One of three ways to slice the record

Lithostratigraphy

Lithostratigraphy divides rocks by their physical character — their lithology. It ignores age and fossils and asks a single question: what is this rock, and where does it change? The units it produces — groups, formations, members and beds — are what a geologist walks out and maps in the field, and they are the framework every formation on this site is built around.

The hierarchy of units

Lithostratigraphic units nest inside one another, from the broad down to the single layer. The formation is the fundamental unit — large enough to show on a geological map, consistent enough in character to trace across an area, and formally named after a place (its type locality).

The formation, defined

A formation is set up on lithology alone. To be formally named it needs three things: a distinctive and consistent rock character that sets it apart from the units above and below; enough mappability to be shown at ordinary map scale; and a type section — a designated exposure where the unit is defined and best displayed. The name pairs a geography with either a rock term or the word "Formation" — Chiltan Limestone, Chinji Formation, Pab Sandstone.

Crucially, a formation need not be one rock type. It can be an interbedded package — say, a rhythmic alternation of sandstone and shale — as long as that package is distinctive and traceable. The boundaries between formations are called contacts, and they are placed at the level where the lithology changes.

Why lithostratigraphy is not a clock

Here is the idea beginners most often miss: a lithostratigraphic boundary is not a time line. Rock type follows environment, and environments migrate. As a shoreline advances or retreats, the same sandy beach deposit is laid down at one time near the coast and at a later time further inland. The resulting sandstone is a single named unit — but it is diachronous: older in one place, younger in another.

Lithostratigraphy in Pakistan

Almost everything catalogued on this site is a lithostratigraphic unit. A few that show the hierarchy in action:

• The Siwalik Group of the Potwar Plateau is split, oldest to youngest, into the Chinji, Nagri, Dhok Pathan and Soan formations — a textbook group-into-formations division based purely on changing sandstone and mudstone character.
• The Salt Range Formation is a single formation of salt, gypsum and marl at the very base of the Salt Range section — defined by its unmistakable evaporite lithology.
• The Eocene carbonates — Sakesar, Chorgali, Kirthar — are formations mapped on limestone character across huge areas of the Kohat–Potwar and Kirthar basins.

Browse them by their rock type on the rock types page, or by region under units.