GEOLOGY AS LEVEL (picture cards)

Orthoclase and Plagioclase in thin section:

• Plagioclase looks stripy because it forms repeated multiple twinning.

• Ortholclase looks like one solid colour because it forms simple twinning.

Physical weathering: Freeze-thaw

produces rock fragments (scree)

Physical weathering: Exfoliation

produces rock fragments

Transportation of boulder clay by ice

Arkose: beige-grey, medium grained, sub-rounded, moderate-high sphericity, moderate-well sorted, more than 25% orthoclase, quartz cement

Greywacke: greeny-grey, medium, sub-angular, low sphericity, poorly sorted, more than 25% clay minerals

Orthoquarzite: beige-yellow (red-brown if desert sandstone), medium grained, rounded, high sphericity, well sorted, more than 90% quartz, quartz cement

Sandstone

Conglomerate: well-rounded, poorly-moderately sorted, coarse grained, contains lithic fragments and quartz 

If red: wadi deposit

If no red: river deposit

Shale: fine grained, well sorted, platey and laminated, dark blue-grey, made of mostly clay minerals

*shales are transported in esgeries, river mouths, deep seas, flood plains (low energy environments) *

beds don't show us anything unless we look at the grains

Cross bedding

• desert dunes/rivers/coastal dunes
• moderate-high energy
• uni-directional current
• if convex has been overturned

Graded bedding

• river flowing into lake/sea/lower energy river
• rapid decrease in energy
• if fining down they have been overturned

Asymmetrical ripple marks

• rivers, desert/beach dunes
• moderate energy
• uni-directional current
• if sand on top and clay underneath it has been overturned

Symmetrical ripple marks

• beaches (waves)
• moderate energy
• bi-directional current
• cross laminations and sand/clay show way-up

Dessication cracks

• flood planes, temporary lakes
• low energy
• if the V in cross section is upside down it has been overturned

Load and flame (sole structure)

• if clay flames down and sand loads up it has been overturned

Flute clasts (sole structure)

• current goes from deep (closed) end to shallow (open) end

Bioclastic limestone: beige/blue-grey, coarse grained, angular,poorly sorted, made of broken fossils (100% calcite), calcite cement/matrix

Oolithic limestone: beige-grey, medium-coarse grained, well rounded, high sphericity, well-sorted, made of ooliths (100% calcite), calcite cement/calcite mud-matrix

Chalk: white, fine, shape is too fine to see, very well sorted, made of coccoliths (dead plankton), 100% calcite

 Goniotite: rounded saddle and pointed lobe

Ceratite: rounded saddle and fluted lobe

Ammonite: fluted saddle and fluted lobe

Cast and mould preservation:

Mould (e) = empty

Cast (f) = solid

Porphyritic texture

euherdral phenocrysts (form slowly and first) surrounded by finer anhedral groundmass (forms fast and last to fill in spaces between phenocrysts)

2 rates of cooling.

Equigranular texture:

all crystals same size

one rate of cooling

Glassy texture:

no crystals

super fast cooling underwater

Vesicular texture

filled with vesicles (gas bubbles)

fast cooling on the surface/near surface

Granite:

• coarse
• light
• made of quartz, orthoclase, biotite/muscovite mica

Gabbro:

• coarse
• dark
• made of plagioclase, augite, hornblende

Dolerite:

• medium
• dark
• made of plagioclase, augite, hornblende

Basalt

• fine
• dark
• made of plagioclase, augite, hornblende

Peridotite

• coarse
• dark
• made of olivine and augite

Columnar joints

• as lava cools it loses volume 
• upstresses in the rock fracture
• rock cools towards hottest points = tension between hottest points = JOINTING

Pillow lava

• crust forms and cracks as lava is pumped into it underwater
• pillows collapse into each other and mould round each other (are soft)

     Pahoehoe lava (ropey)                                     Aa lava (rubble-like)

Granoblastic texture:

crystals are roughly the same size

Poikilitic texture

spotted texture

Porphyroblastic texture

euhedral porphyroblasts surrounded by finer ground mass

Spotted rock

• poikilitic texture
• pelite parent
• low grade (fine grained)
• blebs of biotite/cordierite

Hornfels:

• granoblastic
• made of hornblende
• pelite parent
• high grade

Metaquarzite:

• granoblastic
• made of quartz
• sandstone parent
• contact/regional metamorphism

Marble

• granoblastic
• made of calcite
• limestone parent
• contact/regional

Slate

• slatey cleavage
• made of clay,biotite,chlorite
• can have lumps of pyrite
• regional metamorphic (aligned to p.max)

Schist:

• schistosse texture
• made of quartz, biotite, orthoclase,muscovite
• can have garnet porphyroblasts

Gneiss

• gneissose texture
• made of orthoclase/quartz (light) and hornfels/mafic (dark)
• light and dark crystals stick together
• regional metamorphism (colours aligned to p.max)

Normal fault

Normal fault:

F.U.N (footwall up= normal)

• constructive plate margins
• tensional stress

Reverse fault:

F.D.R (footwall down= reverse)

• destructive plate boundaries
• compressional stress

Thrust fault:

• same as reverse fault but at a low angle (FDT= footwall down = thrust)

Transform fault:

• offset mid ocean ridges at constructive plate boundaries
• shear stress

Transcurrent/tear/shear fault:

• conservative plate boundaries
• dextral- to the right
• synistral- to the left

Horizontal bedding

Vertical bedding

Dipping beds