Abstract: Lower Permian Cisuralian Series in The Type Area: History, Current Status, Sedimentological and Biostratigraphic Characteristics, ans Perspectives For the Lower Permian Global Scale

SPINOSA, CLAUDE, VLADIMIR I. DAVYDOV, (Permian Research Institute, Department of Geosciences, Boise State University, Boise, ID 83725), BRUCE WARDLAW (US Geological Survey, Reston, Virginia), WALTER S. SNYDER (Boise State University), and TAMRA A. SCHIAPPA, (University of Idaho, Moscow, Idaho and Boise State University)

Introduction

The Cisuralian Series was established for the Lower Permian when the present consensus on the Global Permian Scale was first proposed (Jin et al., 1995). The southern Ural Mountain region is the type area, comprising the Asselian, Sakmarian and Artinskian stages. The Orenburgian (uppemost Carboniferous), equivalent of Upper Gzhelian (sensu lato), was also established in the southern Urals. These stages initially were defined and widely recognized on ammonoid phylogenies (Karpinsky, 1889; Ruzhencev, 1937, 1956), however, stage boundaries and subdivisions were established on the basis of fusulinaceans, the most abundant and best studied upper Paleozoic fossil group of the southern Urals. Body stratotypes for all stages were located in the southern Urals (Bashkortostan and Orenburgian province of Russian) and it is expected that the boundary stratotypes (GSSP) for all Cisuralian stages will be formally established in the southern Urals.

History and current status

Asselian as a stage was established by Ruzhenzev in 1954; previoulsy it was considered a horizon (Ruzhenzev, 1937), and subsequently a substage (Ruzhenzev, 1950) of the Sakmarian (s.l.). The Asselian body stratotype (Assel' section) is located on the ridge between Assel' and Uskalyk rivers in southern Bashkortostan. Ruzhenzev distinguished the Asselian by particular ammonoid assemblage which in the Urals corresponds with entire range of the fusulinid Sphaeroschwagerina. The Global Stratotype Section and Point (GSSP) for the base of the Permian and basal Cisuralian Asselian Stage was proposed recently (Davydov et al. 1995) and has been ratified by the International Union of Geological Sciences (lUGS) at Aidaralash Creek, Aktkeibe (formerly Aktyubinsk) region, northern Kazakhstan. It is defined by the first occurrence of the conodont Streptognahtodus isolatus in the S. wabaunsensis chronocline. This level closely approximates the traditional boundary definitions based on ammonoid cephalopods and fusulinacean foraminifers.

The term Sakmarian (as a horizon) was proposed by Frederiks (1918) for deposits with a characteristic southern Uralian ammonoid fauna called by Karpinsky as the “lower belt of Artinskian of Sakmara river” (Karpinsky, 1874). The Sakmarian stage (s.l.) was established by Ruzhenzev (1936) and included the current Asselian and Sakmarian stages. Sakmarian (s. s.) in its present context was proposed by Ruzhenzev in 1955 with its stratotype at the Kondurovka section, (Karamuruntau Ridge, southern Orenburg Province) and based on ammonoid and fusulinid assemblages. Because there where no ammonoids known in the lower portion of Sakmarian, the base of this stage defined by first appearance species of Schwagerina moelleri (“Pseudofusulina” moelleri in Russian nomeclature). Because fusulinid species are provincial, this definition is effective only in the Boreal and in the western Tethyan realms. Conodonts, studied in Sakmarian of the Urals more recently (Chemykh & Reshetkova, 1987; Chuvashov et al., 1993), represent potential tools for a global definition of the base of Sakmarian and establishment of the GSSP. Currently conodont workers suggest a definition for the base of Sakmarian by the first appearance of Streptognathodus barskovi (s.str. = St. postfusus), perhaps lowering the boundary as down as middle Asselian. Also, taxonomic problems with streptognathoid conodonts need to be solved.

