Lab 8: Continental Crust & Passive Margins

Passive margins

zones where oceanic crust and continental crust join within a single lithospheric plate

Cross-sectional diagram of a passive margin

Cobbles of granite and chert would indicate what?

The presence of granite and chert in the bedrock of the source area

Finding an arkose would indicate what?

suggest the dominance of granite or gneiss in the area from which the sand particles were derived

The presence of feldspar would indicate what?

indicate that the source area was not subjected to extensive chemical weathering and that erosion probably took place in an arid environment with high relief

Quartz arenite would indicate what?

probably represents repeated cycles of erosion , transportation and deposition of quartz grains that, although might have ultimately been sourced from igneous or metamorphic rocks, probably subsequently involved the reworking of previous sandstone deposits

Sediment transport

Environments of deposition

1. Alluvial fans

2. River channels

3. Floodplains

4. Lakes

5. Dunes

6. Delta

7. Beaches

8. Lagoons

9. Shelves

10. Reefs

Upland and Glacial environments

included together because their deposits closely resemble one another

Upland environment - sediments

• sediments close to source area

• contain abundant lithic (rock) fragments

• very poorly sorted

Glacial environment - sediments

• very poorly sorted and massive

• large clasts “floating” in a mud matrix

• sediment is called diamicton (rock - diamictite)

• If scratches, rock - tillite

Alluvial fans - sediments

• fan-shaped sediments

• coarse, arkosic sandstones

• conglomerates marked by coarse cross-bedding

• abundance of potassium feldspar

  • points to arid climatic conditions

River channel & floodplains - river sediments

• deposit elongate bars of conglomerate or sandstone

• arkoses (feldspar, quartz, micas)

• asymmetrical ripples and crossbedding provide evidence for the unidirectional current activity is streams

River channel & floodplains - floodplains

• lower velocity of rapids

• finer sediments - fine sand and silt

• thinly interbedded sandstones/siltstones and shales

• evidence of exposure of the floodplain shows mud cracks, root traces or plants or footprints

Lakes - sediments

• fine-grained

• clastic mud (in humid climates)

• preserve seasonal patters

  • dark clay, organic-rich layers (winter)

  • light, silt-rich layers (summer)

• dropstones

Deltas - sediments

• thickly interbedded sandstone/siltstone (with low-angle crossbedding) and shale cut coarser deposits

• swamps - mud with abundant plant fossils

  • carbonaceous mudstone or shale

Beaches, barrier islands, dunes - sediments

• well-sorted quartz sandstone

• well-rounded grains

• low-angle lamination, cross-bedding, and symmetrical ripples

• quartz arenites

• ooilitic limestone (tiny spherical carbonate grains)

Tidal flates - sediments

• features such as mud cracks that indicate altering wet and dry periods

• sandstone with mud cracks

Lagoons - sediments

• fine-grained

• finely laminated (reflecting quiet water conditions)

• marine fossils

• microbial mats can develop

  • when they dry out, they look crinkled

Marine shelves - sediments

• sandstone, siltstone, mudstone/shale, carbonat mud (micrite)

• thin-bedded sandstone with symmetrical ripple marks

• oolitic limestone

• mud with skeletal remains as thin shell beds (shells of marine organisms - brachiopods, trilobites, etc.)

Reefs - sediments

• massive limestone

• limestone breccia

• sand-sized and finer-grained limestones

• limestones full of skeletal fragments (coral, algae_

Deep marine environments - sediments

• clay-rich mudrocks, shale, greywacke

• contain graded bedding

• may preserve ripple marks

• deep-water shales tend to preserve fossils of planktonic organisms

• siliceous ooze (chert)

Graded bedding by a turbidity current

Ripples

Cross-stratification / cross-bedding

Flute marks

Graded bedding

Burrows

Stromatolites

Sedimentary environments