Elsevier Encyclopedia of Geology

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of fossils that actually lived together and (ideally)

interacted with one another to the extent that the

grouping itself achieved a degree of stability – and

therefore recognizability – as a result of these

interactions.

Because properly defined assemblage zones reflect

ecological relations, this type of zone tends to be

restricted to particular facies. The ability of such

zones to track spatial shifts in environments through

time provides them with a distinctive utility in terms

of palaeoenvironmental analyses. However, this util-

ity comes at a price, and that price is a relatively

reduced ability to achieve long-distance chronostrati-

graphical correlations. For this reason, assemblage

zones of benthic organisms tend to be most useful

on local and regional scales. Assemblage zones of

planktonic organisms do perform well in terms of

chronological correlations, but the degree to which

such organismal groupings are maintained by close

inter-specific interactions is debatable.

Like Oppel zones, the specific criteria used to rec-

ognize assemblage zone boundaries are flexible. Not

all of the groups present in the zone’s ‘type area’ need

be present to recognise the zone in remote locations.

Unlike Oppel zones though, there is an objective and

independent rationale underlying this definitional

latitude. In the case of assemblage zones, one seeks

to recognise a set of dependent ecological relations

among species and between organisms and their

environment that transcend mere faunal and/or floral

lists. The objective reality of such patterns in nature is

well established by numerous studies of modern

faunas and floras and is reasonably well understood

from a theoretical point of view. Oppel zones, on the

other hand, are unified only in the vague sense that

the species used to recognize the zone are thought to

be useful in chronostratigraphical analysis. Although

there is certainly ample justification for suspecting

that, in many cases, the biostratigraphic ranges

of the species in different regions and habitats will

coincide, there is much less justification for regard-

ing these organisms as part of transcendent causal

association than is the case with assemblage zones.

As a result of the flexible manner in which assem-

blage zones are defined, the same groups can be used

to define different assemblage zones (e.g., a coral–

bryozoan assemblage zone and a coral–foraminiferal

assemblage zone can have zone-defining taxa in

common) and different members of the same eco-

logical association can be used to define different

assemblage zones. Assemblage zones have been used

frequently in areas where suitably short-ranging taxa

are not present or have not been studied.

By convention, the name of an assemblage zone

should be based on two or more taxa that figure

prominently in the zone’s definition (e.g., Eponides–

Planorbulinella Assemblage Zone, Eodicynodon

Assemblage Zone).

Interval Zone

An interval zone (also referred to as an interbiohor-

izon zone, gap zone, or a partial-range zone) is de-

fined as a body of strata delineated by the region

between two distinctive biostratigraphic horizons,

but that has no distinctive biostratigraphic identity

of its own (Figure 8). The boundaries of interval

zones can be marked by a wide variety of criteria.

These zones typically represent the undefined regions

between other types of zones; especially taxon range

zones.

An interval zone’s existence assumes a complemen-

tary relation with the underlying and overlying bios-

tratigraphically defined horizons that serve as their

inferior and superior boundaries. As with all other

types of biozones, the traditional one-dimensional

concept of biozone geometry (Figure 8A) can mask

the more complex geometries evident in two and

three-dimensional conceptualizations (Figure 8B). In

particular, interval zones are confined geographically

to only those regions in which the defining biozones

overlap. So long as one’s region of interest is confined

to the geographical area encompassed jointly by the

zone’s defining taxa, recognition of the zone can be

made with confidence. Outside this geographic envel-

ope, though, recognition of an interval zone becomes

problematic, if not impossible.

By convention, interval zones are either named for

the taxa used to define their boundaries (e.g., Globi-

gerinoides sicanus–Orbulina suturalis Interval Zone)

or for a taxon that occurs in the interval, but is not

itself used in the zone definition (e.g., Globigerina

ciperoensis Zone).