The Artinskian Stage as originally proposed by Karpinsky (1874) included all the Upper Paleozoic clastic deposits of the Preuralian Foredeep overlying Carboniferous carbonates (Artinskian Sandstone by Murchison, 1845). Artinskian Stage in current context as established by Ruzhenzev (1954) was characterized by ammonoids and fusulinids. The body Artinskian stratotype is in a series of sandstone quarries on Kashkabash Mount near the Arti village on right bank of Ufa river, which actually corresponds only with the upper portion of Artinskian. Most of the characteristics concerning the Artinskian are derived from investigation of clasticrich sections and ammonoids phylogenies (Ruzhenzev, 1954) from the Preuralian Foredeep in the southern Urals and from carbonate sections and fusulinid phyiogenies from the eastern margin of the Russian Platform (Rauser-Chemousova, 1949). Fusulinids and ammonoids from the Urals are provincial at the species and can not be used for GSSP definition. Conodonts may be more appropriate, but their distribution in the Urals is not clear. There are two conodont biofacies in the Urals: deep water gondolellid biofacies and shallow water neostreptognathid biofacies. The first is widely distributed, but correspondence with fusulinid succession remains uncertain. Distribution of the latter is poorly known.

The Kungurian Stage was proposed by Stuckenberg in 1890. The body stratotype consists of several small outcrops of limestones along the tributary of Ufa river to the south-east of Krasnoufims. The biostratigraphy of Kungurian is very poorly known. Fusulinids, ammonoids and conodonts are mostly presented by Artinskian species and became nearly extinct in the Urals during that time. The Kungurian regional biostratigraphy in the Urals is based on very provincial small forums, brachiopods, ostracods and pelecypods. Peculiarities of Kungurian paleogeography in the Urals make difficulties for the definition of the base of Kungurian unresolvable.

Sedimentology and Sequence Stratigraphy along Aidaralash Creek

Upper Paleozoic strata of Aidaralash Creek were deposited on a narrow, but persistent, shallow marine shelf that formed the western boundary of the orogenic highlands. The sequence stratigraphy reflects repeated shelf to fluvial-deltaic cycles. Our general mapping and detailed measurement of stratigraphic sections build on the pioneering work of Ruzhencev (1951, 1952, 1956) and Khvorova (1961). Only a summary is presented here.

Permo-Carboniferous strata exposed along Aidaralash Creek (Fig. 1) can be described using the facies summarized in Table 1. We interpret the mudstone-siltstone (MS) facies, containing abundant fine plant debris, as the background, offshore sediment; this facies reflects the abundant terrigenous input from deltaic sources. Sandstone facies SS1, SS2, SS3 represent event beds. Some beds that display simple grading with tippled tops may represent unreworked sediment gravity flows. However, most of these sandstone beds are horizontally stratified, amalgamated beds 5 to 30 cm thick; the stratification is defined by 0.5 to 5 cm thick graded intervals and many of the beds have tippled tops. These horizontally stratified sandstone beds are interpreted as storm deposits or as wave- and storm-reworked sediment gravity flow deposits. Palcocurrent data suggest that a large and persistent delta complex developed to the northeast which supplied sand that was periodically transported southwestwardly to the Aidaralash area as sediment gravity and storm deposits. The preservation of storm beds and hummocky cross stratification suggests that much of the section was deposited below fair weather wave base. The coarse grain size and sedimentary structures of SS4 gravelite beds suggest mass gravity flow deposition and reworking of the upper portions of the beds by waves and strong currents. The SS1 through SS4 sandstone beds occur in thickening upward cycles with or without discernable coarsening. Fusulinaceans tend to occur near the base of SS3 and SS4 sandstone beds. The interbedded conglomerate and sandstone beds of CG1 are typically poorly cemented and occur as 10 to 100 m thick successions. Khvorova (1961) divided the conglomerate into four types primarily on age and clast composition; the CG1 facies encompasses Khvorova's type 1 (Artinskian) and type 2 (Asselian) conglomerates; type 2 and 3 are mainly Carboniferous and do not occur within the Aidaralash Creek section.

