24 Geological Time Scale

M. Manibabu Singh

epgp books

 

Table of contents:

1.  Introduction

2.  Early Principles Behind Geologic Time

3.  Construction Of Geologic Time Scale (GTS)

4.  The Time Scale Creator

5.  Recognizing Geologic Stages

6.  Divisions Of Geologic Time

6.1 Proterozoic Or Precambrian Eon

6.2 The Palaozoic Era

6.3 The Mesozoic Era

6.4 The Cenozoic Era

 

Learning outcomes

  • To know about the geological time scale
  • To understand the early Principles Behind Geologic Time
  • To know about the construction Of Geologic Time Scale (GTS)
  • To know and understand its divisions

 

1.    Introduction

 

The geography and landscape of a region are always changing. Geological research works reveal that the mountains and valleys that surround us or the position of the coastline today have not always been as we know them now. The land that we walk on, in the majority of cases, has risen up from the depths of an ancient sea, and the distribution of land and sea will change through time. These changes result from complex geological processes: sediments that are transformed into new rocks and erosion of rocks that already exist into sediments; uplift or emergence of land areas, with the consequent retreat of the sea, and flooding of other areas, that are invaded by seas and oceans, where accumulation of sediment starts again that will later be transformed into other rocks, followed by renewed emergence and further destruction, etc. By studying the internal structure and composition of rocks, their age (that measured in millions of years) and the way that they are distributed in a region, geologists can reconstruct the way in which the landscape and geography of the region has changed, when the mountains were uplifted that are emerged now, and so on. All of these geological processes are extraordinarily slow from a human perspective, the duration of which is counted in terms of millions of years.

 

Geologists now able to reconstruct the sequence of events that has shaped the Earth‘s surface from the study of petrology, stratigraphy and palaeontology. Many events have occurred since the formation of the Earth about 4.5 billion years ago (or 4500 million years ago). Some of these events have been recorded in the rocks that make up the crust. A chronological organization of these events is recorded on a geologic time scale as a framework for deciphering the history of our planet, Earth. This system of chronological measurement that relates stratigraphy to time, to describe the timing and relationships between events that have occurred throughout Earth’s history is termed as the Geologic Time Scale (GTS). The geological time has started with the deposition of sedimentary rocks; the oldest stratum was created about some four and half billion years ago.

 

2.    Early Principles behind Geologic Time

 

The history of systematic principle of Geologic time begins with a number of works by Geologists and the like since early centuries. Mention may be made of the work of one Danish physician Nicholas Steno (1638-1687) in 1669 who described how the position of a rock layer could be used to show the relative age of the layer. He devised three main principles that underlie the interpretation of geologic time and these principles have formed the framework for the geologic area of stratigraphy, which is the study of layered rock. The first is the principle of superposition which points out that in an undisturbed pile sediments, layer on the bottom was deposited first and so is the oldest, followed in succession by the layers above them, and the topmost one being the youngest in formation. The second one is the principle of horizontality, which states that all rock layers were originally deposited horizontally. The third, that is, the principle of original lateral continuity refers that originally deposited layers of rock extend laterally in all directions until either thinning out or being cut off by a different rock layer.

 

Contribution of a Scottish physician and geologist James Hutton (1726-1797) was the theory of ‗uniformitarianism‘ – which states that the surface of the earth was an ever-changing environment and ―the past history of our globe must be explained by what can be seen to be happening now‖. The theory of uniformitarianism states that the continuing uniformity of existing processes/physical laws is responsible for present and past conditions on earth. This theory was later well-known with a catch-phrased ―the present is the key to the past‖.

 

Another significant work was by William Smith, a surveyor by profession who was in charge of mapping a large part of England. He was the first to understand that certain rock units could be identified by the particular assemblages of fossils they contained. Using this information, he was able to correlate strata with the same fossils for many miles, giving rise to the principle of biologic succession. This principle states that: each age in the earth‘s history is unique such that fossil remains will be unique. This permits vertical and horizontal correlation of the rock layers based on fossil species.

