Alluvial And Fluvial Environment

is held together by cement that was chemically precipitated, or by a fine-grained matrix of broken rock material. Depending on the rate of lithification, the processes involved in the formation of a rock from sediment, this consolidation can occur within a few years or over the course of millions of years. Sedimentary rocks are varied and are classified on the basis of their composition (types of minerals) and the texture (size, shape, and arrangement) of the sediment and associated cement or matrix (Table 7.1). The two other major categories of rocks, igneous and meta-morphic (Chapter 4), are of peripheral interest to dinosaur studies but nevertheless are important for interpreting the geologic history of the Mesozoic Era. Rare reports of dinosaur tracks, preserved in some former lava flows, are the only known instances of either rock type containing dinosaur fossils; accordingly, dinosaur paleontologists restrict their search image to sedimentary rocks.

Sedimentary rocks comprise about 90% of all exposed rock at the Earth's surface but not every sedimentary environment (places where sediments accumulated) was conducive to the burial and subsequent preservation of dinosaur fossils. Dinosaurs were land-dwelling animals, which is partially corroborated by their most frequent association with sedimentary rocks that have all of the characteristics of continental environments, such as fluvial (river), lacustrine (lake), and deltaic (delta) environments. A few dinosaur body fossils have been found in rocks formed in marine environments, for reasons examined later in this chapter. Nonetheless, the majority

Deltaic Environment
FIGURE 7.2 Common sedimentary environments, showing which ones had relatively low to high preservation potential for dinosaur body and trace fossils. Environments include glacial, alluvial, lacustrine, fluvial, paludal, estuarine, deltaic, coastal, tidal, and shallow marine.

of dinosaur body and trace fossils occur in rocks composed of sediments deposited in areas around and in lakes, rivers, deltas, estuaries (coastal embayments where fresh and salt water mix), and marine shorelines (Fig. 7.2). Another prerequisite for preservation of dinosaur fossils is that the sedimentary environments had to have existed during the Mesozoic Era, specifically from the Late Triassic through to the Cretaceous Periods.

The proper interpretation of sedimentary environments that preserved dinosaur fossils from the Mesozoic depends on a thorough description of their facies, which is all the characteristics imparted by an environment to its sediment at approximately the same time. Facies can be analyzed on the basis of lithologic characteristics (litho-facies), body fossils (biofacies), and trace fossils (ichnofacies). This type of description and the subsequent interpretation of a sedimentary environment comprise facies analysis, which is similar to paleoenvironmental analysis (Chapter 1). Geologists will refer to some rocks as fluvial facies, deltaic facies, or shallow-marine facies, based on a holistic and thorough examination of the many environmental parameters that left their clues in the sediment (Table 7.2). This eclectic approach hints at the breadth of geologic training that is required of a dinosaur paleontologist (Chapter 4); overlooking a single environmental parameter can considerably change the interpretation of the original environment that entombed the dinosaur fossils.

For example, parts of the Late Triassic Chinle Formation in the western USA contain abundant remains of fossil forests (as found in Petrified Forest National Park of Arizona), body fossils of numerous reptiles, a few small theropods, large amphibians, fish, and a few mollusks. All of these fossils are preserved in various muddy and sandy sediments, but no body fossils of insects or most other arthropods were reported. Previous workers did not look for, or otherwise recognize, many of the trace fossils made by invertebrates in the same sediments that contained the body fossils. Consequently, reconstructions of Chinle environments rarely included any invertebrates, showing a bias in interpretations on the basis of lithofacies and

LE 7.2 Parameters in a sediment deposit or sedimentary rock at the outcrop scale that lend to facies | analysis. Keep in mind the two-step method for scientific hypothesis formation: Description - What is it?, and Interpretive significance - What is it used for?

Composition: main mineral components and trace element geochemistry.

Texture: size, shape, and arrangement of sediment grains or cement. Sedimentary structures: features in a rock formed by either biological, physical, or chemical processes.

Geometry of sedimentary body: lenticular (lens-like), tabular.

Physical structures: bedding, cross-bedding, ripples, dunes, graded bedding.

Biogenic structures: burrows, borings, tracks, trails, nests.

Chemical structures: nodules, stalactites, stalagmites, styolites.

Body fossils: microfossils (algae, protozoans), plants, invertebrates, vertebrates.

biofacies. Beginning in the 1990s, finds of numerous trace fossils attributed to insects (termites, ants, bees), crayfish, horseshoe crabs, and mollusks (clams, snails), as well as vertebrate tracks and nests (some made by phytosaurs: Chapter 6), have added an ichnofacies component. As a result, the previously incomplete picture of paleo-environments in the Chinle has been expanded to include fluvial and lacustrine environments in association with forest ecosystems. The ichnofacies thus helped to enrich our understanding of the life history of the theropods and other vertebrates that lived in those same environments.

Dinosaurs used to traverse sedimentary environments, leaving tracks, making nests, dropping feces, and munching on foliage (or each other). In situ evidence for all of these various life activities is restricted to a narrow range of facies, which are fluvial, deltaic, or close to a marine shoreline, such as estuaries, tidal flats, or beaches. After the dinosaurs died, their bodies often remained in the same places as where they had lived or they were buried very close to where they died. Alternatively, entire bodies and the sum of their parts were carried away from their life and death sites before being buried for the first time. Evidence indicates that some dinosaur bones were buried, exhumed, transported, and buried again. Here is where the two major challenges of dinosaur taphonomy are encountered:

1 how far away is a dinosaur body fossil with respect to its original life site (which may not coincide with its death site, let alone its final resting spot)? and

2 how separated in time is a dinosaur body fossil from the strata that represent the time it was alive, as opposed to the strata that contain it?

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