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

Typical marine sediments showing cyclic deposition


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.