 

The English Geologist, Charles Lyell‘s work (―Principles of Geology‖) during the early 1800s had significant contributions in framing the foundation of Geologic time scale – where he proposed two important principles – i) the principle of cross-cutting relationships and ii) Inclusion principle. His first principle states that a rock feature that cuts across another feature must be younger than the rock that it cuts. The second principle, the Inclusion principle, proposes that small fragments of one type of rock but embedded in a second type of rock must have formed first, and were included when the second rock was forming.

 

3. Construction of Geologic Time Scale (GTS)

 

In fact the geologic time scale is the framework for deciphering the history of the Earth and has three important components (Gradstein, et. al. 2004) –

 

(1) The international chronostratigraphic divisions and their correlation in the global rock record,

(2) The means of measuring absolute (linear) time or elapsed durations from the rock record, and

(3) The methods of effectively joining the two scales.

 

In general the rock record of Earth‘s history is subdivided in a ―chronostratigraphic‖ scale of standardized global stratigraphic units, and is based on relative time units, in which global reference points at boundary stratotypes define the limits of the main formalized units, such as ―Devonian‖. The chronostratigraphic scale is an agreed convention, whereas its calibration to absolute (linear) time is a matter for discovery or estimation. No geologic time scale can be final on the fact that there occur continual improvements in data coverage, methodology, and standardization of chronostratigraphic units (Gradstein, et. al. 2004). There have been major developments in Geological Time Scale research has since 1989 by various international forums. Mention may be made of the works of the International Commission on Stratigraphy (ICS) mainly on refining the international chronostratigraphic scale, such as in the Ordovician or Permian periods, traditional European- or Asian-based geological stages have been replaced with new subdivisions that allow global correlation. Moreover, numerous high-resolution radiometric dates have been generated that has led to improved age assignments of key geologic stage boundaries. The use of global geochemical variations, Milankovitch climate cycles, and magnetic reversals has become important calibration tools (Gradstein, et. al. 2004).

 

4.  The Time Scale Creator

 

One goal of ICS is to provide detailed global and regional ―reference‖ scales of Earth history. Such scales summarize our current consensus on the inter-calibration of events, their relationships to international divisions of geologic time and their estimated numerical ages. On-screen display and production of user-tailored time-scale charts is provided by the Time-Scale Creator, a public JAVA package available from the ICS website (www.stratigraphy.org) and www.tscreator.com, (Gradstein and Ogg, 2006).

 

5. Recognizing Geologic Stages

 

Geologic stages are recognized through their fossil content and not by their boundaries. Since the morphology of fossil taxa and their unique range in the rock record form the most unambiguous way to assign a relative age, these are used as the main method to distinguish and correlate strata among different regions. Obviously the evolutionary successions and assemblages of each fossil group are generally grouped into zones. And, the T S Creator program (www.tscreator.com) includes a majority of zonations and/or event datums (first or last appearances) for widely used groups of fossils through time. Trends and excursions in stable-isotope ratios, especially of carbon 12/13 and strontium 86/87, have become an increasingly reliable method to correlate among regions.

 

Geologists recognize two major segments in the geologic time scale called ‗eons‘. Eons is divided into three smaller time units called ‗eras‘. Eons refer to the longest subdivision based on the abundance of certain fossils recorded. Eras are the next subdivision to eons, marked by major changes in the fossil record. Eras are again divided into ‗periods‘ based on types of life existing at the time. Periods, in turn, have ‗epochs‘, the shortest subdivision marked by differences in life forms and can vary from continent to continent. Constant efforts by Geologists could make improvement of our knowledge of Earth history, and simultaneously attaining an advanced state of standardization in naming the units that elucidate this history. The time scale is expressed both in physical rock units and in abstract time units, the latter often with a numerical uncertainty. The two come together in time/rock units, or chronostratigraphic units in the geological vernacular.

 

Thus, eras, periods and epochs are by themselves the subdivisions of Earth‘s geologic timescale; these refer what happens during these intervals that gives each their unique characteristics. The progression from one stage to another is marked by some easily distinguishable, global stratigraphic ‗event‘, such as a mass extinction, bulk change in the composition of sedimentary rocks or shift from one climate regime to another, change in the composition of organism.

