共查询到20条相似文献，搜索用时 281 毫秒
1.
在概述了浙江西部上奥陶统于潜组的沉积学特征,并进而描述了发现的14个遗迹化石属基础上,从4个方面对Spirodesmos为代表的遗迹组合进行了环境分析:(1)14个遗迹属中仅3个属为穿相型分子,其余占遗迹总属2/3以上的遗迹化石均为典型的深水型分子,它们都是Nereites遗迹相的主要成员,因此该遗迹组合可归属于深海Nereites遗迹相中;(2)大部分遗迹化石呈复杂规则弯曲的形态,这种形态的遗迹化石是深海环境的遗迹化石标志;(3)14个遗迹属均为觅食迹和牧食迹,它们的行为习性,尤其是牧食迹主要分布于深海环境;(4)多数遗迹属的水深分布均在200m以下。由此可认为Spirodesmos遗迹组合是半深海-深海环境的产物。 相似文献
2.
Birendra P. Singh 《Journal of the Geological Society of India》2009,74(6):723-737
Lower-Middle Cambrian (Lungwangmiaoan-Hsuchuangian stage) Parahio Formation (Kunzam La Formation) exposed at Purni Village,
Niri-Tsarap Chu Valley of Zanskar region of Tethyan Himalaya has yielded a high diversity and abundant ichnofossils with myriapod
trackways. It includes Cruziana isp., Phycodes palmatum, Diplocraterion isp., Diplichnites isp., Dimorphichnus isp., Tapherhelminthopsis cf. circularis, Teichichnus isp., Monomorphicnus isp., Lockeia isp., Skolithos isp., Planolites isp., Palaeophycus isp., Isopodichnus isp., and arthropod appendage marks. Integration of ichnological and sedimentological studies for measured part of the Parahio
Formation (Cambrian) reveals that alternating energy conditions due to recurring storm events were superimposed on low energy
fair-weather sediments. The post-depositional Arenicolites ichnofacies, preserved within the high-energy storm beds of shoreface deposits, commonly overlies the fair-weather assemblage
of pre-depositional Cruziana ichnofacies of relatively more offshore deposits, which contains more diverse and varied behavioral signatures of various
deposit feeders. The traces of the Cruziana ichnofacies are abruptly replaced by traces of Arenicolites ichnofacies and show reiteration throughout the measured part of the Parahio Formation. The ichnofabric indices in these
deposits range from ii3–ii5. No discrete sets of tiers are observed; perhaps there were two coeval communities. Ichnologically,
an idealized wave-dominated progradational cycle in the Parahio Formation is characterized, from base to top, by a Cruziana ichnofacies (lower-upper offshore to offshore transition) and storm related Arenicolites ichnofacies (shoreface). Each progradational cycle reflects a progressive increase in sand content, degree of oxygenation,
hydrodynamic energy and dearth of food. These environmental factors controlled the vertical distribution of trace fossils
in Parahio Formation. The occurrence of Cruziana ichnoassociation much below the Middle Cambrian trilobite horizon in the Parahio Formation restricts the age of this part
of the formation to the pre-Middle Cambrian, probably series 2 of Cambrian system. 相似文献
3.
