Early Paleozoic Events

CHAPTER 8

In the Beginning (of the Paleozoic…)

• Phanerozoic = "visible life".

• Refers to rocks younger than the Precambrian. 540 m.y. ago to the presentConsists of three eras (from oldest to youngest):

– Paleozoic = "ancient life“

– Mesozoic = "middle life“

– Cenozoic = "recent life"

• Early Paleozoic = Cambrian, Ordovician and Silurian

• Late Paleozoic= Devonian, Mississippian, Pennsylvanian, and Permian

Figure 3-38 (p. 85)Sedimentary facies (lithofacies) developed in the sea adjacent to a land area. The upper surface of the diagram shows present-day facies, whereas the front face shows the shifting of facies through time. Notice that bottom-dwelling organisms also differ in environments having different bottom sediment and

water depth.

Figure 3-41 (p. 87)An illustration of Walther’s Principle, which states that vertical facies changes correspond to lateral facies changes. (After Brice, J. C., Levin, H. L., and Smith, M. S. 1993. Laboratory Studies in Earth History, 5th ed., Dubuque, IA: William C. Brown.)

• The rocks of each facies become younger – in a landward direction during a marine

transgression

Marine Transgression

• One body of rock with the same attributes – (a facies) was deposited gradually at different times – in different places so it is time transgressive– meaning the ages vary from place to place

older shale

younger shale

• During a marine regression, – sea level falls

with respect to the continent

Marine Regression

– and the environments paralleling the shoreline migrate seaward

Marine Regression

• A marine regression – is the opposite of a marine transgression

• It yields a vertical sequence – with nearshore facies overlying offshore

facies– and rock units become younger in the

seaward direction

younger shale

older shale

Sea Level Changes

Paleogeography• The ancient geographic arrangement of the continents

is referred to as paleogeography ("ancient geography").

• Reconstructing the ancient arrangement of the continents requires paleomagnetic, paleoclimatic, geochronologic, tectonic, sedimentologic, and biogeographic fossil data.

• Paleomagnetic evidence provides information on the latitude at which the rocks formed. The orientation of the continent can be determined from the direction to the paleomagnetic pole, as recorded by bits of iron in the rock.

Paleogeography• Longitudes, however, cannot be determined

(which accounts for some of the differences in the paleogeographic reconstructions).

• Paleoclimatic evidence comes from environmentally-sensitive sedimentary rock types (glacial deposits, coal swamp deposits, reef carbonates, evaporites).

Late Neoproterozoic Paleogeography

• Global paleogeography during the Late Neoproterozoic, about 750 mya. Note that the continents are joined, forming the supercontinent Rodinia (or Proto-Pangea). Rodinia had begun to rift apart. The surrounding ocean is Mirovia.

Late Neoproterozoic Paleogeography

Cambrian Paleogeography• continents have moved off

the South Pole• Iapetus Ocean (sometimes

called the Proto-Atlantic) formed as Laurentia drifted away from South America.

• Note the Ouachita trough segment (green on the map above), which separates off as a microcontinent (see Middle Ordovician map), and eventually converges with South America (orange).

Cambrian Paleogeography

Note that shallow seas cover many of the continents. The evaporite deposits are clustered within about 30 degrees north and south of the equator. This is the latitude at which the great deserts of the world occur today.

Figure 8-2 (p. 269)Paleogeographic and tectonic elements of North America during the Cambrian Period, showing position of the Cambrian paleoequator.

•Warm ocean waters •The presence of stromatolites and mudcracks in these carbonate rocks indicate deposition in shallow water. •The water deepens toward the edges of the continent, where deep water shale is deposited (blue-green). Along the edge of the exposed land mass (yellow), sand is deposited.

Ordovician Paleogeography

• Continents distributed along the equator.

• Note the Ouachita Terrane

• Also note the Taconic Orogenic Belt between Laurentia (North America) and Baltica (Europe and western Russia).

Ordovician Paleogeography

• global sea levels were high. Shallow seas nearly cover many of the continents,

• By the Middle Ordovician, Gondwanaland was moving toward the south pole, which led to glaciation in Africa at the end of the Ordovician.

