Clastic Sedimentary Rocks Formation

Nine hundred years ago, a thriving community of Native Americans inhabited the high plateau of Mesa Verde, Colorado. In hollows beneath huge overhanging ledges, they built multistory stone-block buildings that have survived to this day. Clearly, the blocks are solid and durable they are, after all, rock. But if you were to rub your thumb along one, it would feel gritty, and small grains of quartz would break free and roll under your thumb, for the block consists of quartz sand grains cemented together. Geologists call such rock a sandstone. Sandstone is an example of clastic sedimentary rock. It consists of loose clasts, known as detritus, that have been stuck together to form a solid mass. The clasts can consist of individual minerals (such as grains of quartz or ﬂakes of clay) or of fragments of rock (such as pebbles of granite). Formation of sediment and its transformation into clastic sedimentary rock takes place via the following ﬁve steps.

· Weathering: Detritus forms by disintegration of bedrock into separate grains due to physical and chemical weathering.

· Erosion: Erosion refers to the combination of processes that separate rock or regolith (surface debris) from its substrate. Erosion involves abrasion, falling, plucking, scouring, and dissolution, and is caused by moving air, water, or ice.

· Transportation: Gravity, wind, water, or ice carry sediment. The ability of a medium to carry sediment depends on its viscosity and velocity. Solid ice can transport sediment of any size, regardless of how slowly the ice moves. Very fast-moving, turbulent water can transport coarse fragments (cobbles and boulders), moderately fast-moving water can carry only sand and gravel, and slow-moving water carries only silt and clay. Strong winds can move sand and dust, but gentle breezes carry only dust.

· Deposition: Deposition is the process by which sediment settles out of the transporting medium. Sediment settles out of wind or moving water when these fluids slow, because as the velocity decreases, the fluid no longer has the ability to carry sediment. Sediment is deposited by ice when the ice melts.

· Lithiﬁcation: Geologists refer to the transformation of loose sediment into solid rock as lithiﬁcation. The lithification of clastic sediment involves two steps. First, once the sediment has been buried, pressure caused by the weight of overlying material squeezes out water and air that had been trapped between clasts, and clasts press together tightly, a process called compaction. Compacted sediment may then be stuck together to make coherent sedimentary rock by the process of cementation. Cement consists of minerals (commonly quartz or calcite) that precipitate from groundwater and fill the spaces between clasts.

Classifying clastic sedimentary rocks

Say that you pick up a clastic sedimentary rock and want to describe it sufﬁciently so that, from your words alone, another person can picture the rock. What characteristics should you mention? Geologists ﬁnd the following characteristics most useful.

· Clast size. Size refers to the diameter of fragments or grains making up a rock. Names used for clast size, listed in order from coarsest to finest, are: boulder, cobble, pebble, sand, silt, and clay.

· Clast composition. Composition refers to the makeup of clasts in sedimentary rock. Clasts may be composed of rock fragments or individual mineral grains.

· Angularity and sphericity. Angularity indicates the degree to which clasts have smooth or angular corners and edges. Sphericity, in contrast, refers to the degree to which the shape of a clast resembles a sphere.

· Sorting. Sorting of clasts indicates the degree to which the clasts in a rock are all the same size or include a variety of sizes. Well-sorted sediment consists entirely of sediment of the same size, whereas poorly-sorted sediment contains a mixture of more than one clast size.

· Character of cement. Not all clastic sedimentary rocks have the same kind of cement. In some, the cement consists predominantly of quartz, whereas in others, it consists predominantly of calcite.

With these characteristics in mind, we can distinguish among several common types of clastic sedimentary rocks. This table provides common rock names specialists sometimes use other, more precise names based on more complex classiﬁcation schemes. The size, angularity, sphericity, and sorting of clasts depends on the medium (water, ice, or wind) that transports the clasts and, in the case of water or wind, on both the velocity of the medium and the distance of transport. The composition of the clasts depends on the composition of rock from which the clasts were derived, and on the degree of chemical weathering that the clasts have undergone. Thus, the type of clasts that accumulate in a sedimentary deposit varies with location. To see how, let’s follow the fate of rock fragments as they gradually move from a cliff face in the mountains via a river to the seashore. Different kinds of sediment develop along the route. Each kind, if buried and lithiﬁed, would yield a different type of sedimentary rock.

To start, imagine that some large blocks of granite tumble off a cliff and slam into other blocks already at the bottom. The impact shatters the blocks, producing a pile of angular fragments. If these fragments were to be cemented together before being transported very far, the resulting rock would be breccia (a in above figure). Later, a storm causes the fragments (clasts) to be carried away by a turbulent river. In the water, clasts bang into each other and into the riverbed, a process that shatters them into still smaller pieces and breaks off their sharp edges. As the clasts get carried downstream, they gradually become rounded pebbles and cobbles. When the river water slows, these clasts stop moving and form a mound or bar of gravel. Burial and lithiﬁcation of these rounded clasts produces conglomerate (b in above figure). If the gravel stays put for a long time, it undergoes chemical weathering. As a consequence, cobbles and pebbles break apart into individual mineral grains, eventually producing a mixture of quartz, feldspar, and clay. Clay is so ﬁne that ﬂowing water easily picks it up and carries it downstream, leaving sand containing a mixture of quartz and some feldspar grains this sediment, if buried and lithiﬁed, becomes arkose (c in above figure). Over time, feldspar grains in sand continue to weather into clay so that gradually, during successive events that wash the sediment downstream, the sand loses feldspar and ends up being composed almost entirely of durable quartz grains. Some of the sand may make it to the sea, where waves carry it to beaches, and some may end up in desert dunes. This sediment, when buried and lithiﬁed,

becomes quartz sandstone (d figure below). Meanwhile, silt and clay may accumulate in the ﬂat areas bordering streams, regions called ﬂoodplains that become inundated only during ﬂoods. And some silt and mud settles in a wedge, called a delta, at the mouth of the river, or in lagoons or mudﬂats along the shore. The silt, when lithiﬁed, becomes siltstone, and the mud, when lithiﬁed, becomes shale or mudstone (e figure below).