 

‗Ages‘ are expressed in units of million years measured back in time. Geologists use the designation ‗Ma’ to mean ‗millions of years ago‘ and ‗My’ to indicate a ‗million years of time‘. It is sometimes hard to imagine how long the geologic ages were and how far away from us in time they are. We can get a better idea if we scale the time. We can compress millions of years in a meter of space. To get the best idea of the different lengths of the ages we need to make two different scales, one for all of geologic time (all 4500 my) and the other for the fossil record.

 

6. Divisions of Geologic Time

 

The developing framework for discussing the Earth‘s history is known as the geologic time scale (GTS) and has been based largely on the fossil record. The subdivisions based on the hierarchical system of time intervals are identified by a characteristic assemblage of fossil forms and plate tectonics. The modern geologic time scale, in which the interval boundaries are also identified by their age as resolved through radiometric dating, was pioneered by Arthur Holmes.

 

In the GTS, Earth‘s history is divided broadly into two Eons, – Proterozoic or Precambrian Eon (4500-635 Ma), the earlier and Phanerozoic (635 Ma till present), the later. Within the first, Proterozoic or Precambrian Eon, there has three components, that is, Hadean, Archean and Proterozoic. The Hadean refers to a period of time which has no rock record. The Archean that followed corresponds to the ages of the oldest known rocks on earth. The Phanerozoic Eon has three eras – Palaeozoic, Mesozoic and Cenozoic, which in turn are divided into ‗Periods‘ and ‗Epochs‘.

 

6.1 PROTEROZOIC or PRECAMBRIAN EON

 

Precambrian that spans 88 percent of Earth history is taken to be started its time period from the time of initial accretion and differentiation (ca. 4560 Ma) to the first appearance of abundant hard-bodied fossils (the onset of the Cambrian Period at 541 Ma) (Gradstein, et.al. 2005). And, there is no coherent view of a geological time scale to help describe, analyze, calibrate, and communicate the evolution of planet Earth (ibid).

 

The Precambrian time scale, being incomplete and flawed (e.g., Cloud, 1987; Crook, 1989; Nisbet, 1991; Bleeker, 2003), is generally defined in terms of arbitrary, strictly chronometric, absolute age boundaries that are divorced from the only primary, objective, record of planetary evolution: the extant rock record (Gradstein, et.al. 2005). NUNA Conference (Geological Association of Canada), 2003 under the co-sponsorship of the International Committee on Stratigraphy (ICS), had a broad consensus that this arbitrarily defined Precambrian time scale fails to convey the richness of the Precambrian rock record. It has been suggested that ‗the Precambrian time scale should be (re)defined in terms of the only objective physical standard we have, the extant rock record. Boundaries should be placed at key events or transitions in the stratigraphic record, to highlight important milestones in the evolution of our planet. This would be analogous to the ―golden spike‖ GSSP approach employed in the Phanerozoic‘ (Gradstein, et.al. 2005). ‗Golden spike‘ is an informal term for a physical point in a stratigraphic section that defines a chronostratigraphic boundary. Pioneered by the Stratigraphic Committee of the London Geological Society, the ―golden spike‖ has been useful in resolving intractable and unending problems arising from conceptual definitions. (Delson, et.al. 2000: 607)

Precambrian eon comprises three Ages, namely, Hadean, Archean, and Proterozoic. Earth takes ten million years to cool: initial atmosphere escapes into space (H and He) and the core forms (Fe and Ni). This stage is characterized by the volcanic outgassing of water and carbon dioxide that occurred for millions of years, helping to build atmosphere and then oceans. At about three billion years ago, banded iron formation rocks appear due to rising oxygen levels in the atmosphere and sea. No life possible as the Earth initially forms 4.6 billion years ago. However, simple, single-celled forms of life appear 3.8 billion years ago and they will become more complex and successful over the next 3 billion years: Prokaryotes then Eukaryotes Cyanobacteria begins producing free oxygen (photosynthesis).

