Lab 6:
Sedimentary Rocks
Objectives:
- Weathering
- Lithification and precipitation
- Sedimentary rocks’ composition and texture
- Relation between texture and transport history
- Sedimentary structures
- Depositional environments
Introduction:
Sediments: loose grains and chemical residues of Earth’s materials
i.e.- rock fragments, mineral grains, parts of plants or animals and rust
Source: origin of sediments; biochemical (processes of plants and animals), weathering
(chemical and physical of organics), inorganic materials (rocks, minerals)
Weathering:
Chemical: decomposition or dissolution of Earth materials
Physical (mechanical): cracking, crushing, abrasion or disintegration of materials
Product of weathering is sediments and aqueous solutions
Sedimentary rocks: compaction and cementation of sediments or precipitation of crystals
Textures:
Sediments can be transported great distances by wind, water and ice
Overhead of Figure
6.1
Grain size:
Gravel = >2mm
Sand = 1/16 mm - 2 mm
Silt = 1/256 mm - 1/16 mm
Clay = <1/256 mm
Microcrystalline = small crystals
Crystalline = visible crystals
Grain shape:
Angular: points and angles on grains
Rounded: generally round grains, no sharp edges
Well-Rounded: very round, almost circular
Sorting (grain arrangements): separation of different densities and sizes of sediment
Poorly-sorted: composed of many different size grains
Moderately-sorted: in-between well and poorly-sorted
Well-sorted: composed of the same size grains
Composition:
-Description of the kinds and abundances of grains
Biochemical: organisms, shells, plant fragments and carbon
Chemical: intergrown minerals and crystals precipitated from aqueous solution
Detrital: detrital grains, weathered and transported parts of other rocks
Formation of Sed Rocks:
Compaction of sediments
Cementation of sediments
Cement – thin films of chemical residues or microscopic crystals that precipitate
from water in the sediment and act as glue
Precipitation of crystal aggregates from aqueous solutions
Identification of Sed
Rocks:
1. Determine the rocks general composition (biochemical, chemical. or detrital)
2. Describe the rock’s texture
3. Determine the name
4. Infer the origin of the rock
Biochemical rocks: limestone, coquina, chalk
Chemical rocks: rock gypsum, rock salt, chert
Detrital: breccia, conglomerate, sandstone, siltstone, mudstone, shale
Sedimentary
Structures and Environments
Form from physical process, activities of plants or animals
Indicators of how and where the rock formed
Figures 6.11 and 6.12 if you want
to follow along
Stratification: layering of sediments
Laminations: <1 cm
Beds: >1 cm
Bedding planes: surfaces between strata
Cross-stratification: inclined stratification, indicate current, ripple marks, bimodal
Grading: sorting from coarse to fine (normal v. reverse)
Flutes: scoop shaped or V shaped depressions from scouring
Mudcracks: drying and shrinking of cohesive sediment
Raindrop impressions, animal tracks, burrows, etc
Environments: Overheads