MICHAEL C. RYGEL CHRISTOPHER R. FIELDING KERRIE L. BANN TRACY D. FRANK LAUREN BIRGENHEIER STUART C. TYE 《Sedimentology》2008,55(5):1517-1540
The Lower Permian Wasp Head Formation (early to middle Sakmarian) is a ～95 m thick unit that was deposited during the transition to a non‐glacial period following the late Asselian to early Sakmarian glacial event in eastern Australia. This shallow marine, sandstone‐dominated unit can be subdivided into six facies associations. (i) The marine sediment gravity flow facies association consists of breccias and conglomerates deposited in upper shoreface water depths. (ii) Upper shoreface deposits consist of cross‐stratified, conglomeratic sandstones with an impoverished expression of the Skolithos Ichnofacies. (iii) Middle shoreface deposits consist of hummocky cross‐stratified sandstones with a trace fossil assemblage that represents the Skolithos Ichnofacies. (iv) Lower shoreface deposits are similar to middle shoreface deposits, but contain more pervasive bioturbation and a distal expression of the Skolithos Ichnofacies to a proximal expression of the Cruziana Ichnofacies. (v) Delta‐influenced, lower shoreface‐offshore transition deposits are distinguished by sparsely bioturbated carbonaceous mudstone drapes within a variety of shoreface and offshore deposits. Trace fossil assemblages represent distal expressions of the Skolithos Ichnofacies to stressed, proximal expressions of the Cruziana Ichnofacies. Impoverished trace fossil assemblages record variable and episodic environmental stresses possibly caused by fluctuations in sedimentation rates, substrate consistencies, salinity, oxygen levels, turbidity and other physio‐chemical stresses characteristic of deltaic conditions. (vi) The offshore transition‐offshore facies association consists of mudstone and admixed sandstone and mudstone with pervasive bioturbation and an archetypal to distal expression of the Cruziana Ichnofacies. The lowermost ～50 m of the formation consists of a single deepening upward cycle formed as the basin transitioned from glacioisostatic rebound following the Asselian to early Sakmarian glacial to a regime dominated by regional extensional subsidence without significant glacial influence. The upper ～45 m of the formation can be subdivided into three shallowing upward cycles (parasequences) that formed in the aftermath of rapid, possibly glacioeustatic, rises in relative sea‐level or due to autocyclic progradation patterns. The shift to a parasequence‐dominated architecture and progressive decrease in ice‐rafted debris upwards through the succession records the release from glacioisostatic rebound and amelioration of climate that accompanied the transition to broadly non‐glacial conditions. 相似文献
4.
新化、湘潭等地跳马涧组的遗迹化石经笔者鉴定与描术者共15个遗迹属24个遗迹种(包括4个新遗迹种)。它们大部分属于跳马涧组上部,可分为Arenicolites-Palaeophycus组合,Skolithos-Planolites组合,Palaeophycus-Gordia组合,Rhizocorallium-Planolites组合,代表近滨环境,属Cruziana遗迹相。部分属于顶部Zoophycus-Cochlichnus组合和Zoophycus-Skolithos组合,属Zoophycus遗迹相上部。综合前人资料,湖南跳马涧组遗迹化石现已发现22属45种,可归并为居住迹、觅食迹、爬行迹、休息迹、啮食迹、钻孔、逃逸构造、生物扰动构造八种生态类型。由跳马涧组下部至顶部,由以居住迹为主的遗迹组合,转变为居住迹、觅食迹同等发育或以觅食迹为主,转变为啮食迹为主的组合。反映由前滨转变为近滨,再转变为远滨,代表海水加深、海侵扩大的过程。 相似文献
5.
An ichnofossil assemblage is preserved in sediments of the Middle Bhuban Formation,Surma Group,of Miocene age from the Durtlang and Chanmari sections,Aizawl district,Mizoram,northeastern India.The studied sections comprise a thick,bioturbated and thinly bedded sandstone–silty shale facies.The preserved ichnofossils show high density and low diversity with the assemblage represented by feeding and resting traces,i.e.,Psilonichnus upsilon Frey et al.,1984,Ophiomorpha isp.,Teichichnus spiralis Mikulá.1990,Skolithos isp.,Palaeophycus isp.,and other horizontal burrows.Sedimentary structures associated with these traces are mainly cross bedding,flaser bedding and slump structures.Considering the distribution of the Psilonichnus ichnogenus across an integrated ichnological–sedimentological framework,the stratigraphic interval of the Middle Bhuban Formation,in which they occur,are interpreted to have been deposited under a shallow,marginal-marine channel complex dominated by tidal channels developed in quiet,brackish-water portions of a delta plain. 相似文献
6.
Abstract: The Lower Cambrian Nagaur Sandstone (Marwar Supergroup) has yielded trace fossils Treptichnus, Cruziana, serially repeated Rusophycus, Diplichnites, Monomorphichnus, Bergaueria, arthropod swimming traces and leap frogging marks of Cruziana ichnofacies. This ichnofossils assemblage is dominantly preserved in fine to medium grained red sandstone beds of the Mohra Member (Nagaur Sandstone). The presence of graded rip-up clasts, current ripples, dune cross-stratification with mud drapes and tidal bundles indicates a subtidal paleoenvironment for the deposition of Nagaur Sandstone Formation, which is corroborated by the Cruziana ichnofacies trace fossil assemblage. 相似文献
7.
