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Abstract

The Ruyang (Mesoproterozoic; Pt2) and Luoyu (Neoproterozoic; Pt3) groups in the southern North China Platform are dominated by peritidal siliciclastic rocks and contain abundant microbially induced sedimentary structures (MISS), including multidirected and mat-protected ripple marks, mat chips, mat-smoothed ripple marks, various sand cracks, and irregular growth ridges. These MISS can be grouped into four MISS associations, each of which has distinctive MISS morphology resulting from changes in depositional environments. The upper subtidal to lower intertidal zone commonly lacks in situ MISS but contains redeposited mat chips. The upper intertidal zone is characterized by mat-protected ripple marks and mat chips. The lower supratidal zone is rich in various MISS, especially the large and morphologically complex sand cracks indicative of growth of thick mats in microenvironments with low hydrodynamics, sufficient moisture level, and frequently exposed substrates. The upper supratidal zone abounds with small sand cracks formed from relatively thin microbial mats. The morphological associations from tidal-flat deposits of the Ruyang and Luoyu groups are similar to those found in modern siliciclastic coastal environments. Similarities in MISS between modern and Proterozoic tidal flats suggest that the MISS morphological associations can be used for paleogeographical and paleoenvironmental reconstructions, especially in Precambrian siliciclastic successions.

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