 

Hadean: Hadean (4.5 to 4 billion years ago, – GTS 2012) is not a geological period as such, and is characterized by the intense bombardment and its consequences, but no preserved supracrustals (Cloud, 1972). There is no evidence of rock formation during the time, except the meteorites. During Hadean time, the Solar System was forming, probably within a large cloud of gas and dust around the sun, called an accretion disc. The relative abundance of heavier elements in the Solar System suggests that this gas and dust was derived from a supernova, or supernovas – explosion of an old, massive star. It is a general observation that the sun formed within such a cloud of gas and dust, shrinking in on itself by gravitational compaction until it began to undergo nuclear fusion and give off light and heat. Surrounding particles began to coalesce by gravity into larger lumps, or planetesimals, which continued to aggregate into planets and the “left-over” materials formed asteroids and comets. The Earth and other planets would have been molten at the beginning because of collisions between large planetesimals release a lot of heat. Solidification of the molten material into rock happened as the Earth cooled. The oldest meteorites and lunar rocks are about 4.5 billion years old, but the oldest Earth rocks currently known are 3.8 billion years. It is also suggested that the surface of the Earth changed from liquid to solid sometime during the first 800 million or so years of its history. Once solid rock formed on the Earth, its geological history began. This most likely happened prior to 3.8 billion years, but hard evidence for this is lacking. The advent of a rock record roughly marks the beginning of the Archean eon.

 

Archean: The Archean eon (spanned about 1.5 billion years) has four eras: the Neoarchean (2.8 to 2.5 billion years ago), Mesoarchean (3.2 to 2.8 billion years ago), Paleoarchean (3.6 to 3.2 billion years ago), and Eoarchean (4 to 3.6 billion years ago). The GTS 2012 gave the time span of this eon within 4000-2500 Ma. During Archean there increased crustal record from the oldest supracrustals of Isua greenstone belt to the onset of giant iron formation deposition in the Hamersley basin, likely related to increasing oxygenation of the atmosphere. Earth’s crust cooled enough that rocks and continental plates began to form. During its early period life first appeared on Earth and the oldest fossils comprise that of bacteria microfossils, date to roughly 3.5 billion years ago. Stromatolites, the photosynthetic bacteria, as fossils are found abundantly at the Archean coast, in the early Archean rocks of South Africa and Western Australia. These fossils increased in abundance throughout the Archean, began to decline during the Proterozoic.

 Fig:2- Stromatolite fossil (Archean)

 

Proterozoic: Proterozoic Eon experienced first stable continents, nearly modern plate-tectonic Earth but without metazoan life, except at its very top. Land masses gather to make up a continent called ―Rodinia‖ First abundant fossils of living organisms (bacteria, archaeans, eukaryotic cells) are evident with first evidence of oxygen build-up in the atmosphere. Cyanobacteria begins producing free oxygen (photosynthesis) GTS 2012 gave its time span between 2500-635 Ma.

 

6.2 THE PALAOZOIC ERA(―Age of Invertebrates‖)

Next to Proterozoic is the Palaeozoic Era which is divided into seven different periods, such as – Vendian, Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.

 

1. Vendian or Ediacaran: Vendian or Ediacaran refers to the first period of the Palaeozoic era lasting from about 635 to 541 million years ago (Ma), and is characterized by macroscopic fossils of soft-bodied organisms. Kimberella, Dickinsonia and Pteridinium are some of the Vendian fossils of great attention to the palaeontologists which occur in the Vendian rocks. Kimberella, a bilaterally symmetric animal that had rigid parts appears to be somewhat like a mollusc, and these are known from Vendian rocks of South Australia, and also Pteridinium having an elongated, ribbed body from Namibia and North Carolina.

 

A period first proposed by one Boris Sokolov, a Russian geologist and palaeontologist in 1952, the Vendian concept was developed stratigraphically top-to-down, that is, the lower boundary of the Cambrian became the upper boundary of the Vendian (Sokolov 1952, 1997). Vendian Period overlaps the Ediacaran period which is also taken to be synonymous to the former. Unlike later portions of the geologic time scale, the Vendian has neither formal subdivisions nor distinct early boundary. This is in large part due to the fact that it has only recently become a subject of interest to paleontologists. The genesis of the term Ediacaran period traced back with an exploratory work in 1946 by an Australian mining geologist named Reginald C. Sprigg in the Ediacara Hills on the western edge of the Flinders Ranges to north of Adelaide city, South Australia – where he found well-preserved fossilized imprints of eponymous biota of what were apparently soft-bodied organisms (which Sprigg referred to as ‗medusoids‘) mostly on the undersides of slabs of quartzite and sandstone. He could also establish their chronology to the late Precambrian period. In fact, the Ediacara Hills gave a name to the entire “Ediacara biota” of the late Precambrian. The Ediacaran Period as a geological period was officially ratified in 2004 by the International Union of Geological Sciences (IUGS) (Knoll, et.al 2004a, b; 2006).

