Sedimentary Rocks

Chalk Composition

 A biochemical rock composed of the microscopic tests (skeletons) of singlecelled organisms belonging to the phylum Haptophyta; common name coccoliths or coccolithophorids. Although we provide a close-up of this rock there is nothing more to see than in the larger image. Haptophyte skeletal elements can only be seen with a scanning electron microscope

Description     Soft, white, powdery, gritty rock that easily rubs off on you fingers.

Reacts with dilute HCl producing large, foaming bubbles.      May be confused with kaolinite or alabaster gypsum. Neither of these

, however, react with acid.

Chalk Continued• Formation & Environments•      The Haptophyte organisms that produce the coccolith skeletons

that become chalk are pelagic organisms living in the surface waters of the world's oceans. The skeletons eventually settle to the bottom and accumulate to become chalk - if conditions are favorable.     Coccolith skeletons are settling most everywhere out of today's oceans, and should be accumulating everywhere in the oceans. But, if the water is too deep it becomes too cold, and the skeletons dissolve. Therefore, chalk accumulates only at shallower depths, and today that is along the oceanic rift systems (divergent plate boundaries).     We might also expect chalk to accumulate in shallow waters next to continents, such as on the continental shelves, but here there is too much clastic sediment, and whatever skeletons get to the bottom are typically lost in the clays, silts, and sands.

Chert

Description     Extremely hard (H=7), tough, dense, silica rock with conchoidal fracture. Color varies extremely from opaque black, to gray, white, yellow, brown, red.     Chert is resistent to weathering and so tends to stand out of the ourcrop. This picture shows an outcrop of the Shriver Chert in West Virginia. The next two images (click pictures) focus in closer and closer to see the nature of the chert.

Chert Continued• Formation & Environments     Many kinds of chert form from

recrystallization of siliceous skeletons (glass sponges, diatoms, radiolarians). Some chert may be produced chemically.     Two broad types of chert form, nodular and bedded, with abundant variations in between. Bedded chert typically forms in clastic starved basins (i.e. those with little sediment influx), along the edge of continents (such as divergent continental margins) where strong upwelling occurs. Deep water upwellings bring nutrient rich waters to the surface resulting in algal blooms, population explosions of microscopic organisms, many of whom produce silicious skeletons which settle to the bottom and recrystallize to form form thick bedded chert.     Nodular chert is more associated with shallow water environments, especially in carbonates. Here the silica chemcially replaces the carbonate, often times including fossils.   

Conglomerate• Description•      This is a sawed slab of rock that has been wet with water before

photographing.     Rounded gravel particles mostly toward the upper size range of granules (4-64 mm). Lithic fragments include chert, meta-sedimentary rocks, milky (metamorphic) quartz, and occasional igneous rocks and micrite limestones. The matrix (material filling the spaces between the granules) is mostly sand, mixed with silt, and clay.      This rock is extremely immature, being low in quartz, high in gravel, poor in sphericity, and poorly sorted.     The gravel fragments tend to be elongated, but do not show a preferred orientation (i.e. are not imbricated), indicating they were probably deposited during mass transport (e.g. debris flow such as a landslide or underwater avalanche). In context of the outcrop this

rock was deposited subaqueously (below water).

Conglomerate Continued

Dolomite• . Description

     Dense, uniform, fine grained rock with conchoidal fracture. Faint, weak laminations running parallel to bedding. The laminations are probably algal laminates, produced on tidal flats by colonies of blue-green algae

One way or another, dolomitic rocks are formed in many of the environments limestones form in - just depending on the limestone. This particular exposure is algal laminated, and found with other evidence of tidal flat feature. It most likely formed in the

Gypsum•   •Laminated alabaster gypsum.

Gypsum takes

•on many forms, such as alabaster, selenite,

•satin spar, and rose gypsum.

     

Formation & Environments     Sedimentary gypsum forms under high evaporative conditions

]in sedimentary rocks, both clastic and carbonate.

It usually requires desert conditions in shallow marine basins, or along

coastal tidal flats. Gypsum commonly forms in association with halite and dolomite in evaporation basins.

Sometimes the gypsum appears simply as gypsum roses in the sediment; other times it can form

beds tens of feet thick.

Sandstone• . Description

     Pure, coarse grained quartz sand with cross bedding. The slight pink color staining along the cross bedding is iron contamination, and was not part of the original composition. The cross beds are probably large scale trough type resulting from the migration of large ripples.

Antietam formation, Cambrian, Virginia

Formation & Environments     Quartz sandstone results from the extreme weathering and sorting of a

sediment until everything that can be removed has been removed. Complete chemical weathering is required to remove all feldspars and lithics,

but the final removal of the clay takes place in high energy environments, typically beach environments, although other high energy situations such as

tidal sand bars can accumulate large bodies of quartz sand.     Mature quartz sands such as this are not common in the geologic

record because the required conditions are difficult to achieve.

Shale• Description•    Fine grained rock composed of lithified clay making the rock shale. Pure

clays (such as kaolinite) tend to be white or tan, although varying amounts of other components are usually present. Other components may be iron oxides or organic matter. The dark color of this specimen respresents the presence of incompletely decayed organic matter (humus) making the rock dark gray (the rock looks light colored because of the way light is reflected; the detail picture shows its true coloration).   Shales normally have a fine lamination structure. This image is looking at the side of the rock, and you will note no layering or laminations visible. Technically that makes this a mudstone. Mudstone is made of the same stuff as shale, but it has been distrupted by something that destroyed the laminations. Often the disruption is caused by bioturbation; organisms burrowing through the sediment and ingesting it to extract food.

Shale Continued

• Formation & Environments•      Shale = quiet water deposition. Aside from

this it is hard to say anything definitive about the environment of a shale since most environments have periods and places of quite water deposition. For example, shales are common in basins, shelves, deltas, meandering rivers, flood plains, etc.      The dark gray of this specimen indicates a low oxygen environment. From its geologic context we know it was a relatively deep water, quiet, marine basin.