5 BIOCLASTIC ROCKS

䊳 7.5 BIOCLASTIC ROCKS

Carbonate rocks are made up primarily of carbonate minerals, which contain the carbonate ion (CO 3 ) 2– . The most common carbonate minerals are calcite (calcium carbonate, CaCO 3 ) and dolomite (calcium magnesium

Figure 7–12 An evaporating lake precipitated thick salt de- 1 The 2H 2 O in the chemical formula of gypsum means there is water posits on the Salar de Uyuni, Bolivia.

incorporated into the mineral structure.

Sedimentary Structures 117

carbonate, CaMg(CO 3 ) 2 ). Calcite-rich carbonate rocks are called limestone, whereas rocks rich in the mineral dolomite are also called dolomite. Many geologists use the term dolostone for the rock name to distinguish it from the mineral dolomite.

Seawater contains large quantities of dissolved cal- cium carbonate (CaCO 3 ). Clams, oysters, corals, some types of algae, and a variety of other marine organisms convert dissolved calcium carbonate to shells and other hard body parts. When these organisms die, waves and ocean currents break the shells into small fragments, called bioclastic sediment. A rock formed by lithifica- tion of such sediment is called bioclastic limestone, in- dicating that it forms by both biological and clastic processes. Many limestones are bioclastic. The bits and pieces of shells appear as fossils in the rock (Fig. 7–14).

Organisms that form limestone thrive and multiply in warm, shallow seas because the sun shines directly on the ocean floor, where most of them live. Therefore,

(a)

bioclastic limestone typically forms in shallow water along coastlines at low and middle latitudes. It also forms on continents when rising sea level floods land with shal- low seas.

Coquina is bioclastic limestone consisting wholly of coarse shell fragments cemented together. Chalk is a very fine-grained, soft, white bioclastic limestone made of the shells and skeletons of microorganisms that float near the surface of the oceans. When they die, their remains sink to the bottom and accumulate to form chalk. The pale-yellow chalks of Kansas, the off-white chalks of Texas, and the gray chalks of Alabama remind us that all of these areas once lay beneath the sea (Fig. 7–15).

Dolomite composes more than half of all carbonate rocks that are over a billion years old and a smaller, al- though substantial, proportion of younger carbonate rocks. Because it is so abundant, we would expect to see dolomite forming today; yet today there is no place in the world where dolomite is forming in large amounts. This dilemma is known as the dolomite problem.

(b)

The general consensus among geologists is that most Figure 7–14 Most limestone is lithified shell fragments and dolomite did not form as a primary sediment or rock. other remains of marine organisms. (a) A limestone mountain

Instead, it formed as magnesium-rich solutions derived in British Columbia, Canada. (b) A close-up of shell fragments from seawater percolated through limestone beds.

in limestone. (© Breck P. Kent)

Magnesium ions replaced half of the calcium in the cal- cite, converting the limestone beds to dolostone.

understand how the sediment was transported and de-

䊳 7.6 SEDIMENTARY STRUCTURES

posited.

The most obvious and common sedimentary struc- Nearly all sedimentary rocks contain sedimentary struc-

ture is bedding, or stratification—layering that devel- tures, features that developed during or shortly after

ops as sediment is deposited (Fig. 7–16). Bedding forms deposition of the sediment. These structures help us

because sediment accumulates layer by layer. Nearly all because sediment accumulates layer by layer. Nearly all

Cross-bedding consists of small beds lying at an angle to the main sedimentary layering (Fig. 7–17a). Cross-bedding forms in many environments where wind or water transports and deposits sediment. For example, wind heaps sand into parallel ridges called dunes, and flowing water forms similar features called sand waves. Figure 7–17b shows that cross-beds are the layers formed by sand grains tumbling down the steep downstream face of a dune or sand wave. Cross-bedding is common in sands deposited by wind, streams, ocean currents, and waves on beaches.

Ripple marks are small, nearly parallel sand ridges and troughs that are also formed by moving water or wind. They are like dunes and sand waves, but smaller. If the water or wind flows in a single direction, the ripple marks become asymmetrical, like miniature dunes. In other cases, waves move back and forth in shallow water, forming symmetrical ripple marks in bottom sand (Fig. 7–18). Ripple marks are often preserved in sandy sedimentary rocks (Fig. 7–19).