The Lower Cambrian Chilhowee Group of northeastern Tennessee consists of the Unicoi, Hampton and Erwin Formations, and is divided into four facies. The conglomerate facies occurs only within the lower 200 m of measured section (the Unicoi Formation) and consists of fine-grained to pebbly quartz wacke with rare thin beds of laminated siltstone. Low-angle to horizontally laminated, fine-grained sandstone with laminae and lenses of granules and pebbles represents upper flow-regime, overbank deposition within a braided stream system that was close to a coastline. Medium-scale, planar-tabular cross-stratified conglomerate in which megaripple bedforms are preserved is interpreted as representing deposition in interbar pools of braided channels, as flood stage waned and larger bedforms ceased to migrate. Large-scale, planar-tabular cross-stratified conglomerate beds represent migration of large transverse bars within a broad braided stream channel during high flood stage. The sandstone facies occurs throughout the Chilhowee Group, and is therefore interbedded with all other facies. It consists of mainly medium- to very coarse-grained, subarkosic to arkosic arenite. Thinly interbedded, laminated siltstone and sandstone, which may exhibit wavy or lenticular bedding, represents deposition during slack water periods between ebb and flood tides. Large-scale planar-tabular and trough cross-stratification reflects deposition within the deepest areas of subtidal channels, whereas medium-scale cross-stratification represents deposition in shallower water on shoals separating channels. Fining- and thinning-upward sequences most likely resulted from the longshore migration of channels and shoals. The hummocky facies occurs only in the Erwin Formation and consists of horizontally laminated to hummocky stratified, fine-grained arkosic to subarkosic arenite interbedded with equal amounts of bioturbated mudstone. It represents deposition between storm and fairweather wave-base by combined-flow storm currents. The quartz arenite facies is characterized by an absence of fine-grained units and lithologically consists of a super-mature, medium- to coarse-grained quartz arenite. Large-scale planar-tabular cross-stratification and abundant low-angle cross-stratification with rare symmetrical ripples (lower quartz arenite facies) occurs interbedded with the braided fluvial conglomerate facies, and was deposited within either a ridge-and-runnel system or a system of nearshore bars. Large-scale, planar-tabular cross-stratification (upper quartz arenite facies), which forms the top of two 40 m-thick coarsening-upward sequences of the type: hummocky faciessandstone faciesquartz arenite facies, probably represents deposition on sand ridges that formed on a sand-starved shelf as transgression caused the detachment and reworking of shoreface channel-shoal couplets. Palaeocurrent data for the Chilhowee Group are unimodal but widely dispersed from 0° to 180°, and exhibit a minor mode to the west. The data are interpreted to reflect the influence of longshore, tidal and storm currents. The ichnofossil assemblage changes upsection from one characterized only by Paleophycus to a Skolithos ichnofacies and finally to a Cruziana ichnofacies. The facies sequence, biogenic and palaeocurrent data reflect the interaction through time of (I) non-marine and marine processes; and (2) transgression coupled with shoreline progradation. The Chilhowee Group represents an overall deepening from terrestrial deposition to a marine shoreface that experienced both longshore and tidal currents, and finally to a storm shelf environment that periodically shoaled upward. 相似文献
8.
The Middle Jurassic rocks of the Kaladongar Formation well exposed in the Kaladongar Hill range of the Patcham Island and Kuar Bet of the Northern Kachchh comprises of ∼450 m thick sequence of mixed siliciclasticcarbonate sediments intercalated with shales. These Mixed siliciclastic-carbonate sediments show wide variation in textural and mineralogical composition and represent genetically related six rock types: micritic sandstone, allochemic sandstone, sandy allochemic limestone, micrtic mudrock, sandy micrite and muddy micrite; which are highly bioturbated and show behaviourally diverse groups of trace fossils. Total 34 ichnogenera are identified, which includes, Arenicolites, Asterosoma, Beaconites, Bergaueria, Chondrites, Cochlichnus, Dactylophycus, Daedalus, Didymaulichnus, Diplocraterion, Gordia, Gyrochorte, Gyrolithes, Ichnocumulus, Laevicyclus, Lockeia, Margaritichnus, Monocraterion, Nereites, Ophiomorpha, Palaeophycus, Phoebichnus, Phycodes, Pilichnus, Planolites, Plug Shaped Form, Protovirgularia, Rhizocorallium, Scolicia, Skolithos, Taenidium, Teichichnus, Thalassinoides and Walcottia. These trace fossils are classified into six morphological groups namely, circular and elliptical structures; simple structures; branched structures; rosette structures; spreiten structures; and winding and meandering structures. These trace fossils are further group into eight assemblages which occurred together into mixed siliciclastic-carbonate sediments, include, Asterosoma assemblage, Gyrochorte assemblage, Rhizocorallium assemblage, Thalassinoides assemblage, Planolites-Palaeophycus assemblage, Phycodes assemblage, Ophiomorpha assemblage and Skolithos assemblage. The recurring pattern of these assemblages through the sequence displays the development of Skolithos and Cruziana ichnofacies and at places the mixed Skolithos-Cruziana ichnofacies which suggest a low wave and current energy conditions with intervening period of high wave and current energy conditions and an intermediate period of stressful environments, respectively. Sedimentological and ichnological data suggest that the deposition of the mixed siliciclastic-carbonate sediments of the Kaladongar Formation took place in the foreshore to offshore environment under fluctuating wave and current energy condition. 相似文献
9.
