic field prior to the Matuyama-Brunhes transition: Radiometric dating of a 820 ka excursion at La Palma. Geophysical Journal International, 151: F6–F10. [Crossref], [Web of Science ®] , [Google Scholar], Dipole/quadrupole family modeling of paleosecular variation: Journal of Geophysical Research, v. 93, no. B10, p. 11583–11588; 1991, Reversals of the Earth's magnetic field and temporal variations of the dynamo families: Journal of Geophysical Research, v. 96, no. B3, p. 3923–3933] to the Johnson et al. [2008 Johnson, C.L., Constable, C.G., Tauxe, L., Barendregt, R., Brown, L.L., Coe, R.S., Layer, P., Mejia, V., Opdyke, N.D., Singer, B.S., Staudigel, H. and Stone, D.B. 2008. Recent investigations of the 0–5 Ma geomagnetic field recorded by lava flows. Geochemistry, Geophysics, Geosystems, 9(4) ID Q04032, doi:10.1029/2007GC001696[Crossref], [Web of Science ®] , [Google Scholar], Recent investigations of the 0–5 Ma geomagnetic field recorded by lava flows: Geochemistry, Geophysics, Geosystems, v. 9, no. 4, ID Q04032, doi:10.1029/2007GC001696] databases for the last 5 million years. In those cases in which age determinations are available, the polarity obtained for the studied flows is consistent with their stratigraphic positions, except for the Huitepec site, which probably reflects the transitional geomagnetic regime prior to the Matuyama–Brunhes geomagnetic reversal.     

蜀南地区二叠系茅口组古岩溶地区水系分布及岩溶地貌单元特征*

蜀南地区茅口组为一套沉积稳定的巨厚层生物碎屑灰岩,基质致密性脆。中二叠世末的东吴运动使蜀南地区茅口组顶部发育古风化壳岩溶,古岩溶地区水系控制着古岩溶的发育和演化。在研究古地貌恢复方法的基础上,分析研究区地层特征和区域构造背景,采用印模法进行古地貌恢复,进而研究了古岩溶地区的水系发育特征。在此基础上,结合风化壳表面侵蚀溶蚀特征、沉积物性质、地貌组合形态、钻井和地震资料,把研究区划分为岩溶台地、岩溶陡坡、岩溶缓坡及岩溶盆地4种二级地貌单元和溶丘洼地、岩溶槽谷、峰林平原等10种三级地貌单元,进一步分析了各种地貌单元的岩溶特征,为下一步的储集层预测提供了有利的目标。     

Aspartic acid racemization dating of Holocene brachiopods and bivalves from the southern Brazilian shelf, South Atlantic

The extent of racemization of aspartic acid (Asp) has been used to estimate the ages of 9 shells of the epifaunal calcitic brachiopod Bouchardia rosea and 9 shells of the infaunal aragonitic bivalve Semele casali. Both taxa were collected concurrently from the same sites at depths of 10 m and 30 m off the coast of Brazil. Asp D/L values show an excellent correlation with radiocarbon age at both sites and for both taxa (r²_{Site 9 B. rosea} = 0.97, r²_{Site 1 B. rosea} = 0.997, r²_{Site 9 S. casali} = 0.9998, r²_{Site 1 S. casali} = 0.93). The Asp ratios plotted against reservoir-corrected AMS radiocarbon ages over the time span of multiple millennia can thus be used to develop reliable and precise geochronologies not only for aragonitic mollusks (widely used for dating previously), but also for calcitic brachiopods. At each collection site, Bouchardia specimens display consistently higher D/L values than specimens of Semele. Thermal differences between sites are also notable and in agreement with theoretical expectations, as extents of racemization for both taxa are greater at the warmer, shallower site than at the cooler, deeper one. In late Holocene marine settings, concurrent time series of aragonitic and calcitic shells can be assembled using Asp racemization dating, and parallel multi-centennial to multi-millennial records can be developed simultaneously for multiple biomineral systems.     

