Glossary

Stratigraphy terms

Plain-language definitions of the 79 terms used across this site. New to the subject? The field guide walks through the core principles in order.

79 terms

A

Accretionary wedge: A pile of ocean-floor sediment scraped off a descending plate and stacked up at a subduction zone — the Makran coast is a living example.
Ammonite: An extinct group of coiled, chambered marine molluscs; their fast evolution makes them prime index fossils for the Mesozoic.; See also Index fossil
Angular unconformity: An unconformity where older, tilted or folded strata are overlain by younger, more horizontal layers.; See also Unconformity
Aquifer: A body of rock or sediment porous and permeable enough to store and transmit useful amounts of groundwater.; See also Reservoir rock

B

Bed: The smallest formal lithostratigraphic unit — a single distinct layer within a member or formation.; تہہ سب سے چھوٹی باقاعدہ سنگی طبقاتی اکائی — کسی رکن یا فارمیشن کے اندر ایک واحد ممتاز تہہ۔; See also Formation Member
Biostratigraphy: Dating and correlating strata using their fossil content — the practical application of faunal succession.; See also Faunal succession Index fossil Correlation
Bioturbation: The churning and reworking of sediment by burrowing and other activity of organisms.; See also Trace fossil
Breccia: A coarse rock of angular, broken fragments cemented together; their sharp edges show the pieces were not carried far.; See also Conglomerate

C

Chert: A hard, fine-grained rock made of silica, often occurring as nodules or beds within limestone; flint is a variety.; See also Limestone
Chronostratigraphy: Subdivision of rocks by their age and time of formation, independent of rock type.; زمانی طبقاتیات چٹانوں کو اُن کی عمر اور زمانۂ تشکیل کے مطابق تقسیم کرنا، قطع نظر اُن کی نوعیت کے۔; See also Lithostratigraphy Geochronology (era, period, epoch)
Conformable: Describes strata deposited in an unbroken sequence, with no significant gap in time between them.; See also Unconformity
Conglomerate: A coarse sedimentary rock made of rounded, gravel-sized clasts cemented together.
Correlation: Establishing that strata in different places are equivalent in age or stratigraphic position.; See also Faunal succession
Craton / shield: The ancient, stable core of a continent that has resisted deformation for a very long time; where it is exposed at the surface it is called a shield.; See also Gondwana Sedimentary basin
Cross-bedding: Internal layering set at an angle to the main bedding, built by migrating ripples or dunes; it records the direction of the current or wind that formed it.; See also Stratum (strata)

D

Deposition: The laying down of sediment by water, wind or ice — the process that builds up sedimentary strata.; See also Erosion Stratum (strata)Diagenesis
Diachronous: Describes a rock unit whose age differs from place to place, because the environment that formed it migrated over time.
Diagenesis: The physical and chemical changes — compaction, cementation and more — that turn loose sediment into solid rock and keep altering it afterwards.; See also Deposition Sedimentary rock
Disconformity: An unconformity between parallel strata, where the missing time sits along an irregular or buried erosion surface.; See also Unconformity
Dolomite (dolostone): A carbonate rock like limestone but rich in magnesium, often formed when limestone is chemically altered after deposition.; See also Limestone Diagenesis

E

Erosion: The wearing away and removal of rock and sediment by water, wind, ice or gravity.; See also Weathering Deposition Unconformity
Evaporite: A rock such as rock salt or gypsum formed when mineral-rich water evaporates; the Precambrian Salt Range Formation is a classic example.

F

Facies: A body of rock whose characteristics reflect the conditions under which it formed, distinguishing it from neighbouring rock of the same age.; فیشیز (سحنہ) چٹان کا وہ حجم جس کی خصوصیات اُن حالات کی عکاسی کرتی ہیں جن میں وہ بنی، اور جو اسے اسی عمر کی پڑوسی چٹان سے ممتاز کرتی ہیں۔; See also Lithology
Fault: A fracture in rock along which the two sides have moved relative to each other.; See also Thrust fault Fold
Faunal succession: The principle that fossil organisms succeed one another through time in a predictable order, so strata can be dated and correlated by the fossils they contain.; See also Law of superposition Correlation
Flysch: Deep-marine sand and mud — typically turbidites — deposited in a basin during the early stages of mountain building, before molasse.; See also Molasse Turbidite
Fold: A bend in originally flat rock layers, produced when they are compressed.; See also Fault
Foraminifera (forams): Mostly microscopic single-celled marine organisms with mineral shells; their rapid evolution makes them invaluable for dating marine strata.; See also Nummulites Index fossil
Foreland basin: A basin that forms in front of a growing mountain belt, pushed down by the weight of the rising range and filled with debris eroded from it.; See also Sedimentary basin Orogeny
Formation: The fundamental mappable unit of lithostratigraphy — a distinctive body of rock extensive enough to show on a geological map, formally defined at a type locality.; فارمیشن (تشکیل) سنگی طبقاتیات کی بنیادی قابلِ نقشہ اکائی — چٹان کا ایک ممتاز حجم جو اتنا وسیع ہو کہ ارضیاتی نقشے پر دکھایا جا سکے، اور کسی معیاری مقام پر باقاعدہ متعین کیا گیا ہو۔; See also Member Group Bed Type locality / type section
Fossil: Any preserved trace of past life — bones, shells, imprints, tracks — used to date rocks and reconstruct ancient environments.; See also Index fossil Trace fossil Biostratigraphy