The Kaskapau Formation spans Late Cenomanian to Middle Turonian time and was deposited on a low‐gradient, shallow, storm‐dominated muddy ramp. Dense well log control, coupled with exposure on both proximal and distal margins of the basin allows mapping of sedimentary facies over about 35 000 km2. The studied portion of the Kaskapau Formation is a mudstone‐dominated wedge that thins from 700 m in the proximal foredeep to 50 m near the forebulge about 300 km distant. Regional flooding surfaces permit mapping of 28 allomembers, each of which represent an average of ca 125 kyr. More than 200 km from shore, calcareous silty claystone predominates, whereas 100 to 200 km offshore, mudstone and siltstone predominate. From about 30 to 100 km offshore, centimetre‐bedded very fine sandstone and mudstone record along‐shelf (SSE)‐directed storm‐generated geostrophic flows. Five to thirty kilometres from shore, decimetre‐bedded hummocky cross‐stratified fine sandstone and mudstone record strongly oscillatory, wave‐dominated flows whereas some gutter casts indicate shore‐oblique, apparently mostly unidirectional geostrophic flows. Nearshore facies are dominated by swaley cross‐stratified or intensely bioturbated clean fine sandstone, interpreted as recording, respectively, areas strongly and weakly affected by discharge from distributary mouths. Shoreface sandstones grade locally into river‐mouth conglomerates and sandstones, including conglomerate channel‐fills up to 15 m thick. Locally, brackish lagoonal shelly mudstones are present on the extreme western margin of the basin. There is no evidence for clinoform stratification, which indicates that the Kaskapau sea floor had extremely low relief, lacked a shelf‐slope break, and was probably nowhere more than a few tens of metres deep. The absence of clinoforms probably indicates a long‐term balance between rates of accommodation and sediment supply. Mud is interpreted to have been transported >250 km offshore in a sea‐bed nepheloid layer, repeatedly re‐suspended by storms. Fine‐grained sediment accumulated up to a ‘mud accommodation envelope’, perhaps only 20 to 40 m deep. Continuous re‐working of the sea floor by storms ensured that excess sediment was redistributed away from areas that had filled to the ‘accommodation envelope’, being deposited in areas of higher accommodation further down the transport path. The facies distributions and stratal geometry of the Kaskapau shelf strongly suggest that sedimentary facies, especially grain‐size, were related to distance from shore, not to water depth. As a result, the ‘100 to >300 m’ depth interpreted from calcareous claystone facies for the more central parts of the Interior Seaway, might be a significant overestimate. 相似文献
10.