湖北省巴东栖霞组沉积成岩作用地球化学特征研究

在详细的沉积作用、成岩作用研究基础上,利用多元统计分析方法重点对湖北省巴东水布垭剖面栖霞组碳酸盐岩地球化学特征进行了分析。研究表明,该组碳酸盐岩中 CaO 、Al₂O₃ 、K₂O 、Fe₂O₃ 、P、Ba 和 Mn的含量主要与沉积环境或原岩岩性有关,地层中SiO₂、MgO 、Sr 和 Na 的含量主要与成岩作用有关。建立了栖霞组沉积成岩作用地球化学模式。该研究合理地解释了本组岩石特殊的地球化学特征,深化了对该组岩石形成环境,特别是成岩环境的认识,区分了沉积作用和成岩作用对该组岩石地球化学特征的影响,对合理地选取有关地球化学参数研究该组岩石及类似地层的沉积作用和成岩作用具参考价值。     

Geochronology and geochemistry of Grenvillian igneous suites in the northern Oaxacan Complex, southern Mexico: tectonic implications

Chemical and U–Pb isotopic analyses of metaigneous rocks in the northern Oaxacan Complex in southern Mexico indicate that they form part of two granitic–gabbroic suites intruded at 1157–1130 and 1012 Ma, which were metamorphosed under granulite facies conditions between 1004 and 980 Ma. Although the older suite has both within-plate and arc geochemical signatures, the arc characteristics (enrichment of La and Ce relative to Nb, Ta, and Th) are inferred to result from crustal contamination, a conclusion consistent with their negative _Nd signatures. The younger suite is spatially associated with anorthosites (from which we were unable to acquire a protolith age), suggesting that collectively it forms part of anorthosite–mangerite–charnockite–granite (AMCG) suites. The tholeiitic nature of the mafic rocks along with the within-plate character of the felsic rocks suggests that they were intruded during extension related to either farfield backarc rifting, rifting above a slab window, or anorogenic intercontinental rifting. Potentially correlative AMCG suites are widespread in Mexico, the Grenville Province of eastern Canada and northeastern USA, and the Andean massifs of Colombia, however, Pb isotopic data most closely resemble those in South America. These data are consistent with published hypotheses that suggest Oaxaquia represents an exotic terrane derived from Amazonia.     

Late Ordovician (Sandbian) brachiopods from the Mweelrea Formation,South Mayo,western Ireland: stratigraphic and tectonic implications

A Late Ordovician (Sandbian), Scoto‐Appalachian brachiopod fauna from the Mweelrea Formation in western Ireland confirms a location for the South Mayo Trough adjacent to the Laurentian margin, characterized at this time by a succession of marine excursions over fluviatile environments. The new, younger biostratigraphical data help to constrain the timing of late Grampian folding of this part of the South Mayo Trough. Copyright © 2010 John Wiley & Sons, Ltd.     

安徽巢湖地区二叠系瓜德鲁普统孤峰组古环境演化

二叠纪瓜德鲁普世是古海洋条件发生重大变化的转折期。瓜德鲁普世古海洋、古环境的演化对古生代底栖无脊椎动物灭绝的影响仍然是个谜。利用元素地球化学,分析瓜德鲁普统孤峰组的陆源碎屑供应、海洋表层水体的初级生产力以及底部水体的氧化还原条件。结果表明:瓜德鲁普世早期和晚期分别发生了一次陆源碎屑输入的高峰期。瓜德鲁普世早期海洋初级生产力最高,中期海水初级生产力较低,而晚期稍有升高。瓜德鲁普世古海水主要为缺氧至硫化环境。瓜德鲁普世早期以贫氧至缺氧环境为主,间歇性出现硫化环境;中晚期则以硫化环境为主,间歇性出现缺氧环境。这些氧化还原环境的演化主要受到水动力条件的影响。瓜德鲁普世深水环境水体的持续缺氧硫化引发浅水台地底部水体的持续贫氧甚至缺氧,造成海洋生态系统变得脆弱,引发生物危机事件。     

