Tectonics and sedimentation:

Individual sedimentary rock formations can indicate the local environment in which a rock formed (eg. alluvial fan, marginal marine, etc. Large scale sedimentary sequences, however, are related to plate motions and tectonics and essential in their reconstruction, especially prior to the sea-floor paleomagnetic record.

Cratonic sedimentation

Craton: The thick, ancient, stable center of a continent. Cratonic sedimentation is characterized by:


Peripheral rocks - Orogenic belts: More recent rocks record the history of subduction, continent collisions, and microplate terrane collisions along peripheral belts that can be hundreds of km wide.

Geosynclinal sedimentation:

Geosynclines: A relict term from prior to the age of plate tectonics. Geosynclines are troughs that parallel mountain belts on continental margins. Their origins were a vexing enigma until the advent of plate tectonics. Their sedimentology depends on the type of margin: Divergent, convergent, or transform.

Sedimentation on divergent margins

Divergent boundaries, by definition, separate plates of oceanic lithosphere, however they can originally form as a continent is stretched and rifted apart. In this case, a new sea floor spreading center and new oceanic crust ultimately form between the resulting pieces. Our concern here is what happens near the continental margin. Typically, as a continent begins to stretch prior to rifting, clusters of roughly parallel rift valleys (like the Tanzanian Rift Valley at right) form. Only one will eventually become a new sea-floor spreading zone. The rest - the failed rifts - will be preserved as elongate troughs that eventually fill with continental sediment (usually lacustrine).
Stages:

In ancient rocks, we recognize failed intracontinental rifts by ancient parallel rift valley deposits. EG. Newark Supergroup of Eastern North America. Newark Supergroup outcrop

Sedimentation on convergent margins (subduction zones)

Accretion of island arcs: Oceanic - Oceanic convergences

The image at right is of Mt. Redoubt, one of the four major island arc volcanoes of the Alaska Peninsula. (We still call them part of a volcanic island arc, even though they slightly overlap the continental mainland, because they are cause by subduction melting in which magma rises through oceanic crust.) In the modern world, such active arcs are easy to recognize. Millions of years from now, after this subduction zone shuts down, they will remain recognizable as the traces of an oceanic - oceanic convergence.

Sedimentary features:

In ancient rocks, we recognize oceanic - oceanic convergences by the parallel juxtaposition, in map view, of:

Oceanic - Continental convergences:

In this case, the situation is complicated by the much greater amount of sediment being shed from the continent into the forearc basin. The scene above is from the Cascade Range, a volcanic - magmatic mountain belt (as opposed to a volcanic island arc) formed by the subduction of the Juan de Fuca Plate beneath North America. As before, the traces of such convergences are preserved long after the subduction zone ceases to be active.

Features:


In ancient rocks we recognize parallel remnants of volcanic-magmatic mountain belts and subduction melanges. E.G. Jurassic - Cretaceous Sierras parallel the subduction melanges of the Coast Range in California.

Sedimentation on transform margins:

Transtensional basins: Shearing motion is seldom perfectly straight. Bends in the fault can produce deep, narrow, fault-bounded troughs. Termed transtensional basins or "pull-apart troughs".


Tectonic provenance of sandstones

In the absence of severe diagenesis, sandstone composition can suggest the tectonic origin of a sediment.

But note: This signal can be overprinted by a climate signal:

Secular variations in the rock record


"The present is the key to the past, but the key turns more smoothy if you don't push it in too far." - William Galloway

Outcrop du jour