Characteristics and primary mineralogy of fibrous marine dolomite cements in the end-Ediacaran Dengying Formation, South China: Implications for aragonite–dolomite seas

Abstract

Fibrous marine cements are widely distributed in the terminal Ediacaran Dengying Formation of South China and are an important record for reconstructing paleoseawater chemistry. However, the morphological characteristics and filling sequence of these cements, and their formation mechanism and paleoceanographic significance, are poorly understood. Our investigation of five outcrop sections in the northern upper Yangtze region allows four types of fibrous dolomite cement to be identified: fascicular fast dolomite (FFD), radial fast dolomite (RFD), radial slow dolomite (RSD), and fascicular slow dolomite (FSD), where “fast” and “slow” refer to the optical character of cement crystals. FFD cements possess original aragonitic mineralogy, whereas RFD cements had high-Mg calcitic precursors that subsequently underwent mimetic dolomitization via syntaxial replacement. RSDs and FSDs are primary marine precipitates, whereas fibrous dolomite cements with length-fast optical characteristics (FFDs and RFDs) are of secondary origin. These fibrous cements are widely distributed laterally in the Dengying Formation, although the RSDs and FSDs occur only in the second member of this formation. Therefore, the seawater chemistry of the terminal Ediacaran period in the upper Yangtze area was dominated by an aragonite sea, with a regional short-lived “dolomite sea” during the late depositional stage of the second member of the Dengying Formation, from which dolomite may have directly precipitated.