HISTORICAL GEOLOGY
LECTURE 8. EARLY PALEOZOIC GEOLOGY I.
Introduction
The Paleozoic is long – 7 periods covering almost
300 million years.
Therefore, we divide it into an early and late period. Cambrian +
Ordovician + Silurian = early paleozoic (126 m.y)
Devonian +
Mississippian +
Pennsylvanian +
Permian = late paleozoic
(165 m.y).
Harry Williams, Historical Geology

Both these "periods" have a similar geologic history characterized mainly by large scale transgressions, orogenic events and regressions.
Each period can be examined in terms of major factors that shaped the geologic record - these factors are:
1. Paleogeography
2. Transgressions and Regressions
3. Orogenies
Why are these important?......
Harry Williams, Historical Geology

1. Paleogeography:
Location of land and sea areas.
Land - often erosion, non deposition; exception = back-arc basin sedimentation, coastal plains, arkosic sandstone, volcanics.
Sea (including epeiric seas) - deposition of clastics and carbonates.
Paleolatitude - refers to the location of the North American craton in relationship to the Equator and Poles.
Controlled paleoclimate, rock types e.g. warm/hot climates -> carbonates, coral reefs, evaporates, sand dunes. Cold climates -> glacial deposits.
Harry Williams, Historical Geology

2. Transgressions and Regressions
Occurred mainly because of tectonic plate movements.
TRANSGRESSION -> shallow ocean in PLATFORM areas
("Epeiric seas" - sea over a continent), which underwent
DEPOSITION and formation of sedimentary rocks.
REGRESSION -> land exposed - usually underwent
EROSION (with some exceptions, including arkosic sandstones) and removal of rocks.
Because marine conditions often meant deposition and terrestrial conditions often meant erosion, a major transgression followed by a regression created a sequence of strata capped by an UNCONFORMITY.
(on board)
Harry Williams, Historical Geology

The green zone represents a cratonic sequence (rocks laid down during a transgression and regression). The lower boundary of the green zone is a transgression; the upper boundary is a regression.
Harry Williams, Historical Geology

The sequence is thicker and more complete near the craton edges since these areas are the first to be covered by the transgressing sea and the last to be uncovered by the regressing sea. The Paleozoic contains a number of these major depositional sequences - the SAUK in Cambrian time and the
TIPPECANOE in Ordovician/Silurian time.
OROGENIES
Mostly resulting from plate convergence and continentcontinent collisions. Mountain chains were constructed along craton margins, consisting of uplifted, folded, metamorphosed marine and terrestrial deposits - often accompanied by intrusive and extrusive igneous rocks (i.e. distinctive suites of rocks) .
These mountain chains were subject to erosion and became sediment sources for later deposits shales + greywackes in deep marine basins; quartz sands and shales in shallow platform seas; fluvial sandstones and conglomerates in terrestrial areas (i.e. more rocks) .
Harry Williams, Historical Geology

1. Paleogeography
750 million years ago
Late Cambrian (514 million years ago):
Rodinia had broken up. The major land masses (Laurentia, Baltica,
Kazakhstania, Siberia, China and
Gondwana) were moving apart.

The North American craton lay along the
Equator during
Cambrian time.
Harry Williams, Historical Geology 8

CAMBRIAN
1. Paleogeography
The North American craton lay along the Equator during Cambrian time. The climate was warm/hot. The craton land area was towards the “northeast” (today) and eroding.
Shallow platform seas existed around the margins of the craton, especially in the south. The deposits forming in these seas were gradational from sand-shale-carbonate, depending on depth of water/distance from shore.
The break-up of Rodinia had crated linear marine basins (seaways) around the edge of the craton:
Appalachian seaway to the S.E./E.
Cordilleran seaway to the W.
Franklin seaway to the N.
These were sites for SHALE deposition.
Harry Williams, Historical Geology

2. Transgressions/Regressions
The Cambrian was a time of TRANSGRESSION - the
SAUK TRANSGRESSION.
As the coastline moved inland, a typical transgressive sequence was deposited:
Old at bottom
COASTAL SANDS
SHALLOW MARINE SHALES
DEEPER MARINE LIMESTONES
Young on top
A good example is provided by the Cambrian deposits exposed in the Grand Canyon:
Harry Williams, Historical Geology

Harry Williams, Historical Geology
Transgressive sequence:
TEPEATS
SANDSTONE
(coastal sand)
BRIGHT ANGEL
SHALE (shallow marine shale)
MAUV
LIMESTONE
(deeper marine limestone)

O: lower Cambrian. G: middle Cambrian. The sequence records the transgression and the migration of the coastline from west to east.
12

Due to the slow progress of the transgression, the same sequence did not develop everywhere at the same time.
E.g. Bright Angel Shale is lower Cambrian in California; middle Cambrian in the Grand Canyon. This is an example of a “Diachronous Rock Unit” (a layer of rock of non-uniform age).
Above: one layer of sandstone; younger to the left, older to the right.
Harry Williams, Historical Geology

Sedimentary environments near the peak of the Sauk transgression (late
Cambrian). This map shows inland areas where the late Cambrian coastlines would have been located. There are coastal sandstones, shallow marine shales and deeper marine carbonates. Orogenic activity is recorded by volcanic rocks.
14

The end of the
Sauk transgression
(Sauk regression) is marked by a widespread erosional unconformity, which in turn is overlain by the
Tippecanoe cratonic sequence. The erosion is prominent above an uplift that formed in the craton.
15

Broad warping of the craton formed these basins and domes during and after deposition of these rocks. Other uplifts are related to orogenic activity.
16

Tepeats sandstone in the Grand
Canyon records lower Cambrian
Suak transgressive sands. St. Peter sandstone records middle
Ordovician, Tippecanoe transgressive sands.
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Are there any lower
Paleozoic rocks around
DFW?
Not exactly, they are west of
Denton and they are upper
Paleozoic (Pennsylvanian and Permian). They are in the Fort Worth Basin.
Lower Paleozoic rocks are underneath them and are exposed around the Llano
Uplift in the Hill Country.
Harry Williams, Historical Geology 18

Section through the western part of the Fort Worth Basin
Harry Williams, Historical Geology 19

3. Orogenies
Harry Williams, Historical Geology

3. Orogenies
The Cambrian was mostly a stable period for North
America. The fragments of Rodinia were moving APART, so the edges of the craton were mainly PASSIVE MARGINS -> no convergence; no subduction; no orogenies.
Harry Williams, Historical Geology

Seaways formed at the craton edges; these were sites for coastal sands and shales, shallow water carbonates and deeper marine shales offshore.
E.g. a thick shallow water carbonate layer formed along the Appalachian
Seaway from
Newfoundland to
Alabama in
Cambrian time.
Harry Williams, Historical Geology