7 - Sedimentation and stratigraphy

Summary

Make mountains level, and the continent,

Weary of solid firmness, melt itself

Into the sea!

William Shakespeare, Henry IV, Part 2

Introduction

Given the growing emphasis on environmental studies (Chapter 10), stratigraphic (temporal) resolution is no less critical to reconstructing Earth history now than it has been during the past two centuries. Correlation establishes the age equivalence of rocks and fossil assemblages and is the basis for reconstructing paleogeography and facies relationships. A precise stratigraphic framework is also critical to establishing any semblance of cause and effect preserved in the stratigraphic record. Unfortunately, physical reworking (Chapters 1 and 2), dissolution and diagenesis (Chapter 3), and bioturbation (Chapter 4) typically mix (time-average) – and sometimes destroy – stratigraphic signals and temporal resolution, the exception being Lagerstätten (Chapters 5, 6).

The stratigraphic record has long been viewed as being notoriously incomplete because of non-deposition and erosion. Darwin used incompleteness to explain why there are typically no transitional fossils found between new taxa, an idea (along with a number of others) borrowed from Charles Lyell. Say that a particular spot on the Earth's surface started out with an uninterrupted sediment accumulation rate of 10 cm ka⁻¹. This is not an unusual rate for river deltas, which are sites of significant sediment influx (Enos, 1991). Then over the course of the Phanerozoic (approximately the past 540 million years), about 54 km of sediment would have accumulated, which is several times that of the average thickness of continental crust! Clearly, much of the sediment must have bypassed the site of deposition or was eroded.