Page 184 - Laboratory Manual in Physical Geology

constituent grains of a biochemical (bioclastic) limestone Calcite will effervesce (fizz) in dilute HCl, but dolomite
can also be used to call it a coquina , calcarenite will effervesce only if it is powdered first. Rock gypsum
( fossiliferous limestone ), micrite , or chalk ( FIGURE 6.9 ). is an aggregate of gypsum crystals, and rock salt is
Peat is a very porous brown rock with visible plant an aggregate of halite crystals ( FIGURE 6.7 ). Two other
fragments that can easily be pulled apart from the rock. chemical sedimentary rocks are chert (microcrystalline
Lignite is brown but denser than peat. Its plant fragments or even cryptocrystalline quartz) and ironstone (rock
cannot be pulled apart from the rock. Bituminous coal made mostly of hematite, limonite, or other iron-bearing
is a black rock made of sooty charcoal-like or else shiny minerals or chemical residues).
brittle layers of carbon and plant fragments.
Detrital Rocks
Chemical Rocks The main kinds of detrital (siliciclastic) sedimentary rocks
There are seven main kinds of chemical (inorganic) are mudstone, sandstone, breccia, and conglomerate

sedimentary rocks in the classification in FIGURE 6.9 . ( FIGURE 6.9 ). It is very difficult to tell the percentage of

Chemical limestone refers to any mass of crystalline clay or silt in a sedimentary rock with the naked eye, so
limestone that has no color banding or visible internal sedimentary rocks made of clay and/or silt are commonly
structures. Travertine is a mass of intergrown calcite called mudstone . Mudstone that is fissile (splits apart
crystals that may have light and dark color banding, easily into layers) can be called shale . Mudstone can also
cavities, or pores ( FIGURE 6.8C ). Oolitic limestone be called siltstone or claystone, depending upon whether
is composed mostly of tiny spherical grains (ooids, silt or clay is the most abundant grain size. Any detrital

FIGURE 6.2 ) that resemble beads or miniature pearls rock composed mostly of sand-sized grains is simply
and are made of concentric layers of microcrystalline called sandstone ( FIGURES 6.5 and 6.9 ); although you can

aragonite or calcite. They form in intertidal zones of distinguish among quartz sandstone (made mostly of quartz
some marine regions ( FIGURE 6.10 ) where the water grains), arkose (made mostly of feldspar grains), lithic

is warm and detrital sediment is lacking. Dolostone sandstone (made mostly of rock fragments), or wacke (made
( FIGURE 6.9 ) is an aggregate of dolomite mineral crystals of a mixture of sand-sized and mud-sized grains). Breccia

that are usually microcrystalline. It forms in very salty and conglomerate are both made of gravel-sized grains and
lagoons and desert playa lakes ( FIGURE 6.10 ). Because are often poorly sorted or moderately sorted. The grains in

calcite and dolomite closely resemble one another, breccia are very angular and/or subangular, and the grains
the best way to tell them apart is with the “acid test.” in conglomerate are subrounded and/or well rounded.

ACTIVITY ACTIVITY

6.7 Grand Canyon Outcrop 6.8 Using the Present to
Analysis and Interpretation Imagine the Past—Dogs to
|
THINK What can sedimentary rocks tell Dinosaurs
About It us about Earth’s history and past THINK What can sedimentary rocks tell
environments and ecosystems? |
About It us about Earth’s history and past
environments and ecosystems?

OBJECTIVE Analyze and interpret sedimentary
rocks from the edge of the Grand Canyon.

OBJECTIVE Infer characteristics of an ancient
PROCEDURES environment by comparing modern dog tracks in
mud with fossil dinosaur tracks in sedimentary rock.
1. Before you begin , read about Ancient
Environments and Ecosystems and Indicators of PROCEDURES
Ancient Environments next. Also, this is what
1. Before you begin , read about Ancient
you will need :
Environments and Ecosystems and Indicators
___ Activity 6.7 Worksheet (p. 183 ) and pencil of Ancient Environments next. Also, this is what
2. Then follow your instructor’s directions for you will need :
completing the worksheets. ___ Activity 6.8 Worksheet (p. 184 ) and pencil
2. Then follow your instructor’s directions for
completing the worksheets.

Sedimentary Processes, Rocks, and Environments ■ 163