2.    Cambrian (spanning between 541 – 485.4 Ma) is characterized by the explosion of life where all existing phyla come into being at this time, yet does not yield anything higher than invertebrate animals. Different life forms are seen in warm seas as oxygen levels raise enough to support life and dominant animals include marine invertebrates (trilobites and brachiopods). A significant development is the creation of Gondwana as a supercontinent near the South Pole.

3. Ordovician:  The  third  period  under  the  Paleozoic  eon  bears  the  first  traces  of vertebrates.  The   first   animals   with   bones  appear,   though   dominant   animals   are   still    trilobites, brachiopods and corals. Primitive fish, seaweeds and mollusks appear at this time. The beginning of this period has been dated between 485.4 to 443.8 Ma (GTS 2012). This epoch experienced very cold climatic condition. Important developments include the beginning of the construction of South Carolina and formations of four main continents, namely, Gondwana, Baltica, Siberia and Laurentia.

 

4. Silurian: The fourth Silurian period traces the fishes with bony skeleton. Ancestors of shark and dogfish were found in this period. First land-plant appeared which begin to colonize barren land, and also followed by land animals. Coral reefs expand as well. First millipede fossils and sea scorpions (Euryptides) found during this epoch. Another significant feature is Laurentia collides with Baltica and closes Iapetus Sea. The period began about 443 Ma and continued for about 25 my.

 

Fig:8 – Sea scorpion (Euryptides) fossil

 

5. Devonian: This period shows fish as a dominant aquatic animal, and shows the presence of ancestors of the present day air-breathing lungfish as well as some typical bony fishes. Hence, Devonian is also known as ‗the age of Fishes‘. Oceans were still freshwater and fish migrate from southern hemisphere to North America. Besides a transitional form of life between fish and four-footed land-dwelling vertebrate (amphibian) was found to appear in this age. Spreading of the forest was the other important feature of this epoch and hardwoods began to grow. Also amphibians, evergreens and ferns appear. Present-day Arctic Canada was at the equator. The epoch started by about 419 Ma and persisted for about 60 my.

Fig:9 – an amphibian species

 

6. Carboniferous: The significant features of Carboniferous period include formation of Gondwanaland and small northern continents, and there had abundant occurrences of fish along with primitive forms of amphibians and of early reptiles. Others include also the appearance of different types of insects, spiders. There had extensive forests of early vascular plants, especially lycopsids, sphenopsids, ferns. This epoch began about 359 Ma and lasted for 60 My.

 

Two series of development within this epoch, namely Mississippian and Pennsylvanian sub-system, have been ratified in 2000 and internationally approved in the year 2004. The time spans of these series are recorded between 326.4 – 359.2 Ma and 309.9 – 318.1 Ma (GTS 2004) respectively.

 

The name Mississippian was first coined by an American geologist Alexander Winchell in 1869 referring the rock outcrops along the drainage basin of the Mississippi River. He distinguished these limestone-rich Lower Carboniferous rock layers from the coal-bearing beds of the Upper Carboniferous (or Pennsylvanian). Important features of the Mississippian series include appearance of first seed plants. During this period much of North America is covered by shallow seas and sea life flourishes (including coral, brachiopods, blastoids, and bryozoa). And also first seed plants appear.

 

Pennsylvanian Subsystem was named for Pennsylvania, home of some of North America‘s richest coal seams. The name was coined in 1891 by Henry S. Williams for the Upper Carboniferous rock layers of North America. This series showed the initial formation of modern North America. Ice covers the southern hemisphere and coal swamps formed along the equator. Major animal species appeared include lizards, reptiles develop from amphibians. Flying winged insects also appear.

