"Rock of the Month # 219, posted for September 2019" ---

A quick scan of the literature indicates that "the Jixian" is a mid- Proterozoic time slice, named for a locality called Jixian near Tianjin, the port city on the east side of Beijing. So, in China and perhaps beyond, Pt₂Jx means mid-Proterozoic era, Jixian stage. The Museum at CUGB also has a nice stromatolite (layered colonial fossil structure, of algal origin) from Jixian, and the National Geological Museum has a late Proterozoic fossil called Scopulimorpha regularis from Jixian. The Jixian locality was apparently suggested and then withdrawn as a possible world heritage site for Proterozoic stratigraphy, back in 1987. Jixian time (not part of the official world time chart by the International Commission on Stratigraphy, as of 2018/08) is 1600-1400 Ma: rocks forming a Jixian Group were deposited in this time interval. The ICS chart gives that period as the basal Mesoproterozoic and labels it the "Calymmian".

Here (Fig. 1) we see a banded carbonate sediment, a dolostone (i.e., the principal carbonate mineral in the rock is dolomite, as opposed to calcite in a typical limestone). Note the layers (Figs. 1-2) in the samples, and in particular those curious layers that appear to be composed mainly of small (roughly 5-10 mm) sheets and wisps. These small flat sheets often appear to be layered, each one subparallel to the next, laid down in an imbricate fashion (i.e., like tiles on a roof). These flat sheets are probably shredded fragments ("rip-up clasts") of microbial mats (or algal mats), layers of very fine-grained sediment composed of alternate layers of fine silt and organic (microbial or filamentous algal) matter.

Why are the mats shredded into these little pieces? By analogy with modern sediments forming in the Earth's seas today, this dolostone probably accumulated in a shallow, equatorial to subtropical sea, comparable to the coasts of modern Florida or Saudi Arabia. The subtropical latitudes, such as the modern Caribbean, are prone to severe summer storms such as hurricanes. Unconsolidated sediments in shallow waters, down to the effective wavebase, can be torn up in such storms (tempests), and then the fragments will be redeposited when the water calms down after the storm. This is why such sediments are referred to as tempestites. Sedimentary structures are reviewed in the concise field guide by Tucker (2011).

Work in Jixian county (now also termed the Jizhou district), at the northern end of Tianjin municipality, circa 100 km east of Beijing, led to discovery of a Mesoproterozoic fossil biota, in thin sections of carbonaceous black chert (Schopf et al., 1984). One community is in bedded, flat-laminated stromatolitic chert while the other is in silicified conical stromatolites (two forms of Conophyton). In addition to such cherts, chemical sediments also formed around submarine hydrothermal vents (fumaroles), some of which are preserved in jaspilite (Li and Lu, 1996). Jixian strata (Chen et al., 2016) are widespread around the Ordos block, extending well to the west and southwest of Beijing. The Jixian strata include platform dolostone and siliceous (clastic) sediments. The sedimentary environments of platform edge facies, shelf facies, and slope facies are respectively marked by 1) intraclastic dolostones, algae and stromatolites (as seen at the National Geological Museum in Beijing), 2) dolomites, nodular chert and fan-shaped tempestite breccias, and 3) carbonate debris flow sediments.

Figs. 3-4: Part of a metre-scale block of decorative stone, most probably also from northern Tianjin municipality and, below, a close-up showing microbial mat fragments (the 0.5-yuan coin is 20 mm in diameter). Rectangular, rough-hewn blocks of this attractive grey landscaping stone, typically on the order of 50 cm thick, are used to line paths within the ruins of Yuanmingyuan, the Old Summer Palace, of late Qing (i.e., 19th century) date, in northwestern Beijing.

This class of sediments seems exotic, compared to the much larger volumes of "humdrum", gently-deposited limestones and sandstones preserved in the geological record worldwide. Here are more examples from other countries. A detailed study of Silurian strata on Anticosti Island, Quebec, Canada, describes fining-upwards storm units (tempestites), 5-80 cm thick, which consist of a sharp, scoured base grading up (in "sediment-speak") from intraclastic / bioclastic rudstone to medium-grained grainstone, finely laminated calcisiltite and mudstone, and shale. These tempestites are interbedded with low-energy, fairweather mudstones and calcareous shales (Sami and Desrochers, 1992). Such deposits can give an indication of the climate in times past (Long, 2007). In the Upper Ordovician of southern Ontario, there is a recessive, and so indifferently- exposed, shale sequence deposited under storm influence. The lower member of this Whitby Formation includes dark grey shale with black kerogen flakes aligned parallel to bedding, and tempestite with rounded phosphatized pelmicrite intraclasts in dark grey shale, shell lags of brachiopod valves, and medium-grey shale with fining-upwards silty laminations (Ludvigsen and Tuffnell, 1994). Tempestites are described also in the early Tertiary sequences of northern India (e.g., Singh and Srivastava, 2011; Bhatia et al., 2013).

References

Bhatia,SB, Bhargava,ON,Singh,BP and Bagi,H (2013) Sequence stratigraphic framework of the Paleogene succession of the Himalayan foreland basin: a case study from the Shimla Hills. J.Palaeontological Society of India 58, 21-38.

Chen,Y, Fu,X, Xiao,A, Yu,L, Tang,Y and Mao,L (2016) Type and evolution of carbonate platforms in Jixian period Mesoproterozoic: southwestern margin of Ordos Basin. J.Petrol.Explor.Prod.Technol. 6, 555-568.

Li,H and Lu,S (1996) Geology and geochemistry of the hydrothermal jaspilite in the Mesoproterozoic Tieling Formation, Jixian, China. Proc. IGC 30, vol.17, 311-323, China University of Geosciences - Beijing.

Long,DGF (2007) Tempestite frequency curves: a key to late Ordovician and early Silurian subsidence, sea-level change, and orbital forcing in the Anticosti foreland basin, Quebec, Canada. Can.J.Earth Sci. 44, 413-431.

Ludvigsen,R and Tuffnell,PA (1994) The last olenacean trilobite: Triarthrus in the Whitby Formation (upper Ordovician) of southern Ontario. New York State Museum Geol.Surv.Bull. 481, 380pp. 183-212.

Sami,T and Desrochers,A (1992) Episodic sedimentation on an early Silurian, storm-dominated carbonate ramp, Becscie and Merrimack formations, Anticosti Island, Canada. Sedimentology 39, 355-381.

Schopf,JW, Zhu,W-Q, Xu,Z-L and Hsu,J (1984) Proterozoic stromatolitic microbiotas of the 1400-1500 Ma-old Gaoyuzhuang formation near Jixian, northern China. Precambrian Research 24, 335-349.

Tucker,ME (2011) Sedimentary Rocks in the Field: a Practical Guide. Wiley-Blackwell, 4th edition, 276pp.