Conglomerate beds mark the most obvious Aidaralash sequence boundaries (sequence boundaries IV through X, Figure 2), The conglomerate successions occur as thin but laterally extensive deposits, typically 10-50 m thick, 10-30 km long on strike, and extending to the eastern edge of the Pre-Uralian Foredeep. These fluvial-deltaic conglomerate-sandstone successions grade into transgressive marginal marine sequences (beach and or upper shoreface), to a maximum flooding unit (typically with ammonoids, conodonts, and radiolarians). This gradation usually occurs within a stratigraphic thickness of a few to 10 meters. The maximum flooding zone is overlain by a regressive sequence (offshore to shoreface to delta front), which in turn is capped by an unconformity with the overlying conglomerate. The 5-15m thick SS3 sandstone at beds B22, B25, and B29 (Figure 2) reflect relative shallowing to above fair weather wave base but show no evidence of subaerial exposure. These sands have been interpreted as possible sequence boundaries I, II and III in Figure 2, but it is unclear whether these reflect true eustatic sea level lowstand deposits, autogenic fluctuations in sediment supply, or perhaps, merely decreased tectonic subsidence rates. It is interesting to note that there is no indication of a sequence boundary within 200 meters stratigraphically above and below the Carboniferous-Permian boundary at the GSSP.

Fusulinacean biostratigraphy in the Southern Urals

The southern Pre-Uralian Foredeep, paleogeographically situated between Boreal and Tethyan provinces, comprises taxa of Boreal and Tethyan affinities and thus is a key for resolving problems of upper Paleozoic global correlation. Fusulinacean biostratigraphy historically has employed the biozone concept. Fusulinacean zones utilized in the study of the Russian Platform and of the Urals were established in southern Ural sections - however, without reference to any particular section. In light of this, an opportunity exists to reexamine Lower Permian fusulinacean zones to help develop the GSSP definitions for the Cisuralian stages.

The evolutionary framework for most late Paleozoic fusulinacean lineages of the southern Urals is well established. Fusulinacean zones we utilize for the southern Urals are based on evolutionary successions and are defined by assemblages of species (Fig 2). The base of each zone is established by the advent of significant evolutionary changes in several lineages. The top is defined by the base of the next zone. Each fusulinacean zone is characterized by 30-70 species, and 5-15 genera.

The fusulinacean zones from any single section reflect the character of local, regional, provincial, and global environmental changes. Occurrence or absence in a section or even a region may differ from true first appearance and extinction. The occurrence of a species in a section or region may be influenced by local environmental factors.

However, origination and extinction of species are generally influenced to a greater degree by global rather than by local forces. The study of phylogenetic successions can be used to help distinguish between local occurrences and true speciation. The southern Urals fusulinid zonation is based on phylogenies, it thus reflects true speciation and extinction events.

The correlation of southern Ural zones with sequence boundaries for the Aidaralash Creek section is shown in Figure 2. Most sequence boundaries coincide with the bases of fusulinid zones. We can suggest that sequences I, II, III, IV, VII, VIII and IX are eustatic. Sequence boundaries V and VI, located within fusulinid zones, perhaps reflect local tectonism or local climatic changes; conversely the fusulinid phylogenies for those particular zones may be still poorly understood or poorly documented. It is interesting to note, however, that some important stage boundaries, such as the base of Asselian and the base of Sakmarian, do not coincide with recognized sequence boundaries. We can suggest from this that fusulinid speciation appears to be associated with both highstands and lowstands. Sea level lowstands may have been very stressful for fusulinid assemblages and may have been a catalyst for both speciation and extinction. Highstands also may have created environmental opportunities and appear to be more closely associated with fusulinid speciation than extinction.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah