Sedimentary Rocks: Formation, Types, and Fossil Preservation

Sedimentary rocks

Rocks formed from layers of sediment. They can be clastic (pieces of other rocks) or non-clastic (chemical/biological). Fossils are usually found here.

Clastic sedimentary rocks

Made from pieces of other rocks. Example: sandstone. Formed by WTDCC steps.

WTDCC

Steps to form sedimentary rocks: Weathering, Transport, Deposition, Compaction, Cementation.

Mechanical/physical weathering

Breaking rocks into smaller pieces without changing what they're made of.

Chemical weathering

Rocks change by chemical reactions (water, acid, rust, etc.).

Deposition

When sediment stops moving and settles in one place.

Bury

New layers cover older ones.

Compaction

Squeezing water out of buried sediment.

Cementation

Minerals fill spaces and glue sediment together.

Lithification

Turning loose sediment into solid rock (compaction + cementation).

Non-clastic sedimentary rocks

Not made from rock pieces. Examples: coal (plants), evaporites (from drying water), limestones (CaCO₃).

Carbonates

Rocks made from calcium carbonate (CaCO₃).

Limestones

Common carbonate rock, often from shells of tiny sea organisms.

Chalk

Soft limestone made of microscopic shells.

Diatomite

Rock made of tiny diatom skeletons (silica, not CaCO₃). Found in Monterey Formation.

Monterey Formation

Famous diatomite rock in California, formed offshore.

Newport Back Bay, Orange County

Place where Monterey Formation outcrops are found.

Burial

Important for fossilization; quick burial helps preserve remains.

Depositional environments

Places where sediment is laid down (rivers, lakes, oceans, wind).

Fluvial

River environments; good for land fossils.

Lakes and lagoons

Calm waters where fine sediment and delicate fossils can be preserved. Example: Archaeopteryx fossil.

Body fossil

Preserved hard or soft parts of an organism (bones, shells, feathers).

Ichnofossils (trace fossils)

Evidence of activity (footprints, burrows), not actual body parts. Example: "Noah's Raven."

Amber

Hardened tree resin that can trap insects and sometimes DNA.

Sap

Tree fluid that carries nutrients (different from resin).

Xenophanes

First person to use fossils as evidence of ancient seas.

Nicolas Steno

Early geologist; studied shark teeth ("tongue stones"). Created 3 laws of stratigraphy.

Principle of superposition

Younger layers are on top of older layers.

Principle of original horizontality

Sediment layers start flat.

Principle of cross-cutting relationships

A layer is older than what cuts through it.

Principle of lateral continuity

Layers stretch sideways until they thin out or hit something.

Giovanni Arduino

Classified rocks into four groups: Primary (igneous), Secondary (Paleo/Meso), Tertiary + Quaternary (Cenozoic).

Neptunists

Believed all rocks came from an ancient ocean.

Plutonists

Believed igneous rocks form from magma (correct).

Volcanic (extrusive) rocks

Igneous rocks that form from lava on the surface.

Plutonic (intrusive) rocks

Igneous rocks that form from magma underground.