The uppermost portion of the Taciba Formation, Itararé Group, Paraná Basin, Brazil, records a succession of depositional environments tied to the demise of late Paleozoic glaciation. In the study area, Teixeira Soares county, state of Paraná, the unit is dominated by massive to laminated diamictites with inclusions of sandstones and other coarse-grained lithotypes, representing re-sedimented material in proximal areas. These are succeeded by fine to medium-grained sandstones with tabular cross-stratification and pectinid-rich shell pavements, interpreted as nearshore deposits. Above this, laminated and intensely bioturbated siltstones with closed articulated bivalve shells are recorded, probably deposited in inner shelf settings. Fine to very fine sandstones/siltstones with hummocky cross-stratification and intercalated mudstones, including infaunal in situ shells, are interpreted as stacked storm deposits, generated in distal shoreface environments. These are succeeded by fossil-poor, massive to laminated siltstones/mudstones or gray shales (=Passinho shale) that are inferred to be outer shelf deposits, generated in organic-rich, oxygen-deficient muddy bottoms. In this sedimentary succession dropstones or ice-rafted debris are missing and locally the Passinho shale marks the maximum flooding surface of the Itararé succession. These are capped by the fluvio-deltaic deposits of the Rio Bonito Formation (Sakmarian–Artinskian). Six facies-controlled, bivalve-dominated assemblages are recognized, representing faunal associations that thrived in aerobic to extreme dysaerobic bottoms along a nearshore–offshore trend. Within these assemblages, nineteen bivalve species (three of them new) were recorded and described in detail. The presence of Myonia argentinensis (Harrington), Atomodesma (Aphanaia) orbirugata (Harrington) and Heteropecten paranaensis Neves et al. suggests correlation with bivalve assemblages of the Eurydesma-bearing Bonete Formation, Pillahuincó Group, Sauce Grande-Colorado Basin (Buenos Aires Province), Argentina, indicating a possible Asselian age for this diverse post-glacial bivalve fauna. Despite that, typical members of the icehouse-style Eurydesma–Trigonotreta biota (stricto sensu) have not yet been found in the studied bivalve assemblages. 相似文献
11.
In Jabalpur area about 18 m to 45 m thick Lameta Formation is stratigraphically divisible into five lithounits namely, Green
Sandstone, Lower Limestone, Mottled Nodular Beds, Upper Limestone and Upper Sandstone. Having differentiated lithofacies constitution
and here grouped as facies associations, these units are intensively burrowed and sparingly fossiliferous. Ichnogenera including
Arenicolites, Calycraterion, Fucusopsis, Laevicyclus, Macanopsis, Ophiomorpha, Paleomeandron, Rhizocorallium, Stipsellus,
Thalassinoides and Zoophycos are recovered from the Lower Limestone, Mottled Nodular Beds and Upper Limestone associations of the Lameta Formation of
Jabalpur area.Among these, Arenicolites, Calycraterion, Laevicyclus, Ophiomorpha, Rhizocorallium, Stipsellus and Thalassinoides belong to mixed Skolithos and Cruziana ichnofacies and indicate sandy backshore to sublittoral condition of deposition. Additionally rhyzocretes, some times chertified,
are also present in different parts of the Lameta Formation. Ichnofacies assemblage supported by sedimentological information
suggests that the Lameta Formation of Jabalpur area was deposited in coastal marine settings where sediments were subaerially
exposed intermittently. 相似文献
12.
ERNESTO SCHWARZ 《Sedimentology》2012,59(5):1478-1508
The interpretation of sharp‐based shallow‐marine sandstone bodies encased in offshore mudstones, particularly transgressive units, has been a subject of recent debate. This contribution provides a multiple‐dataset approach and new identification criteria which could help in the recognition of transgressive offshore sandstone bodies worldwide. This study integrates sedimentology, ichnology, taphonomy and palaeoecology of Mulichinco Formation strata in the central Neuquén Basin (Argentina) in order to describe and interpret sharp‐based sandstone bodies developed in ramp‐type marine settings. These bodies are sandwiched between finer‐grained siliciclastics beneath and thin carbonates above. The underlying sediments comprise progradational successions from offshore mudstones to offshore transition muddy sandstones, grading occasionally into lower shoreface sandstones. The surfaces capping the regressive siliciclastics are flat and regionally extensive, and are demarcated by skeletal concentrations and a Glossifungites suite; they are also marked by sandstone rip‐up clasts, with encrustations and borings on all sides. These surfaces are interpreted as composite discontinuities, cut during a relative sea‐level fall and remodelled during the initial transgression. The overlying transgressive sandstone bodies are 3 to 7 m thick, >4 km long and about three times longer than wide; they are composed of fine‐grained sandstones with little lateral change in grain size. Cross‐stratification and/or cross‐lamination are common, typically with smaller‐scale structures and finer grain size towards the top. Large‐scale, low‐angle (5° to 8°) inclined stratification is also common, dipping at ca 30° with respect to body elongation and dominant currents. These sandstone bodies are interpreted as offshore sand ridges, probably developed under the influence of tidal currents. Intense burrowing is typical at the top of each unit, suggesting an abandonment stage. Final deactivation favoured colonization by epibenthic‐dominated communities and the formation of skeletal‐rich limestones during the latest transgressive conditions. As partial reworking of pre‐existing ridges occurred during this stage, the Mulichinco sandstone bodies are considered the remnants of transgressive offshore sand units. 相似文献
13.
