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