Acme Zone

An acme zone (also referred to as a peak zone, flood

zone, or epibole) is defined as a body of strata delin-

eated by the region of ‘maximal development’ of a

taxon (e.g., species, genus, family), but not its total

range (Figure 9). In this context, the term ‘maximal

development’ is meant to be used flexibly. In some

cases it might refer to an initial increase and subse-

quent decrease in the relative abundance of a taxon

that takes place within the confines of its biostrati-

graphic-geographic range. In others, it might refer to

an increase/decrease in body size, an increase or de-

crease in diversity, etc. Since these aspects of a taxon’s

evolutionary/ecological history tend to be strongly

associated with local and regional conditions, it is

on these spatial scales that acme zones have their

302 BIOZONES

Figure 8 One-dimensional (A) and two-dimensional (B) representations of an interval biozone. In this example the zone begins at

the geographic acme of the hypothetical species Exus alphus and encompasses all strata between this extinction datum and the

overlying Exus betus geographic expansion datum. Globally, this interval between taxon range biozones defines a spatio-temporal

region within which the interval biozone’s inferred Total Stratigraphic Range (vertical) and Total Geographic Range (horizontal) are

defined (dashed box). In local sections however, the biozone will be represented by a one-dimensional interval whose boundaries are

included within this global biozone. Note the strong diachrony of the biozone boundaries. Intervals of strata that lie outside the

biozones’ geographical boundaries cannot be referred to the biozone

per se, but can be placed within the corresponding biochrono-

zone (¼ time interval represented by the Total Stratigraphic Range). Many other definitional geometries are also possible (e.g.,

interval between two species’ last appearances, interval between two species’ first appearances).

Figure 9 One-dimensional (A) and two-dimensional (B) representations of an acme biozone. In this example the zone begins at the

first substantial increase in the relative abundance of the species (symbolized by dark shading) of the hypothetical species

Exus alphus

and encompasses all strata in which the species is regarded as abundant. Globally, these abundance-based data define a spatio-

temporal region within which the interval biozone’s inferred Total Stratigraphic Range (vertical) and Total Geographic Range

(horizontal) are defined (dashed box). In local sections, however, the biozone will be represented by a one-dimensional interval

whose boundaries are included within this global biozone. Note the strong diachrony of the biozone boundaries. Intervals of strata that

lie outside the biozones’ geographical boundaries cannot be referred to the biozone

per se, but can be placed within the corresponding

biochronozone (= time interval represented by the Total Stratigraphic Range). Many other definitions of acme are also possible (e.g.,

increased size, increased ornamentation).

BIOZONES 303

greatest utility. Care must be taken when defining

such zones, that the recognition criteria employed

are as explicit and objective as possible. Nevertheless,

the popularity of such zones in regional basin ana-

lyses (particularly in commercial biozonations)

stands as testimony to their practical utility. By con-

vention, interval zones are named for the taxon used

in its definition (e.g., Emiliana huxleyi Acme Zone,

Tylosaurus Acme Zone).

Other Types of Biozones

Although the foregoing descriptions represent the

most frequently used biozone types, other types do

exist. These include such exotica as barren zones,

coiling-direction zones, negative association zones,

species pre-lap zone, species post-lap zone, etc. In

addition, biozone types exist that are not defined on

fossils per se, but rather on the trace fossils left in the

sediment as a result of animal and plant activity (e.g.,

track zones). All such zones are valid only to the

extent that they are useful to the stratigrapher and

geologist, and only to the extent that their definition

is based on unambiguous observational evidence. As

was noted by Hedberg in 1971, ‘Time and usage will

then be the surest means of determining whether it is

desirable for such [zones] to persist’.

