T255 Lawrence A. Hardie Commemorative Session II

Tuesday, 3 November 2015
1:30 PM - 5:30 PM
Room 319/320

THE PRECAMBRIAN WUMISHAN CYCLOTHEMS OF CHINA: A 60+ MILLION YEAR LONG PERITIDAL CARBONATE RECORD OF MULTIPLE-SCALE SEA LEVEL OSCILLATIONS, STROMATOLITE EVOLUTION AND AN ARAGONITE-CALCITE SEA TRANSITION

Paper number: T255-8
Program: Technical Sessions
Time: 3:20 PM – 3:35 PM

Authors

Abstract

The Wumishan Formation was deposited in the North China sector of the Nuna Supercontinent during the Mesoproterozoic Era. In exposures near Tianjin, North China the formation is characterized by peritidal carbonate cyclothems, in beds averaging 3.8 m. Its impressive size–a 3.3 km succession of 626 cyclothems deposited over 60+ myr–qualifies the middle to upper parts of the Wumishan Formation as one of the greatest cyclothem sequences of all time.

The cyclothems are marked by: (1) subtidal stromatolite biostromes, (2) subtidal thrombolite-oncolite bioherms, (3) tidal-flat dolomites, and (4) lagoonal-supratidal dolomitic shales. Within a cyclothem (1) and (2) constitute the central part, (3) the lower and upper parts, and (4) the topmost part, indicating initial deepening, then shallowing of the depositional environment. Predominant 4:1 bundling suggests that cyclothems were forced by Earth’s ~100 kyr orbital eccentricity cycles, with 4:1 bundling reflecting the 405 kyr eccentricity cycle. A lithofacies rank series highlights quasi-periodic evolution consistent with this interpretation; fluctuations within cyclothems may record precession and/or obliquity-forced variations.

A Fischer plot of the cyclothems confirms prevalent 4:1 bundling, and reveals 35:1 and 155:1 bundling representing 3.5 myr and 15.5 myr cycles. Redbeds midway through the formation marks a fundamental change, with stronger 35:1 bundling above the redbeds. An incomplete bundle projecting to 1000:1, or a 100 myr cycle, grounds the formation at Wilson cycle scale.

The Wumishan captures a key event in stromatolite evolution. Cm-scale, cemented Mopanyu stromatolites thought to be aragonite precipitates developed on the sea floor, undergo decline and eventual “extinction” just below the redbeds. Successor m-scale, uncemented Shanpoling stromatolites developed in low to moderate-energy subtidal environments. This change in microbial carbonate accumulation coincides with an aragonite-calcite sea transition predicted by L.A. Hardie.