A Chronological Survey of Sedimentary Landforms in the Continental United States/Printable version

A Chronological Survey of Sedimentary Landforms in the Continental United States

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Introduction

Typical marine sediments showing cyclic deposition

Introduction

Sedimentary landforms are prominent in every region of the United States. Each time period has at least one exposure that typifies the environment that existed when these deposits were formed. Most sedimentary deposits were laid in water, but there are also subaerial forms of sedimentation, including most notably the aeolian (wind-blown) sand and loess deposits of the Great Plains. In general, sediments of recent time will be unconsolidated, while those of older times tend to be well cemented. Each sedimentary layer represents an episode of deposition. A local succession of sedimentary beds can be read like the pages in a book, giving information about the environment at that locality at that time. The normal sequence is from oldest at the bottom to youngest at the top, but folding and faulting can and often does obscure this relationship.

Sedimentary units composed of beds that bear a stratigraphic relationship to one another are designated formations. There may be few or many individual beds within a formation, and these may be divided into members. Sometimes a single rock unit is sufficiently prominent to stand alone as a stratigraphic unit, in which case it will be named for the rock type. (Example: the Bolsa Quartzite). Stratigraphic units may grade vertically from one to another without visible break, or they may be unconformable upon one another. Unconformities and disconformities may be erosional or angular in nature. Erosional unconformities represent a time when the unit was exposed to the actions of weathering, either because the land was uplifted or the sea level declined. Structural (angular) breaks may represent tilting, faulting, slumping, or other mechanisms that caused a hiatus in deposition. These are usually obvious in marine sediments, but may be difficult to distinguish in cross-bedded aeolian sandstones.

Sediments, like all rock types, can undergo metamorphism. Heat and pressure produced either by deep burial or tectonic events can cause the solution and recrystallization of mineral grains, and drastically alter the appearance of the rocks. Thus sandstones may become very hard quartzites, mudstones become shales and slates, limestones become marble or dolomite. Igneous activity far below can send hot fluids and gases up through the layers, sometimes producing commercially valuable ore deposits. For purposes of this survey, metamorphosed sediments are still sediments, even though they may properly fall into the category of metamorphic rocks.

Fossils are the special feature of sediments. Fossils rarely survive any great degree of metamorphism, so they are found only in the unaltered sediments. Shales and limestones are the most abundant fossil sources, but other sediments occasionally yield good specimens. In this book, fossils will be briefly described when they constitute a significant feature of the period or the locality.

While each chapter of this book will cover the time period for which it is named, it must be kept in mind that the geologic column is a continuum. With the possible exception of the famed KT Boundary, the time periods graded rather slowly from one to the other, such that it may be difficult to distinguish the "boundary" in any given cross-section. In addition, the example localities presented here must not be taken as representative of the global (or even regional) conditions of the time. These events were localized to a specific set of conditions, and wide differences are likely as one travels to distant areas. With that, let us proceed to open the book and read the last 540 million years of Earth's history.

Ordovician

Ordovician period 488.3± 1.7 Ma to 443.7±1.5 Ma
Mean atmospheric O2 content over period duration	ca. 13.5 Vol % (68 % of modern level)
Mean atmospheric CO2 content over period duration	ca. 4200 ppm (15 times pre-industrial level)
Mean surface temperature over period duration	ca. 16 °C (2 °C above modern level)
Sea level (above present day)	180m; rising to 220m in Caradoc and falling sharply to 140m in end-Ordovician glaciations

The Ordovician period opened with no sedimentary break from the Cambrian, but is nevertheless distinct because the fossil assemblage changes drastically. Many Cambrian marine species disappeared or were greatly reduced in numbers in what has been called the Cambrian-Ordovician Extinction Event (Wikipedia article). The most useful group for dating the Cambrian-Ordovician boundary is the Brachiopods. This group was dominantly composed of inarticulated forms (lacking a specific hinge structure between the shell halves) during the Cambrian, but the relative abundance is reversed at the beginning of the Ordovician. The hinge may have provided some added protection against predators, which were now beginning to appear in the fossil record.

The Ordovician period opened with the Cambrian Sauk Seas still dominating the landscape. The Sauk transgression reached its maximum extent during Early Ordovician time, then began a slow regression that culminated in the complete removal of the epeiric sea at the end of the Early Ordovician. It is thought that this regression represents a period of cratonic uplift due to tectonic forces. The regression of the Sauk Seas left behind a blanket deposit of shoreline sandstones that represent erosion of the newly emerged craton.

This exposure of the craton to subaerial erosion appears to have lasted for a very long time. No depositional sequence exists by which we may determine exactly how much time is missing, but the fossil record suggests it was many millions of years. During this time, what little geographic relief the craton possessed was eroded down, and the clastic remnants of that erosion are present in the sand layer. The flat, sand covered surface was extensively reworked by erosional forces, and eventually became quite uniform in composition across wide areas of the craton.

