Unraveling overprinted formation mechanisms of massive dolostone in the Lower Triassic sequence of an isolated carbonate platform in Nanpanjiang Basin, south China

Massive dolomitization is common in carbonate platforms but determining the causes of dolomitization remains challenging. A particular difficulty lies in identifying cases where petrographic and geochemical attributes of dolostone related to one mechanism could be obscured by a later, different one. To better understand whether traditional approaches are sufficient to unravel the origins of dolostone resulting fromsuccessive, different mechanisms, this study investigates the formation mechanism(s) of dolostone along a platform-to-basin transect of a Permian-Triassic isolated platform in the Nanpanjiang Basin. The dolostone in the Lower Triassic succession comprises three dolomite phases that can be distinguished through field relationships, petrography, Sr-87/Sr-86 ratios, and microthermometry. Dolomite type 1 formed due to the reflux of platform-top evaporated seawater that flowed through the platform interior in the Early Triassic. Dolomite types 2 and 3 are interpreted to have formed at elevated temperatures during or after Late Triassic platform burial and to have played a secondary role in forming the dolostone. The dolomitizing fluids that resulted in the formation of dolomite types 2 and 3 were derived from Early Triassic seawater-like fluid that was expelled from the Lower Triassic basinal carbonate sediments and moved updip to the platform interior. Dolomitized clasts in partially or non-dolomitized slope breccias demonstrate pre-burial timing of dolomite type 1, and distinguish the earlier dolomitization from later, post-burial dolomitization represented by dolomite types 2 and 3. Dolomite type 1 retains its Early Triassic seawater delta C-13 and Sr-87/Sr-86 signatures, whereas overlapping geochemical fields of the three types of dolomite (trace element concentration, delta O-18) imply that burial dolomitizing fluids locally reset the geochemistry of dolomite type 1. This finding suggests that the same dolomite archive may retain well-preserved or altered data depending on the specific geochemical proxy and that identifying individual dolomitization mechanisms using geochemical proxies is possible only in some cases. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

This study investigates the formation mechanism(s) of dolostone in the Nanpanjiang Basin. It finds that dolostone formation consists of three phases, with the first phase formed by the reflux of platform-top evaporated seawater in the Early Triassic. The second and third phases of dolostone are interpreted to have formed at elevated temperatures during or after Late Triassic platform burial. The study also suggests that the same dolomite sample may retain well-preserved or altered data depending on the specific geochemical proxy used.