Figure 7-9 (p. 248)In its long history, the Earth has experienced several major episodes of widespread continental glaciation.

Figure 8-27 (p. 289) Paleogeographic map of Ordovician North America.

•In North America during the Ordovician, sea levels were high, and the craton was flooded.

•In the Appalachian area, the Cambrian and early Ordovician were dominated by shallow water carbonate deposition, as indicated by the presence of mudcracks and stromatolites.

Change in Ordovician Depositional Setting

• The depositional setting changed dramatically during the Middle Ordovician. Carbonate sedimentation ended. The carbonate platform collapsed or was downwarped.

• This was caused by the narrowing of the Iapetus Ocean along a subduction zone. Large volumes of black shales with graptolites, and immature sandstones spread westward over the carbonate rocks. As the Iapetus Ocean narrowed, a volcanic island arc approached the North American craton, leading to deformation and orogenic activity.

Figure 8-24 (p. 286)

Plate tectonic forces that resulted in the Taconic orogeny. (Adapted from Rowley, D. B. and Kidd, S. F. 1981. J. Geol. 89: 199-218.)

Compare Cambrian paleography to Ordovician

Taconic Orogeny

• There are two main highlands areas, the higher of the two is in the northern Appalachians.

• Erosion of Taconic mountains created the Queenston Delta (or clastic wedge)

Silurian Paleogeography

• Note that the Iapetus Ocean is beginning to close as Laurentia and Baltica converge.

Figure 8-30 (p. 291) Paleogeographic map of Silurian North America.

Shallow marine deposits formed in the epicontinental sea, including carbonates with reefs, and the Michigan basin evaporites. Note the Silurian Tuscarora Sandstone in the central Appalachian region.

Early Paleozoic Sedimentary Deposits

The Cambrian-Precambrian Boundary• The base of the Cambrian was long identified by the

first-occurrence of shell-bearing organisms such as trilobites.

• In the 1970's, a distinctive group of small shelly fossils was found below the first trilobites in Siberia and elsewhere, and dated at 544 my. This small shelly fauna includes sponge spicules, brachiopods, molluscs, and possibly annelids.

Tiny Early Cambrian fossils with shells from Siberia.

New classification of the Cambrian base

• The new classification of Lower Cambrian chronostratigraphic units is based on the discoveries of the small shelly fauna and a variety of trace fossils. The base of the Cambrian is now placed at the oldest occurrence of feeding burrows of the trace fossil Phycodes pedum, and dated radiometrically at 544 my using uranium-lead isotope dates from rocks in NE Siberia .

Figure 8-7 (p. 271)

The trace fossil Phycodes. (After Crimes, T. P. 1989. Trace Fossils, in The Cambrian- Precambrian Boundary. Oxford Monographs on Geology and Geophysics 12. Oxford, England: Clarendon Press.)

Early Paleozoic Sedimentary Sequences

• Transgressions and regressions of seas occurred across North America in the Early Paleozoic as the glaciers melted and enlarged. These sequences are bounded by (or separated by) unconformities.

• Lawrence Sloss used the sedimentary record across the North American craton to divide the Paleozoic rock record into unconformity-bounded sequences, sometimes called cratonic sequences or Sloss sequences.

Early Pz Cratonic Sequences

Two major transgressions occurred in North America in the Early Paleozoic, which Sloss named:

1) Sauk sequence (older - primarily Cambrian)

2) Tippecanoe (Ordovician-Silurian)

Cratonic Sequences

• Similar sequences are found on other continents, suggesting that worldwide (or eustatic) sea level change was responsible for the transgressions and regressions.

• These eustatic sea level changes were probably related to glaciations and sea floor spreading. During times of rapid sea floor spreading, ridge formation displaces sea water onto the continents.

Figure 3-42 (p. 88)Sloss sequences correspond to Vail curves of global sea level change. Vail curves are derived from seismic stratigraphic profiles, which permit tracing of unconformities across the craton and into thick continental margin sedimentary rocks.

