W6L1 biochemical sedimentary rocks

Question 1

sea level rises if

Answer 1

if I stand on an island and the ocean crust melts (denser, cooler) the sea level rises

Question 2

Coral Reefs form where

Answer 2

form in tropical and subtropical environments

because requires energy in cold water to biomineralize so confined to narrow margine

Question 3

lagoons

Answer 3

sheltered, calm, but most biological activity
laminated sediments,not much erosion ghoing on
but high productivity in sediment so often anoxic in sediment (black)

Question 4

reefs where in ocean

Answer 4

Reefs typically build on the outer shelf, fronting the open ocean
storms bring debris that can actually stabilize reefs so disadvantage → benefit

Question 5

Coral polyps sensitive to

Answer 5

sea level 
 suspended sediment
 salinity/water temperature
 daily growth rings record changes in these parameters and are registers of climate change
 organisms can be preserved in situ

Question 6

situ

Answer 6

deep sediment loose rock

Question 7

Biochlastic limestones

Answer 7

reefs also support echinoids, molluscs & calcareous algae
after death, debris tumbles down the reef front
bioclastic limestones (limestone with lots of shells and transported material)
if sea level were to rise entire system would move up → good record of sea level changes

Question 8

CHemically precipitated carbonates

Answer 8

After death, coral and calcareous skeletons are subject to physical & chemical weathering break-up & dissolution -
evaporation in the lagoon leads to a concentration of dissolved calcium direct precipitation
micrite mud micrite

Question 9

Sea urchins and sea lilies

Answer 9

crinoids most often found as disarticulated stems
echinoids often found intact
because rather resistant material, harder weathered
bivalves, gastropods & brachiopods also live in the productive environment of the reef

Question 10

Carbonate Platforms

Answer 10

warm, clear, normal salinity, shallow water affected by tides
calcite is precipitated layer by layer around a sand-sized nucleus (quartz grain or shell fragment)
spherical ooids (1-2 mm diameter)
subrounded peloids (fecal pellets or algae)
foirm at passive continental margin, ocean crust stops forming → denser → pulls down continental marghin → creates space → carbonate platform will fill up space created by pull down from sediment
takes place at times of really high sea levels

Question 11

Oozes are

Answer 11

calcite percipitated through organisms

Question 12

Oozes; open- ocean biochemical sediments

Answer 12

microscopic organisms also biomineralise calcite (coccolithophores & foraminifera) or silica (SiO2) (diatoms, radiolaria & sponges)
calcareous and siliceous organisms
exist but not as abundant in high latitudes bcs too cold (biomineralisation rewuires too much energy)

Question 13

which ooze more common

Answer 13

carbonate oozes more common than siliceous oozes because supply of silica is a limiting factor

Question 14

ooze becones

Answer 14

ooze → compact ooze → chalk → compact chalkj → limestones

Question 15

biochemical oozes

Answer 15

phytoplankton ‘blooms’ and zooplankton predators
far from land (little or no terrigenous sediment), deposited above the CCD (calcite compensation caps) – calcareous oozes
high productivity areas where nutrients are stirred up by strong winds and deep water causes dissolution below the CCD - diatom oozes

Question 16

where is calcareous materials

Answer 16

along all mid ocean ridges there is calcareous material

most mid ocean ridges at 2.5 km depth

Question 17

Calcareous oozes

Answer 17

ormed by siliceaos organisms
sponges co-exist with coccolithophores & foraminifera
diatoms & radiolaria prefer high nutrient conditions
biomineralised silica has water in the molecule
SiO2.nH2O
amorphous solid
mobilised by overburden pressure

Question 18

AMorphous silica and chert (flint)

Answer 18

originally dispersed in calcareous ooze
mobilised by overburden pressure (diagenesis)
collects along bedding planes & in burrows (crypotocrystalline nodules)
solidifies to microcrystalline ‘flint’(in chalk), & ‘chert’ (in limestone)
waxy lustre, conchoidal fracture
can be almost any colour (impurities)
pure siliceous oozes deposited below the CCD = chert layers

Question 19

Flint and chert

Answer 19

Bands of flint are typical of the Upper Chalk (Upper Cretaceous)
amorphous silica collects along bedding planes and in sediment altered by bioturbation

Question 20

Replacememnt chert

Answer 20

diagenesis and lithificatin

Question 21

replacement chert

Answer 21

silica-rich fluids originate from mobilisation/dissolution of in situ biogenic silica or from elsewhere, e.g. volcanic ash dissolved in groundwater
silica-rich fluids circulating through rocks can replace:
biological structures
carbonate crystals in limestones & dolomites (chalcedony & jasper - colour depends on impurities)
chert can be precipitated in concentric rings inside rock voids forming striped agate

Question 22

Marl and Marlstone

Answer 22

usually a secondary process after lithification
under pressure and at low T, calcite and limestone dissolve
at depth in the ocean
sea level rise forces in situ dissolution of sediment/rock
results in reduction of calcite & concentration of clay marl
soft
weathers back easily
limestone/marl couplets reflect alternating rise & fall of sea level

Question 23

Dolomitisation

Answer 23

Secondary process after lithification -
limestone is altered to dolomite when fresh or hypersaline water flushes through pores 
 Ca replaced by Mg
 CaCO3  CaMg(CO3)2
 large-scale recrystallisation

Question 24

Dolomite

Answer 24

commonly brown (compared to unaltered limestone)
harder than limestone
does not react with acid unless powdered
 porosity increases
 fossils are destroyed

Question 25

Chemically precipitated calcite rocks- speeleothems

Answer 25

pressurised water + dissolved calcite moves through cracks in limestone
degasses CO2 as it enters a cave – calcite is deposited

Question 26

Chemically precipitated calcareous rocks

Answer 26

travertine
supersaturated hydrothermal waters emerging under pressure
calcite-rich ground water emerging under pressure (slow deposition)