One of three ways to slice the record

Chronostratigraphy

Chronostratigraphy divides rocks by age. It is the time-rock framework — the systems, series and stages — that underlies the geological time scale. Where lithostratigraphy answers "what?" and biostratigraphy helps answer "in what order?", chronostratigraphy answers "when?" — and gives every other framework a shared calendar.

Two parallel hierarchies: rock and time

This is the distinction that confuses almost everyone at first. There are two matched sets of units. Chronostratigraphic units are bodies of rock — all the rock formed during a given time interval. Geochronologic units are the time intervals themselves. Each rock unit has a one-to-one time twin:

Stages and the "golden spike"

The stage is the basic working unit of chronostratigraphy. But how do you fix exactly where one stage ends and the next begins, worldwide? The modern answer is the GSSP — a Global Boundary Stratotype Section and Point. Geologists agree on one specific level in one specific, well-exposed rock section, usually tied to a clear event such as the first appearance of an index fossil, and drive a physical marker into it — the famous "golden spike". That one point defines the boundary for the whole planet.

Putting numbers on it

Chronostratigraphy supplies the ordered framework; the numbers in millions of years come from other methods layered on top. Radiometric dating — for example uranium–lead ages from volcanic ash beds — gives absolute ages at specific horizons. Magnetostratigraphy matches the record of magnetic-field reversals to the global polarity time scale. Astrochronology counts cycles driven by the Earth's orbit. Together these calibrate the international chart that the time scale page is built from.

Chronostratigraphy in Pakistan

• The Salt Range exposes a near-complete section from the Precambrian to the Recent — a rare place to see many systems stacked in order, which is why it recurs throughout this site.
• The Siwalik river deposits of the Potwar Plateau are a showcase for magnetostratigraphy: with few datable ash beds, their age comes largely from matching their magnetic-reversal pattern to the global polarity time scale, tightly dating the fossil mammals within.
• The Cretaceous–Paleogene boundary — the mass extinction that ended the dinosaurs — falls within the Pab-to-Ranikot succession, an example of a global chronostratigraphic boundary recorded in Pakistani rock.

How the three fit together

The three frameworks are complementary, not competing. Lithostratigraphy gives the physical, mappable units. Biostratigraphy correlates them and orders them in relative time using fossils. Chronostratigraphy — fed by those fossil datums and by radiometric and magnetic dating — assembles the shared calendar in which all of it is placed.