Cambrian Sedimentary Deposits - Sauk Sequence

• During the Cambrian, there were no vascular plants on the land, so the landscape was barren. Erosion would have been active and severe without plant roots to hold the soil.

• Transgression of the sea onto the craton followed the Neoproterozoic glaciaton.

• Shoreline (beach) deposition produced a vast apron of clean quartz sand.

• Carbonate deposition occurred farther from land.

Figure 8-9 (p. 274)Upper Cambrian

lithofacies map. (Simplified and adapted from Stratigraphic Atlas of North America and Central America. Shell Oil Company, Exploration Department.)

Grand Canyon Region• The Lower Cambrian Tapeats Sandstone is an

example of the sandy beach deposits unconformably overlying Precambrian rocks

• The Tapeats Sandstone is overlain by the Bright Angel Shale, an offshore deposit.

• The Bright Angel Shale is overlain by the Muav Limestone, indicating deposition farther from the land.

• These rocks form a transgressive sequence.

Cross section of Cambrian strata exposed in the Grand Canyon. The red lines are trilobite zones, which approximate time lines.

Note that these sedimentary units are diachronous (cut across time lines). In each case, the sedimentary units are older in the west than in the east.

• Three formations deposited – in a widespread

marine transgression

– exposed in the walls of the Grand Canyon, Arizona

A Marine Transgression in the Grand Canyon

Ordovician-Silurian Deposits - Tippecanoe Sequence

• When the Tippecanoe Sea flooded North America, it deposited the famous St. Peter Sandstone, an unusually pure, well sorted, well rounded quartz sandstone.

• The sandstone is overlain by extensive limestone deposits, locally replaced by dolomite.

• In the eastern US, the limestones are overlain by and interbedded with shales along the periphery of the Queenston delta or clastic wedge. The Niagara Falls area is a classic locality where these rocks are exposed.

Figure 8-15 (p. 278)

Stratigraphic section (A) and block diagram (B) of Niagara Falls. (Map after E. T. Raisz.)

Niagara Falls

Tippecanoe Evaporites

• Near the end of the Tippecanoe sequence, landlocked reef-fringed basins developed in the Great Lakes area.

• Michigan Basin.

• During the Silurian evaporation led to the precipitation of immense quantities of rock salt and gypsum within the basin, indicating an arid paleoclimate.

Figure 8-17 (p. 279) Isopach map showing thickness and area of evaporite basins during

the late Silurian. Areas of evaporite precipitation were surrounded by carbonate banks and reefs. (Adapted from Alling, H. L., and Biggs, L. I. 1961. Bull. Am. Assoc. Petrol. Geol. 45: 515-547.)

Figure 8-18 (p. 280)Cross-section illustrating a model for the deposition of evaporites in a barred basin.

Sea water flows into the basin over the partially-submerged barrier. Evaporation produces dense brines, which sink to the bottom of the basin and are unable to mix with open sea water. When the brine becomes sufficiently concentrated, evaporite minerals are precipitated.

Early Paleozoic Tectonics

Plate Tectonic Events (in order)1. Breakup of Rodinia (Proto-Pangaea)2. Oceanic closing and orogeny to form

Pangaeaa. Taconic orogenyb. Caledonian orogenyc. Acadian orogenyd. Alleghenian orogenye. Hercynian orogeny

Taconic Orogeny

Taconic orogeny occurred in three phasesa. Appalachian Mountains built in collision with

part of western Europe

b. compression folded shelf sediments into mountains and Logan's thrust formed (48 km displacement)

c. Giant granite batholiths produced by Taconic melting

• Evolution of the Appalachian mobile belt• Late Proterozoic opening of Iapetus Ocean

Appalachian Mobile Belt

– with passive continental margins

– and large carbonate platforms

• Middle Ordovician transition to convergence resulted in orogenic activity

Appalachian Mobile Belt

Early Paleozoic Climate

• Transgressions= generally mild climate

• Regressions= harsher, more diverse climates