Biozones and Biochronozones

Ever since the days of Smith, Lyell, and Oppel, the

purpose of establishing biozones has been to achieve

chronostratigraphical correlations. Other types of

stratigraphic intervals (e.g., key beds, magneto-

chrons, isotope zones) may be based on boundaries

whose emplacement is effectively synchronous over

geological time-scale. None is uniquely identifiable in

the same manner as biozones, however. Indeed, the

use of biostratigraphic methods to place, in an ap-

proximate temporal framework, the observations

upon which these other types of chronostratigraphic

inference depend, is so common it is rarely even men-

tioned. This failure to give biostratigraphical data due

credit for the basic contribution it makes would not

be so unfortunate, were it not for the fact that bios-

tratigraphy has come to be viewed by many as a

lacklustre and unimaginative field of study. Nothing

could be further from the truth. Biostratigraphic data

underpin not only most of what we know about

geological time over the last 600 million years, but

are critical to studies of palaeogeography, palaeoecol-

ogy, extinctions, diversifications, and patterns of

morphological evolution. In order to remind oneself

of the importance of biostratigraphical data, it is

worth recalling that the lack of a detailed time-scale

in which to place Proterozoic and Archaean events is

not due to a lack of palaeomagnetic reversals, cata-

strophic depositional events, isotopic excursions,

etc., but rather to the lack of fossil biotas on which to

base a coherent, finely resolved, and widely accessible

stratigraphy.

This having been said, the distinction between bio-

zones and chronozones must always be borne in

mind. All biozone boundaries are diachronous by

definition and, without recourse to radioisotopic

Figure 10 Results of multivariate biostratigraphic analyses on simulated biostratigraphic data from various geological sections

numbered 1 to 8 using the graphic correlation (A), as implemented by constrained optimization) and ranking and scaling (B) approaches.

These methods focus on the estimation of global, composite, taxon range biozones from datasets containing a series of biostratigraphic

first and last appearance datums for two or more stratigraphic successions or cores. Such approaches offer more consistency,

objectivity, and higher reproducibility of results than either qualitative or pairwise quantitative approaches to biostratigraphic

analysis. In these diagrams, connecting lines between the eight sections represent best-estimate lines of biochronostratigraphic

correlation.

304 BIOZONES

data, it is never correct to regard local biozones as

being equivalent to chronozones. Even when the

physical data necessary for absolute dating are avail-

able, it is no simple matter to accurately interpolate

the ages of biozone boundaries from radioisotopically

dated horizons.

Fortunately, several methods exist whereby strati-

graphers can compare sets of biostratigraphical ob-

servations in different successions or cores and,

through use of a few simple rules, create a summary

of the chronostratigraphically relevant data con-

tained in all such sections/cores. Graphic correlation

(Figure 10A) and ranking and scaling (Figure 10B) are

two of the more common quantitative methods

used for bridging the gap between observational

biostratigraphies and the ideal of a ‘biochronostrati-

graphy’. The ongoing incorporation of raw bio-

stratigraphical data into internally consistent and

synthetic composite successions based on the ap-

plication of these methods, along with improve-

ments in consistent identifying fossil morphologies

(e.g., via automated object recognition) promise to

take the science of biostratigraphy, and its foundation

concept – the biozone – into the twenty-first century.

There it will, once again, serve as the single most

useful contrivance in the practical stratigrapher’s

toolkit.

Glossary

Constrained optimization A mathematical technique

involving the search for the optimal condition or

structure of a system of observations given one or

more consistently applied external rules.

Depositional hiatus A horizon within a body of sedi-

mentary rock that represents a gap in time due to

the nondeposition of sediment, active erosion, or

structural complications.

Diachrony The condition of taking place at different

times.

Facies A stratigraphic body distinguished from other

such bodies by a difference in appearance or com-

position.

Homotaxis The condition of occupying the same

position in a sequence.

Isochrony The condition of being created at the same

time.

Microfossil Fossil shells or other hard parts of tiny

organisms or parts of organisms studied with the

aid of a microscope.

Phylogenetic systematics A method of determining

evolutionary relations between taxa based on the

nested patterns of shared characteristics derived

from the same pre-existing condition.

Stratum (strata) A tabular section of a rock body that

consists of the same type of rock material.

Taxon (taxa) Generalized term for any level within

the Linnean hierarchy of biological classification

(e.g., order, family, genus).

Further Reading

Cubitt JM and Reyment RA (1982) Quantitative strati-

graphic correlation. Chichester: Wiley.

Gradstein FM, Agterberg FP, Brower JC, et al. (1985)

Quantitative stratigraphy. Dordrecht: D. Reidel.

Hedberg HD (1971) Preliminary report on biostratigraphic

units. Montreal, Canada: International Subcommission

on Stratigraphic Classification.

