Facies and depositional environment of the Upper Cambrian of the central Alborz, members 3 and 4 of the Mila Formation (Deh-Molla Formation), with emphasis on their ribbon rocks

Abstract

AbstractThe Late Cambrian was associated with tremendous changes in world oceans and lands including the deposition of specific facies in different parts of the world known as “ribbon rocks” due to their thinly bedded undulatory banded nature. In this study, the Upper Cambrian succession of the central Alborz (members 3 and 4 of the Mila Formation) is sedimentologically investigated with an emphasis on its upper part’s ribbon rocks. The study tries to give an understanding of facies analysis and depositional environment of this succession. In this regard, the strata are divided into nine lithostratigraphic units and 12 microfacies representing three sub-environments of supra- inter-and sub-tidal. The results show that the microfacies were deposited in an epeiric platform with marked changes in the facies distribution and platform physiography in time so that the evolution of this platform can be divided into three stages. During the first stage, dominantly tidal flat facies were deposited in broad belts without significant effects from disturbing agents such as storms. In the second stage, the platform was drowned due to a rapid transgression where deep-marine glauconite-rich green shale was deposited on former platform facies. In the third stage, the environment was strongly affected by disturbing factors such as storms and tides where severe facies changes occurred, represented by the deposition of ribbon rocks. The ribbon rocks of the Deh-Molla Formation show a spectrum of subtidal to intertidal-supratidal depositional settings.Keywords: Upper Cambrian, Mila Formation, Deh-Molla Formation, Ribbon rocks, Depositional environment IntroductionThe Upper Cambrian rocks of the Alborz zone are known as the members 3 and 4 of the Mila Formation, newly suggested to be regarded as a new formation, that is, the Deh-Molla Formation (Geyer et al. 2014). The Deh-Molla Formation can be subdivided into two informal members of 1 and 2. Member 1 is composed of thick-bedded to massive carbonates without significant variations in bedding nature in lateral or vertical directions. Member 2, in contrast, is dominated by thin-bedded ribbon rocks, pointing to significant environmental changes. This difference in the lithology, bedding nature and facies distribution of these two members provides a potential research subject. The current study aims to investigate sedimentologic aspects, facies analysis and depositional environment of the Deh-Molla Formation in the central Alborz with an emphasis on its ribbon rocks. Materials & MethodsThree stratigraphic sections were chosen for this study to encompass all spatio-lateral variations of the formation. These include the Shahmirzad, Tuyeh-Darvar and Deh-Molla sections. During the fieldwork, the sections were measured, logged and sampled carefully with regard to their lithology, bedding geometry and architecture, fossil content, stratal surfaces, sedimentary structures and depositional trends. In total, 257 rock samples were collected from these sections, which were petrographically investigated in the laboratory. The samples were classified according to standard widely-used nomenclatures (Dunham 1962; Embry and Klovan 1971) and were grouped into facies and facies association after comparison with standard facies models as well as with results of coeval studies. Discussion of Results & ConclusionsTwelve microfacies are identified in the studied rocks indicating three facies associations/sub-environments of supratidal, intertidal and subtidal. These facies association and related microfacies include: i) supratidal facies association: MF-1: Dolomudstone; MF-2: Ooid dolograinstone–dolopackstone; ii) intertidal facies association: MF-3: Rounded echinoderm ossicle grainstone; MF-4: Brachiopod packstone or rudstone; MF-5: Oncoid rudstone-grainstone; MF-6: Microbial mudstone; MF-7: Bioclastic packstone-grainstone; and iii) lower intertidal–subtidal facies association: MF-8: Intraclast grainstone-packstone; MF-9: Microbial-lithistid framestone; MF-10: Bioclastic wackestone-bindstone; MF-11: Bioclastic packstone-grainstone; MF-12: Green shale-marl.The domination of peri-tidal facies in the Deh-Molla Formation suggests that they were probably deposited in a gently sloping broad carbonate platform (most likely in an epeiric platform) (Pratt and James 1986). The distribution of the recognized facies in space and time indicates that the environment significantly changed during the deposition of the formation so that the evolution of the platform through time can be considered as a three-stage scenario:Stage 1 includes Member 1 of the studied formation. The sediment was deposited in a gently sloping carbonate platform under calm and arid environmental conditions. The supratidal region was the site of evaporitic processes leading to the deposition of a thick dolomite unit (MF-1). The coastline and adjacent area were occupied by dunes and smaller bedforms of MF-2 to MF-5, pointing to the constant action of surfs and waves, which led to the accumulation of shells and grain-dominated shoal and banks. The rest of the intertidal together with the shallower sub-tidal area was the best region for the development of microbial-dominated structures such as microbial-sponge small buildups and blue-green algal biostromes (MF-9). Sheltered areas between these bioherms were places with a superior accumulation of mud accompanied by debris derived from these buildups (MF-10).Stage 2 is characterized by the deposition of green deeper-marine marl and shales. The unit marks the base of the Member 2 and the drowning phase of the carbonate platform during which deeper subtidal facies (MF-12) accumulated over previous shallower marine carbonates.Stage 3 includes the uppermost part of the formation that is characterized by ribbon strata. These strata have recorded significant environmental perturbations during their deposition represented by the development of thinly bedded rocks and multiple tempestite horizons. During this stage, the platform was characterized by numerous shoals, banks and bedforms mostly with a medium- to small-scale extension, which formed due to repeated action of tides, waves and storms. The alternation of calm and stormy conditions led to the development of couplets of skeletal- (MF-7) and microbial- (MF-6) dominated facies in intertidal settings. Many sedimentary structures and rock aspects are recorded in these strata, pointing to a combined action of tides and storms. These include wavy, lenticular and flaser cross-stratification, graded bedding, shell imbrications, flat-pebble conglomerates, multiple erosional surfaces, ripple cross stratifications, and etc.This study reveals that the ribbon rocks are mainly composed of five microfacies, including MF-6–MF-8 and MF-10–MF-11 associated with interbeds of shale and marls. The facies show a spectrum of environmental conditions from subtidal to intertidal-supratidal. Therefore, these banded thin-bedded rocks were deposited in a broad platform with a gentle slope characterized by recurring dynamic depositional processes such as storms and tides shaping the physiography and morphodynamic of the environment. These processes led to the development of numerous bedforms along the platform. Many similar studies also indicate a peritidal origin for these rocks (e.g., Demicco and Mitchell 1982; Demicco 1983; Cañas 1999; Laughrey and Harper 2012).

Keywords

• upper cambrian
• mila formation
• deh-molla formation
• ribbon rocks
• depositional environment