西藏羌塘北部上二叠统热觉茶卡组 类动物群的特征

西藏羌塘北部热觉茶卡组的 类动物群共3属11种（不含未定种）,包括：Palaeofusulina sinensis Sheng, P. fusiformis Sheng,P. nana Likharev, P. laxa Sheng, P. wangi Sheng, P. minima Sheng et Chang, P. simplex Sheng et Chang, Parananlingella acervula (Sheng et Rui), Reichelina changhsingensis Sheng et Chang, R. pulchra K. M.-Maclay, R. gaqoiensis Wang, Sheng et Zhang。根据 类沿剖面的分布,建立了Palaeofusulina 带,由下至上划分为2个亚带：Palaeofusulina minima亚带和Palaeofusulina sinensis亚带。该 类化石带属种丰度高,所含的 类动物群属华夏特提斯型分子,为晚二叠世的代表分子和典型分子,与中国华南长兴阶 类化石带可进行对比。     

内蒙古锡林浩特二叠系哲斯组腕足动物群特征及其意义

内蒙古锡林浩特克什克腾旗哈尔呼舒剖面哲斯组灰岩中发现了大量腕足化石,经鉴定计有22属42种,以长身贝亚目和石燕贝亚目为主,化石组合面貌具有典型的哲斯动物群特征。根据其与邻区的腕足生物地层对比,确定该腕足生物群时代为中二叠世Wordian期。哲斯动物群中凉水型分子共计10属26种,约占种级总量的61.9%,暖水型分子仅1种,约占种级总量的2.4%,体现了典型的凉水型动物群特征。古生物地理区划属于北方生物地理大区。此外,哲斯组碳酸盐岩中颗粒组合属于苔藓虫-棘皮动物-腕足动物组合,代表凉水沉积环境,也印证了哲斯动物群的凉水型特征。     

Faunas from the upper silurian (upper ludlovian) in the ludlow-much wenlock district,England

Faunal lists for 17 sections in the Overton Formation are presented. Faunal diversity appears to increase towards the formations base while faunal dominance appears to increase towards its top.     

黔南坳陷平塘甘寨二叠系茅口组沉积相与储层特征分析

通过铸体薄片、物性测试、扫描电镜等资料,对黔南坳陷平塘甘寨二叠系茅口组沉积相与储层特征进行深入分析,认为贵州平塘甘寨茅口组主要的岩石类型为亮晶-微晶生屑灰岩、生物灰岩、微晶灰岩、内碎屑亮晶灰岩、硅质岩、瘤状灰岩和含燧石灰岩等。茅口组生物种类较多,有介形虫、腹足、腕足、三叶虫、藻类、棘屑、蜒、珊瑚、海百合茎等。上述岩石学特征和生物组合指示平塘甘寨地区中二叠统茅口组为开阔碳酸盐台地沉积环境,包括台内滩和滩间两个亚相。储层特征分析结果表明,茅口组储集空间以晶间溶孔和晶间孔为主,具有明显的低孔低渗特征。局部溶蚀孔、构造裂缝相对较发育,对改造储层的孔渗性有重要的贡献。储层类型为碳酸盐岩裂缝-孔隙型储层,储层质量及发育部位受沉积相、成岩作用和构造活动多因素的控制。     

A biogeographically mixed late Guadalupian (late Middle Permian) brachiopod fauna from an exotic limestone block at Xiukang in Lhaze county, Tibet

A total of 17 brachiopod species belonging to 15 genera are recorded from a limestone block of about 3×4 km² in the Indus–Tsangbo suture zone at Xiukang in Lhaze County of Tibet. The brachiopod fauna generally indicates a Late Guadalupian age (late Wordian–Capitanian, late Middle Permian) based on its association with the Timorites-bearing ammonoid fauna and the presence of the brachiopod Urushtenoidea crenulata. Palaeobiogeographically, the fauna exhibits transitional/mixed characters between the warm-water Cathaysian and cold to temperate Gondwanan faunas and may have developed on a carbonate build-up or seamount on the oceanic crust.     

东昆仑花石峡北部上二叠统格曲组砂岩碎屑锆石LA-ICP-MSU-Pb年龄及其构造意义

东昆仑南缘上二叠统（乐平统）格曲组为一套由砾岩、砂岩、钙质泥岩构成的扇三角洲-浅海碳酸盐台地相沉积组合,在区域地质演化分析上具有重要的意义。经测定,格曲组砂岩的碎屑锆石LA-ICP-MS U-Pb年龄的范围为412~2448Ma,包括1869~1602Ma、1396Ma~1270Ma、1197~877Ma和572~412Ma四个年龄区间,指示物源区先后经历了古元古代早期的构造-岩浆事件、古元古代晚期的汇聚事件、中元古代的裂解事件、中元古代晚期—新元古代早期的构造-岩浆-变质事件和新元古代晚期—早古生代的裂解-扩张-汇聚事件。其中,与新元古代—早古生代洋陆转化相关的产物是格曲组的主要物源。格曲组砂岩碎屑锆石年龄组成表明沉积盆地具有前陆盆地“冷盆地”的特征。相对于格曲组,下中二叠统存在大量大于2000Ma的年龄信息,而下三叠统开始出现了大量海西末期-印支期的年龄信息。碎屑锆石年龄组成出现两次明显的变化,分别为格曲组和洪水川组底部不整合所代表的构造运动的沉积响应,表现出较好的构造-沉积耦合关系。     

A Gondwanian Palynoflora from the Lower Permian Chubu Formation,Southern Tibet

l.IntroductionThePermianstratacroppingoutintheMt.Jo-moLungmaandnorthwardregionofsouthernTibetwererecognizedbytheComprehensiveScientificExpeditiontotheQinghai-TibetplateauofAcade-miaSinica(l975),andGondwanianGlossOPterisflorawasdiscoveredfromtheterrestrialPermiandeposits(HshJen,l973,l976).AccordingtoYinJixiangandGuoShizeng(l976)andYinJixiang(l997),therelevantstratamaybedividedintothreeformations,indescendingorder,theyare;Chubujeka(Quburiga)FormationfThelowerportionmainlycomposedofgra…     

Reply to Molina-Garza et al. (2019) “Discussion of: Ortega-Flores et al. (2018) provenance analysis of Oligocene sandstone from the Cerro Pelón area,southern Gulf of Mexico”

ABSTRACT

The origin of the Oligocene turbidites from the Cerro Pelón area in south Gulf Mexico proposed by Ortega-Flores et al. (2018) is in disagree with the interpretations made by Molina-Garza et al. (2019), which main criticism is based on U-Pb ages of detrital zircons from the matrix of a conglomerate unit, which they refer to as ‘Nanchital Conglomerate’, as well as on the presence of limestone, gabbros, and mafic protolith-derived clasts. Molina-Garza et al. (2019) basically interpret the Nanchital Conglomerate as Miocene in age, which was sourced mainly from metamorphic complexes including their sedimentary covers located to the west and south of the Cerro Pelón area. For some reason, Molina-Garza et al. (2019) suppose that the Nanchital Conglomerate should have the same provenance sources that the Oligocene turbidites from Cerro Pelón area, reported by Ortega-Flores et al. (2018). Based on the foregoing, we strongly disagree with Molina-Garza et al. (2019) considering that, from the beginning, they intend to compare two units of different age. Additionally, the scarce data reported from both the matrix and the clasts of the Nanchital Conglomerate are not determinant for interpreting the provenance of this conglomeratic unit and subsequently, to consider the same rock sources from the Oligocene through Miocene time.