G

Geochronology (era, period, epoch): The dated divisions of geological time. Eras contain periods, which contain epochs — the framework laid out in the geological time scale.; See also Chronostratigraphy
Gondwana: The southern supercontinent — including the Indian plate — from which the land now forming Pakistan rifted and drifted northward.; See also Tethys
Group: Two or more associated formations that share characteristics, ranked one level above the formation.; گروہ دو یا زیادہ باہم متعلق فارمیشنیں جو مشترک خصوصیات رکھتی ہوں؛ درجہ بندی میں فارمیشن سے ایک درجہ اوپر۔; See also Formation

H

Hiatus: A gap in the rock record — an interval of time not represented by rock, because nothing was deposited or it was later eroded. Its physical surface is an unconformity.; See also Unconformity Diachronous

I

Igneous rock: Rock formed by the cooling and solidifying of molten magma or lava, either underground (e.g. granite) or at the surface (e.g. basalt).; See also Metamorphic rock Nonconformity
Index fossil: A fossil species that was widespread but short-lived, making it especially useful for dating and correlating the rocks that contain it; nummulites are a classic example.; See also Biostratigraphy Nummulites Ammonite

L

Law of superposition: In an undisturbed sequence, each layer is younger than the one below it and older than the one above.; See also Original horizontality Faunal succession
Lignite: A soft, brown, low-grade coal; the Thar coalfield of Sindh is one of the largest lignite deposits in the world.
Limestone: A sedimentary rock made mostly of calcium carbonate, commonly built from the shells and skeletons of marine organisms; Pakistan’s Eocene limestones are widely quarried for cement.; See also Dolomite (dolostone)Nummulites Facies
Lithology: The physical character of a rock — its composition, grain size, texture and colour.; سنگیات کسی چٹان کا طبیعی کردار — اس کی ترکیب، دانے کا حجم، بناوٹ اور رنگ۔; See also Facies
Lithostratigraphy: Subdivision of rocks into units — formations, members and so on — based on their physical, lithological character.; سنگی طبقاتیات چٹانوں کو اُن کے طبیعی، سنگی کردار کی بنیاد پر اکائیوں — فارمیشن، رکن وغیرہ — میں تقسیم کرنا۔; See also Chronostratigraphy Formation

M

Marker bed: A distinctive, easily recognised layer that can be traced widely, giving a reference line for correlating sections.; See also Correlation
Marl: A lime-rich mudstone, intermediate in composition between limestone and clay.
Member: A named subdivision of a formation with its own recognisable character, but not mapped separately at the same scale.; رکن کسی فارمیشن کا ایک نامزد ذیلی حصہ جس کا اپنا قابلِ شناخت کردار ہو، مگر اسی پیمانے پر الگ نقشہ نہ بنایا جائے۔; See also Formation Bed
Metamorphic rock: Rock changed in the solid state by heat, pressure or chemically active fluids — such as schist, gneiss or marble — often deep within mountain belts.; See also Igneous rock Orogeny
Molasse: Thick, mostly non-marine sand and mud shed from a rising mountain belt into the basin at its foot; the Siwalik Group is classic Himalayan molasse.; See also Flysch Foreland basin Siwalik

N

Nappe: A large sheet of rock pushed many kilometres over the rocks beneath it along a thrust or décollement — a hallmark of intense mountain building.; See also Thrust fault Orogeny
Nonconformity: An unconformity where sedimentary rock rests directly on older igneous or metamorphic rock.; See also Unconformity
Nummulites: Large, coin-shaped foraminifera that flourished in the Eocene; their limestones are widespread in Pakistan and quarried for cement.; See also Foraminifera (forams)Limestone Index fossil

O

Ophiolite: A slice of ocean-floor crust and upper mantle thrust up onto a continent, marking a vanished ocean; the Bela, Muslim Bagh and Waziristan ophiolites trace the closing of Tethys.; See also Suture zone Tethys Subduction
Original horizontality: The principle that sediments are originally deposited in roughly horizontal layers; strata now tilted were moved after deposition.; See also Law of superposition
Orogeny: A mountain-building episode, typically driven by the collision of tectonic plates.; See also Foreland basin Suture zone
Outcrop: A place where bedrock is exposed at the surface rather than buried under soil or vegetation.