Hedberg HD (1976) International stratigraphic guide: a

guide to stratigraphic classification, terminology, and

procedure. New York: John Wiley & Sons.

Kauffman EG and Hazel JE (1977) Concepts and methods

in biostratigraphy. Stroudsburg, Pennsylvania: Dowden,

Hutchinson & Ross, Inc.

Mann K, Lane HR, and Stein J (1995) Graphic correlation.

Tulsa: Society of Economic Paleontologists and Mineral-

ogists, Special Publication 53.

Miall AD (1984) Principles of sedimentary basin analysis.

New York: Springer-Verlag.

Rawson PF, Allen PM, Brenchley PJ, et al. (2002) Strati-

graphical procedure. London: The Geological Society.

Reyment RA (1980) Morphometric methods in biostrati-

graphy. London: Academic Press.

Shaw A (1964) Time in stratigraphy. New York: McGraw-

Hill.

BIOZONES 305

BRAZIL

F F Alkmim and M A Martins-Neto, Universidade

Federal de Ouro Preto, Ouro Preto, Brazil

Brazil in the Geological Scenario

of South America

The South American continent comprises five major

tectonic units: the Pacific active margin, the Atlantic

passive margin, the Andean Orogen, and the Patagon-

ian and South American platforms (Figure 1). The

continental margins and the Andean Orogen are the

younger portions of the continent; the platforms, on

the other hand, correspond to the mature and ancient

parts of South America.

Brazil is located entirely on the South American

platform, which is defined as the Precambrian core

of the continent, not affected by the Andean oroge-

nies (Figure 1). The exposures of the South American

platform are collectively referred to as the Brazilian

Shield, which in reality encompasses three distinct

morphotectonic domains: the Guyanas, the Central

Brazil (also called Guapore

) and the Atlantic shields

(Figure 1). Sedimentary basins, including large Palaeo-

zoic sags, Cretaceous passive and transform margins,

and Tertiary rifts make up the Phanerozoic cover of

the South American platform (Figure 1).

Figure 1 Tectonic map of South America, showing the main subdivisions. Modified from Almeida FFM, Hasui Y, Brito Neves BB, and

Fuck RA (1981) Brazilian structural provinces: an introduction. Earth Science Reviews 17: 1–29, with permission from Elsevier.

306 BRAZIL

Two fundamentally distinct lithospheric compon-

ents, namely cratons and Neoproterozoic orogens,

form the Precambrian core of South America. The

cratons correspond to the relatively stable pieces of

the continent that escaped the Neoproterozoic oroge-

nies recorded in all the remaining shield areas of

Brazil. Four cratons have been delimited in the South

American platform: the Sa

o Francisco, Amazon, Sa

Luis, and Rio de la Plata cratons (Figure 2). The non-

cratonic segments of the platform are the Neopro-

terozoic Mantiqueira, Tocantins, and Borborema

orogenic domains (Figure 2). (In the Brazilian

geological literature each component of the South

American platform and its cover (i.e. cratons, Brasi-

liano orogens, and Phanerozoic basins) is referred to

as a tectonic province.)

Together with Africa, the South American platform

once lay in the western portion of Gondwana, the

supercontinent assembled by the end of the Neo-

proterozoic and split apart in the Cretaceous, which

also encompassed Antarctica, India, and Australia

(Figure 3). The assembly of western Gondwana

resulted from a series of diachronic collisions, pre-

dominantly between 640 Ma and 520 Ma, the

Figure 2 Simplified tectonic map of Brazil, showing the subdivisions of the South American platform and its Phanerozoic cover.

Modified from Almeida FFM, Hasui Y, Brito Neves BB, and Fuck RA (1981) Brazilian structural provinces: an introduction. Earth Science

Reviews

17: 1–29, with permission from Elsevier.

BRAZIL 307

so-called Brasiliano or Pan-African orogenies. In this

context, the cratons of South America and Africa

are the preserved and more internal portions of the

plates that collided to build up western Gondwana;

the Neoproterozoic orogenic domains encompass

the margins of these plates and other lithospheric

pieces also involved in the tectonic collage that

made up western Gondwana. The dispersal of

western Gondwana and the opening of the South

Atlantic in the Lower Cretaceous broke apart Neo-

proterozoic orogens and cratons. Consequently, the

Neoproterozoic orogens and cratons of eastern Brazil

have African counterparts (Figure 3).

At first glance the geological panorama of Brazil

reflects only the Neoproterozoic assembly, Early

Palaeozoic to Jurassic permanence, and Cretaceous

Figure 3 Schematic map of western Gondwana, showing the cratons and Neoproterozoic Brasiliano–Pan-African belts.

Table 1 The main thermotectonic events recorded in the South American platform

Event Age Occurrence Manifestation Significance

Tertiary

reactivation

Eocene–Oligocene Eastern and

Northern Brazil

Rifting, alkaline magmatism

South Atlantic 130–78 Ma Continental margin and

interior

Flood basalts, followed by

rifting and a late phase of

alkaline magmatism

Gondwana breakup,

opening of the

South Atlantic

Brasiliano 640–520 Ma Non-cratonic areas Deformation, metamorphism,

magmatism, minor accretion

Western Gondwana

assembly

Macau

bas rifting 900–800 Ma Sa

o Francisco Craton and

adjacent belts

Rifting, bimodal magmatism,

glaciation

Rodinia breakup

Rondonian/

San Igna

cio/

Sunsa

1500–1000 Ma South-western Amazon

Craton

Deformation, metamorphism,

magmatism

Rodinia assembly

Staterian rifting 1750 Ma Sa

o Francisco Craton and

adjacent belts

Rifting, bimodal magmatism Atlantica breakup

Uatuma

1800 Ma Amazon Craton Anorogenic magmatism Atlantica breakup,

plume

Transamazonian 2200–1950 Ma Amazon and Sa

o Francisco

cratons, all Brasiliano

orogenic domains

Deformation, metamorphism,

magmatism, crustal

accretion

Assembly of

Atlantica

supercontinent

Jequie

, Rio Das

Velhas,

Aroense

2900–2780 Ma Sa

o Francisco and Amazon

cratons, some Brasiliano

orogenic domains

Deformation, metamorphism,

magmatism, crustal

accretion

308 BRAZIL

dispersal of western Gondwana. A closer examination

of the cratons and Neoproterozoic orogenic belts

reveals, however, a long and diverse pre-Gondwana

history, as well as a whole series of post-Gondwana

features. Thus, in addition to the Brasiliano and South

Atlantic events, other events of the same significance

and extent are recorded in different ways in the South

American platform and its cover. The most important

of these events, together with their ages, areas of

occurrence, and geotectonic significance, are shown

in Table 1.

Regional Structures and Topography

of Brazil

The topography of Brazil to a large extent reflects the

constitution of the South American platform dis-

cussed in the previous section. The cratons underlie

the low areas, hosting the main river basins; the

highlands, on the other hand, have the Neoprotero-

zoic orogenic domains as their substrata (Figure 4).

Each of the large-scale topographical highs and

lows of the Brazilian territory is the expression of a

particular regional tectonic structure. Some of these

structures had already nucleated by the Palaeozoic.

The majority, however, were initiated in the Meso-

zoic and underwent significant reactivation during

the Cenozoic. The most prominent structural and

topographical lows correspond to the Parana

,Sa

Francisco, Parnaı

ba, and Amazonas basins. The

Borborema Plateau, the Serra do Mar Uplift, and

the Alto Paranaı

ba Arch are the largest structural

and topographical highs (Figure 4).

Cratons

The cratons of the South American platform,

consisting of Archaean crust with substantial

Figure 4 Correlation between the large-scale tectonic structures and the topographical relief of Brazil. Relief map compiled by JBL

Franc¸ olim based on EROS GTOPO 30 dataset; reproduced with the permission of the author.

