SINUSOIDAL COLUMNAR STROMATOLITES FROM THE MESOPROTEROZOIC OF NORTHERN CHINA: ORIGIN AND SIGNIFICANCE

~1.4 Ga sinusoidal stromatolites in the Tieling Formation, near Jixian in North China, form a thin (<1 m) interbed within a thick succession of branched vertical columns. Both sinusoidal and vertical columns, together with the intervening matrix, appear originally to have been mainly composed of carbonate mud and formed extensive aligned systems of closely spaced ridges, separated by narrow runnels that show signs of scouring. The sinusoidal columns are unbranched. Initially they grew vertically, gradually curved steeply sideways, and then returned to vertical before curving equally steeply in the opposite direction. The laminae progressively rotate, maintaining orientation approximately at right-angles to the column axes. As a result, individual laminae faced into currents at up to forty-five degrees or more. Adjacent columns show similar shapes, bending back and forth together. Some are deformed and adjacent laminae are pulled apart. Locally columns are broken, and lie between undisturbed columns. However, lamina arrangement indicates that the sinusoidal shape developed as the columns accreted and was not produced by deformation.

We interpret both the vertical branched and the sinusoidal columns as agglutinated fine-grained stromatolites formed by uncalcified mats that trapped and stabilized carbonate mud. Column morphology reflects dynamic interaction between mat growth and sediment supply. Fluctuations in mat expansion and burial resulted in column expansion, termination and branching. Heliotropic influence is unlikely since the sinusoidal columns form a thin band within a succession of vertical ridged columns, and their curvature is locally angular rather than sine-form. The sinusoidal columns appear to have developed in response to variable current directions due to shoaling. These conditions, and possibly they shape, made them prone to damage by episodic storm scour of adjacent runnels that removed sufficient supporting matrix to break and topple some columns. Similar sinusoidal columns occur in coeval carbonates in South Korea and the Belt-Purcell Supergroup of Laurentia.