 

7. Permian: The last of Paleozoic era is Permian. Continents aggregated into Pangaea It is the age of large amphibians and also the land dwelling reptiles which spread across continents. The rise of Appalachians is another significant feature. Owing to volcanic phenomenon in Siberia, there had major mass extinctions (about 90% of Earth‘s species) especially the marine life at end of period. These include most of the species of fishes, trilobites, ammonoids, blastoids, etc. The epoch started approximately about 300 Ma and continued for 48 My.

 

6.3 THE MESOZOIC ERA(the ‗Age of Reptiles‘)

 

Mesozoic era comprises three periods, Triassic, Jurassic and Cretaceous – each has distinct characters of its own in terms of the development of plate tectonic and of life forms.

 

Triassic: The first period of Mesozoic, the Triassic, is characterized by the initial separation of continents where Pangea breaks apart and formation of Rocky Mountains and Sierra Nevada. Marine diversity increases; ‗gymnosperms‘ become dominant. Other significant features are the diversification of ‗reptiles‘ including first dinosaurs and first mammals, and crinoids, and modern echinoids. Evidenced of first turtle fossil from this period.

 

Jurassic: Continents separating, Pangea still breaking apart; North America continues to rotate away from Africa; Dinosaurs flourish -―Golden age of dinosaurs‖, First birds appear, archaic mammals; ―gymnosperms‖ dominant; evolution of angiosperms; ammonoid radiation; characterized by the ―Mesozoic marine revolution‖.

 

Cretaceous: Cretaceous being the last period of Mesozoic is characterized by the separation of most continents, continued radiation of dinosaurs, angiosperms appear with increasing diversity. T-Rex develops but number of ammonoids and dinosaur species decline at end of period and also demise of about 25% of all marine life caused by a meteorite impact. Snakes, birds and first primates appear, deciduous trees and grasses common. Mass extinction marks the end of the Mesozoic Era.

 

6.4 THE CENOZOIC ERA (the ―Age of Mammals‖)

 

The last and most recent of the geologic time is the Cenozoic Era (66.4 to 0.01 Mya), which means ‗new life‘ – the term of which is derived from two Greek roots kainos, (new) and zoic (life). Among others, the two most important features of this era are – i) the rise of the mammals, and ii) appearance of the angiosperm or flowering plants, besides the insects, the newest fish (teleostei) or modern birds. This final era is divided into two major periods – Tertiary and Quaternary.

 

Tertiary Period

 

The Tertiary Period lasted between 66 – 2.59 Ma (GTS 2012). There had warm and moist climatic condition in the beginning of the period but soon, cooling had lead to an ice age. Major Events of the period include the development of all modern phyla, formation of the current configurations of continents and the rise of mammals. Obviously, there were major fragmentation of two super-continents – the Gondawana of Southern Hemisphere and Laurasia of Northern Hemisphere. Hence, Australia was separated from Antarctica during late Palaeocene, Africa from India (and India collided with Asia), and Greenland from Europe. Life forms developed are the primitive whales, monkeys, cats/dogs, pigs, rhinos, horses, the modern forms of whales, etc. The term Tertiary was introduced first by Giovanni Arduino in 1760 to denote –

 

i) the youngest of a tripartite division of the Earth’s rocks, such as, the Primitive schists, granites, and basalts that formed the core of the high mountains (of Europe);

 

ii) the fossiliferous Secondary, or Mesozoic, in northern Italy (predominantly shales and limestones); and

 

iii) a younger group of fossiliferous sedimentary rocks, the Tertiary rocks, found chiefly at lower elevations.

 

In 1810 Alexandre Brongniart included all the sedimentary deposits of the Paris Basin in his terrains tertiares, or Tertiary, and soon thereafter all rocks younger than Mesozoic in Western Europe were called Tertiary. The recognition of the Quaternary Period in 1829 by Jules Desnoyers—based on the post-Tertiary deposits of the Seine valley—placed a somewhat different connotation on the term Tertiary, particularly in regard to its upper limits. Quaternary is not a satisfactory name in the hierarchy of stratigraphic nomenclature. The terms Primary and Secondary have been supplanted by Paleozoic and Mesozoic, and Tertiary is being gradually replaced by Paleogene and Neogene as formal period names in scientific literature. Some authorities prefer not to use the term Tertiary and instead divide the time interval encompassed by it into two periods, the Paleogene Period (66.4 to 23.7 million years ago) and the Neogene Period (23.7 to 1.6 million years ago).

