FIELD TECHNIQUES
2.10 Stratigraphic Practice
Stratigraphically, rocks are classified on the basis of lithology (lithostratigra- phy), fossils (biostratigraphy), key surfaces (sequence stratigraphy) and time (chronostratigraphy). From field studies, sedimentary rocks are primarily con- sidered in purely descriptive lithostratigraphic terms, shown in Table 2.5.
2.10.1 Lithostratigraphy
The fundamental unit in lithostratigraphy is theformation, possessing an inter- nal lithological homogeneity and serving as a basic mappable unit. Adjacent formations should be readily distinguishable on physical or palaeontological grounds. Boundaries may be gradational, but they should be clearly, even if arbitrarily, defined in a designated type section or sections. Although thick- ness is not a criterion, formations are typically a few metres to several hundred metres thick. Thickness will vary laterally over an area and formations are commonly diachronous on a large scale. Stratigraphically adjacent and related formations, such as those deposited within the same basin, may be associated so as to constitute agroup(typically of the order of 103m thick). A formation may be subdivided intomembers, characterised by more particular lithologi- cal features, and if there is a distinctive bed within a member this can be given a specific name. Lithostratigraphical units are given geographical names.
Table 2.5 Hierarchy of lithostratigraphic units.
Supergroup– a formal assemblage of related or superposed groups.
Group– an assemblage of formations.
Formation– the fundamental lithostratigraphic unit, identified by lithological characteristics and stratigraphic position, generally tabular. Mappable at the Earth’s surface and traceable into the subsurface. Several tens to hundreds of metres thick.
Member– a formal lithostratigraphic unit constituting a formation.
Lens– a geographically restricted member occurring within a formation.
Tongue– a wedge-shaped member.
Bed– a distinctive subdivision of a member; the smallest formal lithostratigraphic unit of sedimentary rock.
To erect a lithostratigraphy yourself there are several publications which give details of the procedure and discuss the International Code (see Refer- ences and Further Reading). In many parts of the world, older stratigraphic names are in use which do not conform with the International Code.
2.10.2 Sequence stratigraphy
An increasingly popular way of dividing up the stratigraphic record is on the basis ofunconformities into sequences. See Figs 2.6 and 2.7 for the basic models. A sequence is defined as a succession of relatively conformable, genetically related strata bounded by an unconformity or its correlative con- formity (see References and Further Reading for more information). An unconformity (thesequence boundary) is a surface separating younger from older strata along which there is evidence of subaerial exposure with a significant hiatus (type A) or of drowning (type B). It will pass laterally (basinwards) into aconformity.
A sequence can usually be divided intosystems tracts(defined as a linkage of contemporaneous depositional systems, i.e. related facies, or facies associ- ation) deposited during a specific part of a cycle of relative sea-level (RSL) change, i.e., falling stage systems tract (FSST), also called forced regressive (FRST), when RSL is falling; lowstand systems tract (LST; RSL is low);
transgressive systems tract (TST; RSL is rising), and highstand systems tract (HST; RSL is high) (see Figs 2.6 and 2.7). Apart from the sequence bound- ary (sb), otherkey surfacesare thetransgressive surface(ts), which may be coincident with the sequence boundary in more proximal (landward) parts of a basin, at the base of the TST, and themaximum flooding surface(mfs) that separates the TST from the HST (see Figs 2.6 and 2.7). In more distal parts of the basin, there is commonly acondensed section (cs) equivalent to the
HST HST mfs FSST
sb ts
sb TST
mfs cs
cs LST
ivf sb/ts sand mud
sb
mfs ts
mfs sb
sea level HST
TST LST HST FSST
Figure 2.6 Sequence stratigraphic model for siliciclastic sediments (simpli- fied), showing arrangement of systems tracts and key surfaces, and location of sands and muds, for a ramp margin. FSST, LST, TST and HST=falling stage, lowstand, transgressive and highstand systems tracts; ivf=incised val- ley fill; sb=sequence boundary, ts=transgressive surface, mfs=maximum flooding surface, cs=condensed section.
sb mfs HST
sb
mfs ts
mfs sb HST LSTTST HST FSST
sb/ts TST FSST
LST megabreccia mudst.
grainst.
reef talus
Figure 2.7 Sequence stratigraphic model for carbonate sediments (simpli- fied), showing arrangement of systems tracts and key surfaces, and location of major facies, for a rimmed shelf margin. See Fig. 2.6 for abbreviations.
upper part of the TST, the mfs and the lower part of the HST. The sequence stratigraphic terms are defined in Table 2.6.
There are some significant differences in the sequence stratigraphic models for clastics and carbonates (see Figs 2.6 and 2.7) in view of the different controls on sedimentation, notably the mostlyin situgeneration of carbonate sediments and the imported nature of clastic sediments.
Table 2.6 Hierarchy of sequence stratigraphic units
Depositional sequence: genetically related strata bounded by surfaces of erosion or non-deposition, i.e., unconformities (sequence boundaries) and their correlative conformities.
Key surfaces: sequence boundary, transgressive surface and maximum flooding surface, which divide sequences intosystems tracts.