In graded bedding, the largest grains collect at the bottom of a layer and the grain size decreases toward the top (Fig. 7–20). Graded beds commonly form when some

Figure 7–15 The Niobrara chalk of western Kansas consists violent activity, such as a major flood or submarine land- of the remains of tiny marine organisms. (David Schwimmer)

Figure 7–16 Sedimentary bedding shows clearly in the walls of the Grand Canyon. (Donovan Reese/Tony Stone Images)

Sedimentary Structures 119

Figure 7–17 (a) Cross-bedding preserved in lithified ancient sand dunes in Arches National Park, Utah. (b) The develop- ment of cross-bedding in sand as a dune migrates.

(b)

slide, mixes a range of grain sizes together in water. The sediment carried in by the next high tide and are com- larger grains settle rapidly and concentrate at the base of

monly well preserved in rocks.

the bed. Finer particles settle more slowly and accumu- Occasionally, very delicate sedimentary structures late in the upper parts of the bed.

are preserved in rocks. Geologists have found imprints Mud cracks are polygonal cracks that form when

of raindrops that fell on a muddy surface about 1 billion mud shrinks as it dries (Fig. 7–21). They indicate that the

years ago (Fig. 7–22) and imprints of salt crystals that mud accumulated in shallow water that periodically dried

formed as a puddle of salt water evaporated. Like mud up. For example, mud cracks are common on intertidal

cracks, raindrop and salt imprints show that the mud mud flats where sediment is flooded by water at high

must have been deposited in shallow water that intermit- tide and exposed at low tide. The cracks often fill with

tently dried up.

Current produces asymmetrical ripples Oscillating waves form symmetric ripples

Sediment Sediment

(a)

(b)

Figure 7–18 (a) Asymmetric ripple marks form when wind or currents move continu- ously in the same direction. (b) Symmetric ripple marks form when waves oscillate back and forth.

Figure 7–19 Ripple marks in billion-year-old mud rocks in Figure 7–21 Mud cracks form when wet mud dries and eastern Utah.

shrinks.

Fossils are any remains or traces of a plant or ani-

䊳 7.7 INTERPRETING SEDIMENTARY

mal preserved in rock—any evidence of past life. Fossils

ROCKS: DEPOSITIONAL

include remains of shells, bones, or teeth; whole bodies

ENVIRONMENTS

preserved in amber or ice; and a variety of tracks, bur- rows, and chemical remains. Fossils are discussed fur-

Imagine that you encounter a limestone outcrop as you ther in Chapter 9.

walk in the hills. Entombed in the limestone you find

Figure 7–20

A graded bed in Tonga, southwestern Pacific. Larger grains collected near the bottom, and smaller particles

Figure 7–22 Delicate raindrop imprints formed by rain that settled near the top of the bed. (Peter Ballance)

fell about a billion years ago on a mudflat. (© Breck P. Kent)

Summary 121

Glacier Sand

Deep sea Flood

floor plain

Delta

Submarine fan

Lagoon

Barrier island Continental

rise Figure 7–23 Common depositional environments.

canyon

Continental slope

fossils of marine clams that lived in shallow water. Geologists answer these questions by analyzing the min- Therefore, you infer that the limestone must have formed

erals, textures, and structures of sedimentary rocks. in a shallow sea. Further, since the limestone is now well

Additionally, the size and shape of a sedimentary rock above sea level, you infer that tectonic forces have lifted

layer contain clues to its depositional environment. this portion of the sea bed to form the hills.

Accurate interpretations of depositional environments Geologists study sedimentary rocks to help us un-

are often rewarding because valuable concentrations of derstand the past. When geologists study sedimentary

oil and gas, coal, evaporites, and metals form in certain rocks, they ask questions such as: Where did the sedi-

types of environments.

ment originate? Was the sediment transported by a Depositional environments vary greatly in scale, stream, wind, or a glacier? In what environment did the

from an entire ocean basin to a 3-meter-long sand bar in sediment accumulate? If it was deposited in the sea, was

a stream. Many small-scale environments may be active it on a beach or in deep water? If it was deposited on

within a single large-scale depositional system (Fig. land, was it in a lake, a stream bed, or a flood plain?