 

Charles Lyell‘s (1833) classification of Tertiary was on the basis of the relative percentages of living species of mollusks to fossil mollusks found in different layers of tertiary rocks. His four-tier division was made mainly from those finds from in West European Basin, such as,

 

iv. New Pliocene (later renamed as Pleistocene) – 90 % of living mollusk,

iii. Pliocene           – 1/3 or over 50 % of living mollusk,

ii.   Miocene         – 20 % of living mollusk,

i.          Eocene– 3 % of living mollusk.

Tertiary Era is classified into five epochs namely – from oldest to youngest Paleocene, Eocene, Oligocene, Miocene, and Pliocene.

 

The earliest epoch, the Paleocene dates back about 66-56 Ma (GTS 2012) with a duration of about ten million years. Palaeocene is defined generally on the basis of fossil flora (that is from non-marine strata) and the term was accepted formally by the United States Geological Survey in 1939. This period witnessed extinction of dinosaurs and the emergence of primates. Primitive lemuroids and tarsioids came into existence along with insectivores. Tropical plants dominate during the epoch. Also seen the appearance of first horses of the size of a cat.

 

Eocene: Started around 56 Ma and continued about 23 million years, this epoch shows the appearance of modern mammalian orders. Besides, an abundance small primate (prosimians) was noted in this epoch for which the epoch is often referred as the ‗Golden Age‘ of the Prosimians. Emergence of angiosperms with their massive radiation, and appearance of many new species of small plants, trees and shrubs are some of the significant features of this epoch. The Eocene is typified in shallow marine strata of the Paris-London Basin, which interfinger with mammal-bearing beds laid down on adjacent coastal plains. This well-documented correlation between marine and nonmarine faunas in the type area supports a reliable worldwide chronostratigraphy.

 

Oligocene: No formations of Oligocene age were included in Charles Lyell‘s review of European stratigraphy when he formulated the Eocene and the Miocene in 1833, and, in fact, he used the great difference between the fossils of these two epochs as a useful demonstration of a hitherto unappreciated vastness of geological time. Spanning during 33.9 to 23 Ma (GTS 2012) the Oligocene epoch is characterized by the replacement of tropical and subtropical forests by the temperate deciduous woodland in some parts of the world. Angiosperms continued their expansion throughout the world. Another significant development in life forms is the appearance of the New World monkeys. The ancestors of the Old World monkeys and primitive anthropoid apes like Propliopithecus, Parapithecus etc also appeared in the Old World. Oligocene is divided into two global stages or ages, the Rupelian and the Chattian, which are typified in shallow marine sequences in the North German Plain.

 

Miocene: The Miocene epoch, that spans the interval between 23.5 and 5.3 Ma, was introduced in 1833 by Sir Charles Lyell for the marine strata representing the time when 40–60 percent of the extinct molluscan species. The Miocene, like all chronostratigraphic units, is framed in a hierarchical series, and its limits are thus defined by its six subordinate stages. The epoch showed divergence of monkeys and apes from their ancestral stock. During its last phase, Dryopithecus, Sivapithecus, Proconsul etc. appeared. Deer, hyenas, giraffes also appeared. Sea whales proliferated and corals and gastropods were plentiful.

 

Pliocene: Pliocene (5.33 -2.59 Ma), the last epoch of Cenozoic era was marked by increasingly wide swings in global climates, but without the intense short-term cyclicity of the Pleistocene. During the warm-climate intervals, the winter frost line retreated virtually to the Arctic Circle, and seasonal variation in rainfall was moderate, in contrast to cold winters and summer-dry seasonality during the progressively more intense cold-climate intervals. Significant expansion of ice caps during the cold-climate intervals is indicated by tillites and ice-rafted debris at high latitudes, as early as 3 Ma in Norway and Iceland, and evidence for worldwide lowering of sea level.

 

This epoch witnessed the appearance of dominant mammalian faunas, like sheep, goat, antelope, cattle, etc. Besides there was rise of higher primates and the forerunners of the present day anthropoid apes also evolved distinctly during this period. Different mountain ranges were formed subsequent to the earthquake, volcanic action, etc.

