پديد آورندگان :
بايت گل ، يارام نويسنده دانشجوي دكتري، زمينشناسي دانشگاه فردوسي مشهد , , محبوبي، اسدالله نويسنده دانشكده علوم- دانشگاه فردوسي مشهد Mahboubi, A , موسويحرمي، رضا نويسنده Moussavi Harami, Reza
چكيده لاتين :
Introduction
High-frequency shallow-water cycles are the basic building blocks of shallow-marine carbonate successions characterized by a hierarchy of stratigraphic cyclicity (e.g., Goldhammer et al., 1990, Montanez and Osleger, 1993; B?denas et al., 2010; Bayet-Goll et al., 2014). Based on this study, microbial mats have verity of morphology and type of growth structure and including laminar to wavy-laminar, domal or hemispheroidal, bulbous, columnar, regular ?abellate columns, unlaminated, loaf- to mound-shaped thrombolities. Facies associations of Deh-Sufiyan Formation are arranged in small-scale of peritidal, shallow subtidal, and deep subtidal cycles and microbial mats are the major features of them. The trends of vertical changes of facies in shallowing-upward and deepening-upward cycles and distribution of various types of microbialites in these cycles had been related to depositional environments and their positions on carbonate ramp. Basal classification method used in this study can provide valuable information for application of microbial mats in paleo-environmental and sequence stratigraphy analysis.
. Material & methods
The Palaeozoic succession in the central Alborz has been divided in ascending order into the Zaigoon, Lalun, Mila, and Lashkarak formations. Based on studies of Geyer et al. (2014) and Peng at al. (1999), the Mila Formation as traditionally applied shows numerous inconsistencies in definition and physical properties which require a complete lithostratigraphic revision. A new lithostratigraphic scheme for the Cambrian–Ordovician strata of the Alborz by Geyer et al (2014) suggested retaining the term “Mila Group” for the lithostratigraphic unit that comprises the Fasham Formation (Base Quartzite), the Deh-Sufiyan Formation (members 1 to 2), the Deh-Molla Formation (members 3 to 4) and the Lashkerak Formation (member 5). Four stratigraphic sections of the Deh-Sufiyan Formation are described and interpreted in order to usage of microbial mats in depositional environment interpretation and sea level changes. These are (from west to east) Shahmirzad, Tuyeh Darvar, Mila Kuhe, and Deh Molla sections. All sections were lithologically logged bed-by-bed according to color, grain-size, sedimentary and biogenic structures, geometry and fossil contents. This study allowed us to propose a palaeoenvironmental model based on Wilson (1975) and Flügel (2010).
This research also developed a practical formative-based approach for the use of microbial mats in depositional environment interpretation and sea level changes. It revealed that the trends of microbial shallow-water cycles, composition, verity of textures as well as various forms of microbialites can be used as a basis for interpretation of other shallow-water cycles occur in the Early Paleozoic successions.
Discussion, results and conclusion
Four depositional zones are recognized on the Deh-Sufiyan Ramp including basin, outer ramp (deep subtidal), mid ramp (shallow subtidal to lower intertidal), and inner ramp (shoal and upper intertidal to supratidal). Their facies associations are arranged in small-scale cycles, i.e. peritidal, shallow subtidal, and deep subtidal cycles. These cycles show spatial differences in the reaction of the depositional system to small-scale relative sea-level changes. Small-scale cycles are stacked into medium-scale cycles that in turn are building blocks of large-scale cycles. Systematic changes in stacking pattern (cycle thickness, cycle type, and facies proportion) allow inferring long-term changes in sea-level. Six large-scale cycles have been identi?ed and interpreted as depositional sequences showing retrogradational (transgressive systems tract) and prograditional (highstand systems tract) packages of facies associations. Microbial shallow-water cycles are the most prominent feature of the carbonate successions of the Deh-Sufiyan Formation, Mila Group in Central Alborz. The regional distribution of these cycles indicate that they can occur within an individual depositional system based on different responses to a single sea-level event (B?denas et al., 2010; Bayet-Goll et al., 2014). Microbial cycles are arranged in small-scale of peritidal, shallow subtidal, and deep subtidal cycles.
Peritidal cycles are composed of supra-/intertidal and/or restricted shallow subtidal facies. Four sub-types have been recognized. Peritidal cycles are regressive (upward-shallowing) facies successions commencing with restricted subtidal-intertidal regime facies, and are capped by supratidal facies. They show a shallowing-upward trend generally beginning with massive dolomite, passing upward into microbial laminites and ending with facies characterized by exposure features such as desiccation polygons, pedogenic horizons, or fenestral dolomite. They are interpreted as the lateral progradation of tidal flats over the subtidal environment (e.g., Burgess et al. 2001). Three sub-types of Shallow subtidal cycles are recognized. The ?rst sub-type starts with a laminar to wavy-laminar microbial dolomite passing upward into columnar and domal stromatolites (similar to the previous cycles) and ends with fenestral microbial laminites characterized by exposure features. These cycles are indicative of progressive shallowing from shallow subtidal to supratidal environments due to filling of the accommodation by carbonate-sediment production or by autocyclic progradation of tidal cycles. The second sub-type begins with laminar to wavy-laminar microbial dolomite or thrombolite or calcareous shale to marl, passing upward into columnar and domal stromatolites with upward-increasing synoptic relief, occasionally capped by dolomitized oncoid grainstone. The cycle is interpreted as progradational from shallow subtidal to lower intertidal environments. These cycles record a progression from low-energy subtidal deposits below storm-wave base to agitated shallow subtidal to lower intertidal deposits near fair-weather wave base. The incomplete shallowing could be explained either by erosion due to the presence of storm currents and waves or a balance between accumulation and accommodation (B?denas et al. 2010) or accommodation increase fast in compared with sediment aggradation (Chen et al. 2001). The third sub-type represents symmetrical deepening–shallowing cycles with a basal oncolitic–intraclast dolograinstone–packstone passing upward into calcareous shale to marl facies that grades into massive dolomite, and occasionally capped by thrombolites and stromatolites. The deep subtidal cycles are asymmetrically deepening-upward generally beginning with coarse-grained, graded skeletal packstone/grainstone or laminar to wavy-laminar microbial and domal stromatolites passing upward into low-energy, offshore marl to shale or nodular lime mudstone. These cycles are interpreted as a deepening-upward cycle because of the progressive upward, retrogradational change from the high-energy facies to the low-energy facies.
Sedimentary trends and facies heterogeneities of the documented microbial high-frequency cycles indicate that the ultimate control on the microbial cycles appears to be ecstasy acting in concert with intrinsic process such as the lateral progradation of tidal flats over the subtidal carbonate factory, which closely related with high frequency sea-level fluctuations.
Keywords: Microbial mats, Microbial cycles, Mila Group, Deh-Sufiyan Formation.