塔里木盆地下志留统塔塔埃尔塔格组痕迹化石与海进期沉积及海进体系域 总被引：9，自引：2，他引：7
在层序地层学研究中,海进（transgrssion）常被认为是快速形成的,序列简单,并以少量沉积为特征。对塔里木盆地下志留统塔塔埃尔塔格组痕迹化石及沉积特征研究表明,海进期沉积及海进体系域比人们想象的要复杂,由三类沉积组成:（1）与海进侵蚀有关,包括海进滞留沉积,发育固底控制Glossifungites痕迹相;（2）与海进初期的海面缓慢上升或总体海进过程中的静止期有关,包括前滨和临滨砂泥岩,发育Skolithos痕迹相和近端Gruziana痕迹相;（3）与海面快速上升有关,包括滨外泥岩,发育远端Gruziana痕迹相。 相似文献
14.
Strata of the Bardas Blancas Formation (lower Toarcian–lower Bajocian) are exposed in northern Neuquén Basin. Five sections have been studied in this work. Shoreface/delta front to offshore deposits predominate in four of the sections studied exhibiting a high abundance of hummocky cross-stratified, horizontally bedded and massive sandstones, as well as massive and laminated mudstones. Shell beds and trace fossils of the mixed Skolithos-Cruziana ichnofacies appear in sandstone beds, being related with storm event deposition. Gravel deposits are frequent in only one of these sections, with planar cross-stratified, normal graded and massive orthoconglomerates characterizing fan deltas interstratified with shoreface facies. A fifth outcrop exhibiting planar cross-stratified orthoconglomerates, pebbly sandstones with low-angle stratification and laminated mudstones have been interpreted as fluvial channel deposits and overbank facies. The analysis of the vertical distribution of facies and the recognition of stratigraphic surfaces in two sections in Río Potimalal area let recognized four transgressive–regressive sequences. Forced regressive events are recognized in the regressive intervals. Comparison of vertical distribution of facies also shows differences in thickness in the lower interval among the sections studied. This would be related to variations in accommodation space by previous half-graben structures. The succession shows a retrogradational arrangement of facies related with a widespread transgressive period. Lateral variation of facies let recognize the deepening of the basin through the southwest. 相似文献
15.
The Upper Cretaceous Twentymile Sandstone of the Mesaverde Group in NW Colorado, USA, has been analysed with respect to its pinch‐out style and the stratigraphic position of tidally influenced facies within the sandstone tongue. Detailed sedimentological analysis has revealed that the Twentymile Sandstone as a whole is a deltaic shoreface sandstone tongue up to 50 m thick proximally. Facies change character vertically from very fine‐grained, storm wave‐dominated shelf sandstones and mudstones to fine‐grained, wave‐dominated sandstones and, finally, to fine‐ to coarse‐grained tidally dominated sandstones. The pinch‐out style is characterized by a basinward splitting of the massive proximal sandbody into seven coarsening‐upward fourth‐order sequences consisting of a lower shaly part and an upper sandy part (sandstone tongue). These are stacked overall to reflect the regressive‐to‐transgressive development of the tongue. Each of the lower sandstone tongues 1–3 are gradationally based, very fine‐grained and dominated by hummocky cross‐stratification and were deposited on the lower to upper shoreface. Sandstone tongues 4 and 5 prograded further basinwards than the underlying tongues, are erosively based, fine‐ to coarse‐grained and mainly hummocky, herringbone and trough cross‐stratified. Especially in tongue 5, tidal indicators, such as bipolar foresets and double mud drapes, are common. These tongues were deposited as upper shoreface and tidal channel sandstones respectively. Sandstone tongues 6 and 7 retrograded in relation to tongue 5, are very fine‐ to fine‐grained and hummocky cross‐stratified. These tongues were deposited in lower shoreface to offshore transition environments. The two lower fourth‐order sequences were deposited during normal regressions during slowly rising or stable relative sea level and represent the highstand systems tract. The three succeeding fourth‐order sequences, which show succeedingly increasing evidence of tidal influence, were deposited during falling and lowstand of relative sea level and represent the falling stage (forced regressive) and lowstand systems tracts. The uppermost two fourth‐order sequences were deposited during rapidly rising sea level in the transgressive systems tract. The maximum tidal influence occurred during lowstand progradation, in contrast to most other published examples reporting maximum tidal influence during transgression. 相似文献
16.
