Cyclic tidal successions of the Middle Ordovician Maggol Formation in the Taebaeg area, Kangwondo, Korea

Abstract

Sedimentologic investigation of the Ordovician Maggol Formation in the Hyeoldong and Gumunso sections, Taebaeg area, has been carried out to delineate the depositional environment, lithostratigraphy, and cyclicity as well as to infer the paleoclimatic conditions under which the formation was deposited. Based on lithology, sedimentary structure, and fossil contents, the Maggol Formation can be divided into nine lithofacies and their corresponding depositional environments. These are 1) bioclastic/intraclastic/peloidal/oolitic packstone to grainstone (lower intertidal to subtidal); 2) biotubated limestone (subtidal); 3) lime mudstone with bioturbation and microbial lamination (lower intertidal); 4) ribbon rock (intertidal); 5) lime mudstone with microbial lamination (upper intertidal to supratidal); 6) stromatolitic limestone (shallow subtidal to lower intertidal); 7) calcareous shale (upper intertidal to supratidal); 8) fine laminated dolomite (upper intertidal to supratidal); and 9) flat pebble conglomerate (subtidal to supratidal). The lithofacies suggests that the Maggol Formation was deposited in a tidal flat environment, which shows repetitive cyclical, tidal successions. Based on the successive nature of nine lithofacies, 28 facies associations can be observed. All of the tidal successions in this study show shallowing-upward sequences, and at least 20 successions from the Gumunso area and 71 successions from the Hyeoldong area can be recognized in the outcrop. Stratigraphically, the Maggol Formation can be divided into three members based on lithology and lithofacies in this study, and this can be applied at least in the southern limb of the Baegunsan Syncline. The lower member is characterized by calcareous shale, bioturbated and/or laminated limestone, ribbon rock, and flat pebble conglomerate. The middle member is composed of about 30 m-thick, grey massive dolomite. The upper member contains bioclastic/peloidal packstone to grainstone, biotubated and/or laminated limestone, fine laminated dolomite, and solution-collapse breccia. The presence of evaporite mineral casts as well as related sedimentary structures suggests that the upper member might have been deposited in a sabkha-type tidal flat under arid climate whereas the lower member in a Bahamian-type tidal flat under humid climate.