Cambrian

Cambrian period 542± 0.3 Ma to 488.3± 1.7 Ma
Mean atmospheric O2 content over period duration	ca. 12.5 Vol % (63 % of modern level)
Mean atmospheric CO2 content over period duration	ca. 4500 ppm (16 times pre-industrial level)
Mean surface temperature over period duration	ca. 21 °C (7 °C above modern level)
Sea level (above present day)	Rising steadily from 30m to 90m

The Cambrian is the first period of the Paleozoic era of the Phanerozoic eon. The beginning of Cambrian time coincides with a mass extinction event that depleted late Precambrian life forms and made way for a virtual explosion of new life. It is thought that the Ediacaran period, which closed Precambrian time, saw the emergence of Earth from a long and bitter ice age. Melting ice and warming temperatures encouraged the growth of algae and other oxygen producing organisms. The atmospheric oxygen content, which had been as low as 8% during the Ediacaran, increased to 12.5%. At the same time, volcanic gases were pumping carbon dioxide into the atmosphere, creating a greenhouse effect that reinforced the warming trend.

The Cambrian climate on the global scale was much warmer than we see today. Other than some primitive algal and lichen species, there were no land plants and certainly no land animals. But life in the warm, shallow seas was abundant and diversified. The Cambrian was characterized by transgressive-regressive episodes during which the sea levels repeatedly shifted. It is thought that this resulted from a combination of rapid continental drift and the melting of the last remnants of the previous ice caps.

One cannot talk about Cambrian life forms without mentioning the famed Burgess Shale (Wikipedia article), but strictly speaking, the bulk of that locality lies in Canada. We will therefore confine our locality discussion to the Sauk Transgression, as typified in the lowermost sedimentary formations of the Grand Canyon in Arizona.

During Cambrian time, shallow epeiric seas covered large portions of the continent. The general trend was toward increasing depth and areal extent of these seas, but there were also numerous episodes of retreating seas as land masses were uplifted by tectonic forces. The Sauk Sequence is, therefore, typically interfingered with units grading laterally into each other, and sequence repetitions are abundantly present. For any given shoreline position, the sequence nearest shore consists of light colored sands, silts, and muds, sometimes containing thin limestone bands. Farther out, pure limestones and dolomites accumulated. Then, in the deeper waters farther offshore, dark colored silts, muds and sands, and impure limestone beds that contain chert are found. It is important to realize that these interlaced facies do not represent different times of deposition, but merely different environments. Thus, in the basal layers of the Grand Canyon, we see the Lower Cambrian Tapeats Sandstone being deposited near shore, grading to the eastward into the Bright Angel Shale, which in turn grades into the Muav Limestone. The Bright Angel Shale is a classic example of a formation that represents different intervals of deposition at different places. Because the trend of Sauk transgression in this region was from west to east, the Bright Angel Shale follows this path laterally with respect to time. Thus, in the westernmost exposures, the Bright Angel Shale is partly of Lower Cambrian age, while the easternmost exposures are entirely Middle Cambrian.

Grand Canyon Geologic Section

The transition from sandstone to shale to carbonates is the classic signature of a transgressive event. Regressive events may show the reverse sequence, if the regression was also a slow event. (Some regressions were the result of rapid crustal movements, and did not allow time for the sedimentation to occur). If an uplifted region became exposed to erosion at any time, there may also be missing sequences and a visible unconformity in the bedding.

Late Cambrian sedimentation patterns shifted more toward carbonate sequences, indicative of stabilizing sea levels and somewhat deeper waters. Stromatolitic algal limestone reefs became common, which means that shorelines had stabilized as well. There is no stratigraphic boundary between the Cambrian and the overlying Ordovician periods. The two are distinguished entirely by means of the fossil assemblages. The Cambrian climate trends therefore continued unchanged into the Ordovician period.

In the diagram at right, unit 3a is the Tapeats Sandstone, lying unconformably upon the Precambrian granite and schist. Unit 3b, which is softer and thus forms an erosional slope, is the Bright Angel Shale. Finally, unit 3c is the Muav Limestone. Together, this sequence is called the Tonto Group.

A Cambrian trilobite

Trilobite fossils are the most famous specimens from the Cambrian period. However, the Archaeocyathids (Wikipedia article) and Brachiopods (Wikipedia article) are, by far, the more useful groups, being marker species for the period. The Archaeocyathids first appear at the Ediacaran-Cambrian boundary, and became extinct by the Middle Cambrian. The Brachiopods appeared in both articulated and inarticulated forms, but only the inarticulated forms were abundant in the Cambrian. The articulated species dominate after the Cambrian-Ordovician boundary. Thus, these two fossil species can be used to date a Cambrian formation. Although all the major invertebrate phyla appear during the Cambrian, nearly all Cambrian macrofossils belong to the three groups mentioned. Other large life forms appear to have been rare, or not to have fossilized well.

The Cambrian period introduced such a rapid expansion of life forms that it has been called the Cambrian Explosion (Wikipedia article). The seemingly sudden proliferation of diverse species is something of a mystery, but interpretation is difficult due to a limited supply of evidence, incomplete fossil records, and the lack of chemical traces left in Cambrian rocks.