The Bridport Sand Formation is an intensely bioturbated sandstone that represents part of a mixed siliciclastic‐carbonate shallow‐marine depositional system. At outcrop and in subsurface cores, conventional facies analysis was combined with ichnofabric analysis to identify facies successions bounded by a hierarchy of key stratigraphic surfaces. The geometry of these surfaces and the lateral relationships between the facies successions that they bound have been constrained locally using 3D seismic data. Facies analysis suggests that the Bridport Sand Formation represents progradation of a low‐energy, siliciclastic shoreface dominated by storm‐event beds reworked by bioturbation. The shoreface sandstones form the upper part of a thick (up to 200 m), steep (2–3°), mud‐dominated slope that extends into the underlying Down Cliff Clay. Clinoform surfaces representing the shoreface‐slope system are grouped into progradational sets. Each set contains clinoform surfaces arranged in a downstepping, offlapping manner that indicates forced‐regressive progradation, which was punctuated by flooding surfaces that are expressed in core and well‐log data. In proximal locations, progradational shoreface sandstones (corresponding to a clinoform set) are truncated by conglomerate lags containing clasts of bored, reworked shoreface sandstones, which are interpreted as marking sequence boundaries. In medial locations, progradational clinoform sets are overlain across an erosion surface by thin (<5 m) bioclastic limestones that record siliciclastic‐sediment starvation during transgression. Near the basin margins, these limestones are locally thick (>10 m) and overlie conglomerate lags at sequence boundaries. Sequence boundaries are thus interpreted as being amalgamated with overlying transgressive surfaces, to form composite erosion surfaces. In distal locations, oolitic ironstones that formed under conditions of extended physical reworking overlie composite sequence boundaries and transgressive surfaces. Over most of the Wessex Basin, clinoform sets (corresponding to high‐frequency sequences) are laterally offset, thus defining a low‐frequency sequence architecture characterized by high net siliciclastic sediment input and low net accommodation. Aggradational stacking of high‐frequency sequences occurs in fault‐bounded depocentres which had higher rates of localized tectonic subsidence. 相似文献
17.
A new locality bearing ichnofossils of the Cruziana Assemblage Zone-Ⅲ from the Mussoorie syncline,Lesser Himalaya,is located in rocks of Member-B of the Dhaulagiri Formation,Tal Group,exposed along the Maldewta-Chhimoli fresh road cut section.The site yielded ichnofossils Bergaueria perata,Cochlichnus anguineus,？Diplocraterion isp.,Dimorphichnus obliquus,diplichnitiform Cruziana bonariensis,Diplichnites gouldi,Glockeria isp.,Helminthopsis isp.,Monomorphichnus lineatus,Phycodes palmatum,Palaeophycus striatus,Planolites beverleyensis,Planolites montanus,Treptichnus cf.T.pedum,scratch marks and an undetermined worm impression.An Early Cambrian age （Cambrian Series 2） is assigned to the ichnofossil-bearing strata based on the stratigraphic position between the Drepanuroides and Palaeoolenus trilobite zones.A revised Cambrian ichnofossil zonation is presented for the Tal Group of the Mussoorie syncline.Together with their occurrence on rippled surfaces,and the lateral displacement of some trackways （due to current action）,a sub-aqueous shallow-marine depositional setting is proposed for the rocks of Member-B. 相似文献
18.