P

Plate tectonics: The theory that Earth’s rigid outer shell is broken into moving plates whose collisions, rifting and subduction drive most large-scale geology — including the building of the Himalaya.; See also Subduction Orogeny Gondwana
Provenance: The source region from which the grains in a sedimentary rock were derived, worked out from their composition and used to reconstruct ancient geography.; See also Sandstone

R

Regression: A relative fall of sea level that moves the shoreline seaward, exposing what was formerly seabed.; See also Transgression
Reservoir rock: A porous, permeable rock — often sandstone or fractured limestone — able to hold and yield oil, gas or water.; See also Source rock Sandstone Aquifer

S

Sandstone: A sedimentary rock made of sand-sized grains cemented together; the Siwalik succession and many petroleum reservoirs of Pakistan are sandstones.; See also Siltstone Conglomerate Reservoir rock Provenance
Sedimentary basin: A region of the crust that subsides over long periods so sediment accumulates in it, building up a thick stratigraphic succession.; See also Foreland basin
Sedimentary rock: Rock formed from sediment — fragments of older rock, mineral grains or the remains of organisms — laid down in layers and hardened. Most units on this site are sedimentary.; See also Stratum (strata)Lithology Deposition Diagenesis
Shale: A fine-grained sedimentary rock of hardened mud and clay that splits into thin layers; organic-rich shales can be petroleum source rocks.; See also Siltstone Source rock
Siltstone: A fine-grained sedimentary rock made of silt particles — finer than sand, coarser than clay.; See also Sandstone Shale
Siwalik: The Neogene-to-Quaternary molasse shed from the rising Himalaya across northern Pakistan, world-famous for its rich fossil-mammal record.; See also Molasse Foreland basin Orogeny
Source rock: An organic-rich rock, usually a shale, in which oil and gas are generated before migrating into a reservoir.; See also Reservoir rock Shale
Stratigraphy: The branch of geology concerned with the order, arrangement and age relationships of rock layers, and what they record about the history of the Earth.; طبقات الارضیات ارضیات کی وہ شاخ جو چٹانی طبقات کی ترتیب، تنظیم اور عمری تعلقات کا مطالعہ کرتی ہے اور یہ جانتی ہے کہ وہ زمین کی تاریخ کے بارے میں کیا بتاتے ہیں۔; See also Lithostratigraphy Chronostratigraphy
Stratum (strata): A layer of sedimentary rock with consistent characteristics that set it apart from the layers above and below. “Strata” is the plural.; طبقہ (جمع: طبقات) تلچھٹی چٹان کی ایک تہہ جس کی خصوصیات اسے اوپر اور نیچے کی تہوں سے ممتاز کرتی ہیں۔ اس کی جمع ”طبقات“ ہے۔
Stromatolite: A layered structure built by mats of microbes trapping and binding sediment — among the oldest evidence of life on Earth.; See also Fossil
Subduction: The sinking of one tectonic plate beneath another into the mantle, building volcanic arcs and accretionary wedges above it.; See also Accretionary wedge Ophiolite Plate tectonics
Suture zone: The boundary along which two continental blocks were welded together after the ocean between them closed.; See also Orogeny Tethys

T

Terrane: A block of crust with its own distinct geological history that became attached to a larger continent along a fault or suture.; See also Suture zone Orogeny
Tethys: The ancient ocean that once lay between Gondwana and the northern continents, squeezed out of existence by the collision that built the Himalaya.; See also Gondwana Suture zone
Thrust fault: A low-angle fault that pushes older rock up and over younger rock, common in mountain belts where the crust is being shortened.; See also Fault Nappe Orogeny
Trace fossil: A fossil of an organism’s activity rather than its body — a burrow, track, trail or boring.; See also Bioturbation Fossil
Transgression: A relative rise of sea level that moves the shoreline landward, depositing marine sediments over what was formerly land.; See also Regression
Turbidite: A sediment bed laid down by a turbidity current — a dense, sediment-laden flow surging down an underwater slope — showing a characteristic grading from coarse at the base to fine at the top.; See also Flysch
Type locality / type section: The specific place where a stratigraphic unit is formally defined, and against which other occurrences of it are compared.; معیاری مقام / معیاری قطعہ وہ مخصوص جگہ جہاں کسی طبقاتی اکائی کو باقاعدہ متعین کیا جاتا ہے اور جس سے اس کے دیگر مقامات کا موازنہ کیا جاتا ہے۔; See also Formation

U

Unconformity: A surface in the rock record representing a gap in time, where deposition paused or erosion removed rock before younger layers were laid down.; See also Disconformity Angular unconformity Nonconformity

W

Weathering: The in-place breakdown of rock at or near the surface by physical and chemical processes, producing the loose material that erosion then carries away.; See also Erosion

Definitions are independent plain-language summaries for learners. For formal usage consult the standard stratigraphic literature and the International Commission on Stratigraphy.