Important Stratigraphic and Geomorphic Concepts

Facies

Facies

• A sediment body or sedimentary rock distinguished by its lithology, geometry, sedimentary structures, proximity to other sediments or sed rocks, and fossil content.

• Formed by and characteristic of a particular depositional environment.

• Facies refers to various sediment characteristics you might refer to a "tan, laminar-bedded, beach sandstone facies".

• Rock unit characteristics result from depositional environment (energy).

• Depositional setting imprints a distinctive sediment characteristics..

• facies is a distinct kind of rock for that environment and area.

• Each depositional environment grades laterally into other environments. We call this facies change when dealing with the rock record.

• A = Sandstone facies (beach environment)

• B = Shale facies (offshore marine environment)

• C = Limestone facies (far from sources of clastic input)

time line

Walther's Law – (continuous deposition)

Sedimentary environments that start out side-by-side will overlap one another over time due to transgressions or regressions. A vertical sequence of beds results.

The vertical sequence of a facies matches the original lateral distribution of sediments and sedimentary environments.

Transgressive sequenceDeeper water facies overlie shallow

water facies.

A "deepening upward“ or “fining upward” sequence.

Offshore LS

Beach sands

Nearshore silt

Offshore Clay

What is this facies

Sequence?

Shallow water facies overlie deeper water facies.

A regressive sequence

"shallowing upward" LS

SS

SH

Lime SH

Laws of Stratigraphy

• Superposition- youngest on top• Original lateral continuity- erosion cuts

valleys• Original horizontality- most sediments

deposited in horizontal layers, or nearly so.

• Cross-cutting relations- A feature must be younger than the rock, sediment, or surface it cuts

• Inclusions- must be older than their host

Basic Stratigraphic PrinciplesNicolaus Steno (1638-1687)

fossil descriptions

first stratigrapher

Steno's Principles

Superposition

Original Horizontality

Original Lateral Continuity

Principle of Superposition

youngest material on top; except if inverted

Fundamental principle for age relationships in sedimentary rocks

(sediments) and extrusive volcanics

Superposition

Younger

Steno's PrinciplesSuperposition

limey mudstone over coal; ND E.E. Gamble

E.E. GambleTwo till units that sandwich an outwash; MN

soil geomorphic analogue

2 Holocene alluviums; fragipan (dryer altithermal), vs. humid environment. MS

Wysocki

Pleistocene Loess Deposits Eastern WA Gamble

Original Continuity Steno’s principle of

Lateral Continuity

Badlands, SD E.E. Gamble

Cross-cutting Relations relative age relationships

A feature is younger than the rock, sediment, surface, or other feature that it cuts.

Relative Age Tools: Cross-Cutting Relations

A rock unit must always be older than any feature that cuts or disrupts it (e.g., faults, metamorphism, igneous intrusions).

Cross-cutting RelationshipsArray features A, B, C, D, E by age.

A

B C

D

E

Cross Cutting Surface

Converging, backwearing side slopes

St. Peter Sandstone (Boone soil) over Prairie Du Chein dolomite; River Falls, WIWysocki

AB

Inset Relationship

( type of cross-cutting relationship )

Wysockiinset relationship; northeast KS

channel fill; note lack of surface expression. / Big Nemaha R., NE

Wysocki

Cross-Cutting Relations and Inclusions

Facing of Strata:how some sedimentary structures indicate if beds are overturned..

Hutton’s “Great unconformity” at Siccar point, eastern Scotland

Relative Time

Superposition (Steno)

Lateral Continuity (Steno)

Original Horizontality (Steno)

Inclusions (Lyell)

Cross-cutting Relationships (Lyell)

Relative Time

Quiz - Array by age: Faults A & B, Dikes A & B,

the Batholith, and the Limestone and Shale units.

Shale Youngest

Dike A

Dike B

Batholith

Fault B

Fault A

Limestone Oldest