One of three ways to slice the record

Biostratigraphy

Biostratigraphy divides and correlates rock by its fossil content. Because life evolves — species appear, flourish, then go extinct, and never come back in quite the same form — the fossils in a rock act as a stamp of its age. Where lithostratigraphy can only be traced as far as a rock type continues, fossils let geologists match strata across whole continents.

The principle of faunal succession

First recognised by William Smith in the early 1800s, faunal succession is the observation that fossil species follow one another through time in a definite, repeatable order — and that the order is the same everywhere on Earth. Find that order once and you can use it as a clock. A fossil that is easy to spot, geographically widespread, and lived for only a short slice of time makes an ideal index fossil: its mere presence dates the rock around it.

Biozones: the working unit

The unit of biostratigraphy is the biozone — a body of rock defined by its fossils. Zones are bounded by datums: a species' first appearance (FAD) as it evolves, and its last appearance (LAD) as it goes extinct.

Correlation: matching rock across distance

This is what makes biostratigraphy so powerful. Two sections hundreds of kilometres apart may be built of completely different rock, yet if they share the same fossil datums, those levels are the same age. The fossils correlate the sections where the lithology cannot.

Biostratigraphy in Pakistan

Pakistan's strata are unusually rich in the fossils that make good zones:

• Larger benthic foraminifera — Nummulites, Assilina, Alveolina, Discocyclina — zone the Paleocene–Eocene carbonates of the Sulaiman and Kirthar basins (the Dungan, Sakesar, Chorgali and Kirthar formations), and are the classic tool for dating them.
• Ammonites and belemnites date the Jurassic–Cretaceous marine shales such as the Sembar and Chichali formations.
• The Siwalik mammal succession of the Potwar Plateau is one of the richest fossil-mammal records on Earth, and its evolving rodents and large mammals provide a fine-grained biostratigraphy for the otherwise fossil-poor river sandstones.

See the fossils page for the units and the taxa recorded in them.

Strengths and limits

Biostratigraphy gives relative age and unrivalled long-distance correlation — but only where fossils are present and preserved, and it can be tripped up by fossils that favour a particular environment (a facies control) or by specimens reworked out of older rock. On its own it orders events; combined with chronostratigraphy and numerical dating, it gets anchored to actual ages in years.