Sequence boundary (sb):two types:
A – characterised by subaerial exposure and erosion associated with stream rejuvenation, a basinward shift of facies and onlap of overlying strata, often biostratigraphic gap;
B – a drowning unconformity; deeper-water facies over shallow-water facies.
Transgressive surface (ts): marks onset of pronounced relative sea-level rise; first significant marine flooding surface above sb, with facies deepening upward above. The ts may coincide with the sb in a landward direction.
Maximum flooding surface (mfs): marks maximum relative sea level, deepest-water facies; distal areas starved of sediment form acondensed section (cs), overlain by shallowing-upward succession.
Systems tract (ST): a linkage of contemporaneous depositional systems.
Four types are distinguished:
(1) falling stage (FSST) (also called forced regressive, FRST) – facies deposited during sea-level fall;
(2) lowstand (LST) – facies deposited during sea-level low;
(3) transgressive (TST) – facies deposited during relative sea level rise;
(4) highstand (HST) – facies deposited during sea-level high.
Parasequence set: succession of genetically related parasequences that have a distinctive stacking pattern (e.g., thinning up); usually bounded by major marine flooding surfaces.
Parasequence (psq): relatively conformable succession of genetically related beds or bedsets bounded by marine flooding surfaces; typically metre-scale.
Marine flooding surface (fs): a surface that separates younger from older strata, across which there is evidence of an abrupt increase in water depth.
Some sequences, especially in platform carbonates, are composed of sev- eral or many metre-scale cycles termedparasequences (defined byflooding surfaces at their bases), and then the systems tracts are defined by the stacking patterns of the parasequences (e.g., whether they thin/thicken-up or fine/coarsen-up). See Section 8.4 for further information on how to recog- nise the key surfaces and systems tracts in the field, and what features to look for in successions of parasequences.
Sequences within an area are generally named by letters or numbers, or a combination of both, working from the base upwards.
2.10.3 Chronostratigraphy
Chronostratigraphy considers the stratigraphic record in terms of time; it can be very useful to think of strata in this way, especially when examining the succession on a basin-scale and there are breaks in sedimentation and periods of uplift. A chronostratigraphic diagram depicts the succession in space and time, and so does not indicate thickness. It will show where and when deposition and subaerial exposure took place, and bring out the relation- ships between different units. Once a sequence stratigraphic analysis has been completed, it is useful to sketch out the chronostratigraphy (see Fig. 2.8).
Chronostratigraphic divisions are time/rock units, i.e., they refer to the succession of rocks deposited during a particular interval of time. The chronostratigraphy of the Cainozoic, Mesozoic and Palaeozoic is shown in Tables 2.7, 2.8 and 2.9 with the system, series and stage names and the approximate age of the beginning of the stage.
HST TST LST FSST sb ts mfs
HST
sand mud exposure
time
Figure 2.8 Chronostratigraphic diagram for the succession shown in Fig. 2.6. This diagram shows the distribution of sediment in time and space, and brings out the times of subaerial exposure.
Table 2.7 The Cainozoic chronostratigraphical scale with approx- imate ages of the beginning of the series. Ma=millions of years before the present
System Series Stage Ma
Quaternary Holocene 0.1
Pleistocene 1.7
Neogene Pliocene Gelasian
Piacenzian
Zanclean 5.5
Miocene Messinian
Tortonian Serravallian Langhian Burdigalian Aquitanian 24 Paleogene Oligocene Chattian
Rupelian 34
Eocene Priabonian
Bartonian Lutetian
Ypesian 54
Paleocene Thanetian Selandian
Danian 65
Table 2.8 The Mesozoic chronostratigraphical scale with approximate ages of the beginning of the series
System Series Stage Ma
Cretaceous Upper Maastrichtian Campanian Santonian Coniacian Turonian
Cenomanian 99
Lower Albian
Aptian Barremian Hauterivian Valanginian
Berriasian 142
Jurassic Upper Tithonian
(Malm) Kimmeridgian
Oxfordian 156
Middle Callovian
(Dogger) Bathonian Bajocian
Aalenian 178
Lower Toarcian
(Lias) Pliensbachian Sinemurian
Hettangian 200
Triassic Upper Rhaetian
Norian
Carnian 230
Middle Ladinian
Anisian
Lower Olenekian
Induan 251
Table 2.9 The Palaeozoic chronostratigraphical scale with approximate ages of the beginning of the series
System Series Stage Ma
Permian Lopingian Changhsingian
Wuchiapingian Guadalupian Capitanian
Wordian
Roadian 270
Cisuralian Kungurian Artinskian Sakmarian
Asselian 290
Carboniferous Upper Gzhelian
Kasimovian Moscovian
Bashkirian 323
Lower Serpukhovian
Vis´ean
Tournaisian 360
Devonian Upper Fammenian
Frasnian 382
Middle Givetian
Eifelian 395
Lower Emsian
Pragian
Lochkovian 417
Silurian Upper Pridolian
Ludlovian 424
Lower Wenlockian
Llandoverian 443
Ordovician Upper Ashgillian
Caradocian 458
Mid Llanvirn 473
Lower Arenig
Tremadocian 490
Cambrian Upper 500
Middle 511
Lower 545