The Cambrian succession in the Zanskar Basin of Tethys Himalaya contains an abundant ichnofossils like in the other Tethyan
Himalayan successions. The ichnofossils are stratigraphically important as they occur below the trilobite body fossils and
are useful to define the basal part of the Cambrian. The ichnofossil assemblage reported from the Zanskar Basin of Ladakh
Himalaya is significant to demarcate the Early Cambrian age due to lack of other faunal elements so far. The body fossils
of trilobites recorded from the overlying beds indicates the earliest part of the Middle Cambrian age. Sixteen ichnogenera
identified include: Bifungites, Cruziana, Diplichnites, Dimorphichnus, Isopodichnus, Lockeia, Merostomichnites; Monomorphichnus, Psammichnites,
Palaeophycus, Planolites, Rhizocorallium, Skolithos, Taphrhelminthopsis, Teichichnus, Trepitichnus and trilobite scratch marks etc. The ichnogenera reported so far from this part of the Tethyan Himalayan region belongs mostly
to the traces of arthropod origin. The ichnofauna ranges in age from Lower Cambrian to late part of the Middle Cambrian. The
ichnofaunal assemblage can be assigned to repichnial, cubichinial, pascichnial, to fodinichnial behaviour. The distribution
of ichnofossils in the studied sections shows that the ichnofossils are predominately less in occurrence in the sections were
trilobites dominates and higher in the successions the abundance of ichnofossils decreases. 相似文献
19.
RAGHAVENDRA P TIWARI CHINMOY RAJKONWAR LALCHAWIMAWII PAUL LALNUNTLUANGA JEHOVA MALSAWMA VICTOR Z RALTE SATISH J PATEL 《Journal of Earth System Science》2011,120(6):1127-1143
A detailed ichnological study performed on the Bhuban Formation, Surma Group (Lower to Middle Miocene) of Mizoram, India reveals
the occurrence of rich and diverse trace fossils. These have been collected from the two localities in Aizawl, i.e., Bawngkawn
and Ropaiabawk, where sandstone—shale sequence is well exposed. Total 20 ichnospecies of 14 ichnogenera have been identified
which include Arenicolites isp., Cochlichnus anguineus, Helminthopsis abeli, Laevicyclus mongraensis, Ophiomorpha borneensis, Palaeophycus tubularis, Palaeophycus heberti, Palaeophycus sulcatus, Palaeophycus alternatus, Pholeus abomasoformis, Pholeus bifurcatus, Planolites beverleyensis, Planolites annularis, Polykladichnus irregularis, Rhizocorallium isp., Skolithos linearis, Taenidium satanassi, Teichichnus rectus, Thalassinoides horizontalis and Thalassinoides paradoxicus. Ethologically these ichnogenera display dwelling and feeding activities of the infaunal organisms. Arenicolites, Ophiomorpha, Polykladichnus and Skolithos are the members of the Skolithos ichnofacies while Palaeophycus, Planolites, Rhizocorallium and Thalassinoides are the members of the Cruziana ichnofacies. The presence of Skolithos ichnofacies indicates sandy shifting substrate and high energy conditions in foreshore zone while the Cruziana ichnofacies indicate unconsolidated, poorly sorted soft substrate and low energy condition in the shoreface/offshore zone.
These ichnogenera indicate foreshore to shoreface-offshore zone of shallow marine environment for the deposition of the rocks
of the Bhuban Formation of Mizoram. 相似文献
20.
Lithofacies in the mid‐Permian Nowra Sandstone indicate a middle/upper shoreface to foreshore environment of deposition under the influence of storm‐generated waves and north‐northeasterly directed longshore currents. Palaeogeographic reconstruction for the Nowra Sandstone portrays a sand‐dominated high energy shelf and offshore shoal forming a sequence thickening seaward away from the western shore of the Sydney Basin. The shoal‐crest at the outer edge of the shelf trends north‐northeast. It is characterized by fine‐ to medium‐grained sandstone with upper flow regime structures and a high proportion of conglomerate, whereas coarser sandstone with lower energy bedforms occurs along the seaward side of the shoal. In the deeper water to the east, the lower Nowra Sandstone becomes rapidly thinner as it passes seaward, via bioturbated storm redeposited sandstone beds, into the shelf deposits of the Wandrawandian Siltstone. This sequence accumulated during a regressive event and the base of the formation becomes progressively younger eastward. The sand may have been supplied by rivers along the western coast but the major source was south of the study area. The lower Nowra Sandstone is separated from the upper part of the formation by an extensive ravinement surface overlain by the Purnoo Conglomerate Member. In contrast to the lower unit, the upper Nowra Sandstone forms a westward thickening wedge that represents a backstepping nearshore sand facies that accumulated during a transgression. The upper Nowra Sandstone passes vertically and laterally eastward into the Berry Siltstone. Thus both boundaries of the Nowra Sandstone are diachronous, first younging eastward and then westward as a response to a regressive‐transgressive episode. 相似文献