 

Quaternary Period

 

Quaternary refers to the last period of Cenozoic era and is divided into two epochs – the Pleistocene and Holocene. This era begins from 2.59 Ma (GTS 2012) and is continuing still today. The significant events include origin and evolution of modern humans, glaciations and pluviation, and flourishing of humans in the later parts.

 

Pleistocene epoch (2.59 – 0.78 Ma): The term Pleistocene (Gr. Pleistos, meaning ―most‖ and kainos, meaning ―new‖ or ―recent‖) was introduced by Sir Charles Lyell in 1839 to describe marine strata in the Mediterranean region that contain molluscan faunas, the species of which are more than 70 percent living. In the light of present knowledge such a definition would include much of the time now universally assigned to the late Tertiary. In 1846 Forbes used the word Pleistocene to apply to the ‗glacial epoch‘ thus giving a climatic implication – a redefinition to which Lyell agreed in 1873. A definition based on climatic change as evidenced by continental glaciations is almost universally used for Pleistocene in central and northern North America as indicated by the official usage of the U. S. Geological Survey (Wilmarth, 1925: 49).

 

Although this new, glacial-age definition seemed reasonable at the time, it is now inaccurate to view the Pleistocene as equivalent to the occurrence of glaciations. The reasons for this are twofold. First, full-scale continental glaciations began around one million years ago, well after the start of the Pleistocene at 1.8 million years ago, and not all parts of the Earth were affected at the same time. Second, the existence of pre-Pleistocene glacial events was not known by Lyell or Forbes, but it is now known that glacial conditions existed periodically throughout Earth’s history, even in Precambrian times.

 

The Pleistocene is a unique epoch because it is the period during which our own species, Homo sapiens, evolved. It is also marked by climatic fluctuations that culminated in widespread continental glaciers and these phenomenons obviously stimulate in human evolution. And many species of vertebrates, especially large mammals, went extinct during the Pleistocene, but much of the modern flora and fauna are survivors from this epoch.

 

Major evidences, that show that there were Great Ice Ages during the Pleistocene times, include – Moraines and Loesses, Changes in flora and fauna, River terraces, Sea-level Changes and so on.

 

Glacial cycles were not the only geological and climatic characteristics of the Pleistocene. Volcanic activity was also occurring in the rift valleys of Africa and in western North and South America. In southwestern North America, the Colorado River began to carve out the Grand Canyon. Although the Pleistocene represents a brief portion of geologic time, it includes detailed records of profound changes in climate and landscape.

Holocene epoch: First proposed in the third International Geological Congress 1885, the term Holocene refers to the most recent division of Earth history. Terms like Recent or Postglacial were also used for this epoch until 1967, when the U.S. Geological Survey formally adopted the term ‗Holocene‘ and discontinued the use of ‗Recent‘. The Holocene, which covers the last 11,500 years of Earth history, is an important chronostratigraphic division that follows the Pleistocene Epoch. Significantly, during this epoch most of our modern landscapes and soils evolved. In addition, significant changes in global climate occurred as the Earth moved into a postglacial or interglacial regime. The name and faunal definition of Holocene are consistent with the criteria for Cenozoic epochs proposed by Charles Lyell in 1833, but the internal subdivision of the Holocene and even its traditional boundary have been identified with climatostratigraphic transitions. Important features include – flourishing varied Human culture, extinction of mastodons and the formation of barrier islands and beaches. In many parts of the world witnessed formation of barrier islands and beaches.

 

Basing on the variations in air-temperature and precipitation regimes, Holocene can be divided into three climatic stages – i) the earliest or the anathermal stage (spanning from 11Ka to 3Ka) which was cooler and mostly wetter than today, ii) the hypsithermal (or altithermal) stage – ranging between 1000 BCE and ca. 1400 CE, when climate was warmer and mostly drier than today, and iii) the medithermal stage, also known as the Little Ice Age (from 1400 to 1900 CE) that reached a relatively cold, wet-climate minimum ca. 1650 CE. However such a division is not widely accepted, owing to current concerns about ozone depletion and carbon dioxide loading.

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