BRAZIL 309

Palaeoproterozoic accretions, are the portions of the

Precambrian basement that were unaffected by the

Brasiliano orogenies. Their boundaries are defined

by the deformation styles of Neoproterozoic cover

strata, patterns of geophysical anomalies, and geo-

chronological data. Accordingly, the cratons of Brazil

are bounded on all sides by Brasiliano basement-

involved fold–thrust belts, but contain Brasiliano

thin-skinned foreland fold–thrust belts. In this regard

they differ from similar features delimited on other

continents. The cratons of Brazil do, however, exhibit

the typical attributes of their equivalents worldwide,

such as mantle roots, low heat-flow values, and high

lithospheric strength.

o Francisco Craton

A map view of the Sa

o Francisco Craton shows a

shape that resembles a horse’s head, with the north-

ern segment merging with the east coast of Brazil

(Figure 5). Except for the Atlantic coast, the Sa

Figure 5 Simplified geological map of the Sa

o Francisco Craton showing the distribution of the major lithostratigraphical units.

310 BRAZIL

Francisco Craton is bounded on all sides by the exter-

nal fold–thrust belts of the Mantiqueira, Borborema,

and Tocantins orogenic domains (Figures 2 and 5). As

indicated by reconstructions of western Gondwana,

the Sa

o Francisco Craton and the Congo Craton of

Africa formed a single lithospheric unit from the

Palaeoproterozoic until the Cretaceous opening of

the South Atlantic.

The basement, made up of Archaean metamorphic

complexes (tonalite–trondhjemite–granodiorite associ-

ation and voluminous calc-alkaline plutons), Archaean

greenstone belts, and Palaeoproterozoic metasedi-

mentary successions, is exposed in the northern lobe

of the craton and in a smaller area close to the south-

ern boundary (Figure 5). In both exposures, Archaean

nuclei are bounded on the east and south-east by

Palaeoproterozoic Transamazonian fold–thrust belts,

which involve the Archaean basement and Palaeopro-

terozoic metasedimentary units. The second-largest

and best-studied mineral province of Brazil, the

Quadrila

tero Ferrı

fero (Iron Quadrangle), lies partly

in the southern portion of the craton and partly in

the adjacent Brasiliano Arac¸uaı

belt. In this pro-

vince, gold is found in an Archaean greenstone-belt

sequence, and high-grade iron-ore deposits occur in a

Palaeoproterozoic banded iron formation.

Alluvial to marine sediments with 1750 Ma acid-

volcanic intercalations at the base (representing the

fill of an ensialic rift system), glacial-influenced

Tonian (ca. 850 Ma) rift sediments, and Cryogenian

(720–600 Ma) foreland strata form the Precambrian

cover of the craton. The foreland-basin strata are

deformed in the areas adjacent to the craton bound-

aries, thereby defining two thin-skinned foreland

fold–thrust belts with opposite vergences. These

belts represent the orogenic fronts of the Neoproter-

ozoic Brası

´lia

and Arac¸uaı

belts, which fringe the

craton to the west and to the east, respectively.

Amazon Craton

The Amazon Craton encompasses an area of approxi-

mately 430 000 km

in north-western South America,

extending far beyond the Brazilian borders into

Colombia, Venezuela, Guyana, Suriname, and French

Guiana. The eastern and south-eastern limits of the

craton are marked by the external portion of the Bra-

siliano Tocantins Orogen, represented by the Ara-

guaia and Paraguay belts, respectively. The western

and north-western boundaries are covered by the sub-

Andean basins, and the northern limit is marked

by the South American equatorial margin (Figures 2

and 6).

More than two-thirds of the Brazilian portion of

the craton is covered by Phanerozoic sediments and

the Amazon forest, so that the geological knowledge

Figure 6 Schematic map of the Amazon Craton, illustrating the main basement components and cover units. Modified from Tassinari

CG, Bettencourt S, Geraldes M, Macambira JB, and Lafon JM (2000) The Amazonian Craton. In: Cordani UG, Milani EJ, Thomaz FA, and

Campos DA (eds.)

Tectonic Evolution of South America, pp. 41–95. 31st International Geological Congress, Rio de Janeiro.

BRAZIL 311

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