Secular changes in sedimentation systems and sequence stratigraphy

Abstract

The ephemeral nature of most sedimentation processes and the fragmentary character of the sedimentary record are of first-order importance. Despite a basic uniformity of external controls on sedimentation resulting inmarkedly similar lithologies, facies, facies associations and depositional elementswithin the rock record across time, there are a number of secular changes, particularly in rates and intensities of processes that resulted in contrasts between preserved Precambrian and Phanerozoic successions. Secular change encompassed (1) variations in mantle heat, rates of plate drift and of continental crustal growth, the gravitational effects of theMoon, and in rates ofweathering, erosion, transport, deposition and diagenesis; (2) a decreasing planetary rotation rate over time; (3) no vegetation in the Precambrian, but prolific microbial mats, with the opposite pertaining to the Phanerozoic; (4) the long-term evolution of the hydrosphere–atmosphere–biosphere system. A relatively abrupt and sharp turning point was reached in the Neoarchaean, with spikes inmantle plumeflux and tectonothermal activity and possibly concomitant onset of the supercontinent cycle. Substantial and irreversible change occurred subsequently in the Palaeoproterozoic, whereby the dramatic change from reducing to oxidizing volcanic gases ushered in change to an oxic environment, to be followed at ca. 2.4–2.3 Ga by the “Great Oxidation Event” (GOE); rise in atmospheric oxygen was accompanied by expansion of oxygenic photosynthesis in the cyanobacteria. A possible global tectono-thermal “slowdown” from ca. 2.45–2.2 Ga may have separated a preceding plate regime which interacted with a higher energymantle from a ca. 2.2–2.0 Ga Phanerozoic-style plate tectonic regime; the “slowdown” period also encompassed the first known global-scale glaciation and overlapped with the GOE.While large palaeodeserts emerged from ca. 2.0–1.8 Ga, possibly associatedwith the evolution of the supercontinent cycle, widespread euxinia by ca. 1.85 Ga ushered in the “boring billion” year period. A second time of significant and irreversible change, in the Neoproterozoic, saw a secondmajor oxidation event and several lowpalaeolatitude Cryogenian (740–630 Ma) glaciations. With the veracity of the “Snowball Earth”model for Neoproterozoic glaciation being under dispute, genesis of Pre-Ediacaran low-palaeolatitude glaciation remains enigmatic. Ediacaran (635–542 Ma) glaciation with a wide palaeolatitudinal range contrasts with the circum-polar nature of Phanerozoic glaciation. The observed change from low latitude to circum-polar glaciation parallels advent and diversification of the Metazoa and the Neoproterozoic oxygenation (ca. 580 Ma), andwas succeeded by the Ediacaran–Cambrian transition which ushered in biomineralization, with all its implications for the chemical sedimentary record.