W6L1 biochemical sedimentary rocks Flashcards
sea level rises if
if I stand on an island and the ocean crust melts (denser, cooler) the sea level rises
Coral Reefs form where
form in tropical and subtropical environments
because requires energy in cold water to biomineralize so confined to narrow margine
lagoons
sheltered, calm, but most biological activity
laminated sediments,not much erosion ghoing on
but high productivity in sediment so often anoxic in sediment (black)
reefs where in ocean
Reefs typically build on the outer shelf, fronting the open ocean
storms bring debris that can actually stabilize reefs so disadvantage → benefit
Coral polyps sensitive to
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
situ
deep sediment loose rock
Biochlastic limestones
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
CHemically precipitated carbonates
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
Sea urchins and sea lilies
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
Carbonate Platforms
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
Oozes are
calcite percipitated through organisms
Oozes; open- ocean biochemical sediments
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)
which ooze more common
carbonate oozes more common than siliceous oozes because supply of silica is a limiting factor
ooze becones
ooze → compact ooze → chalk → compact chalkj → limestones
biochemical oozes
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
where is calcareous materials
along all mid ocean ridges there is calcareous material
most mid ocean ridges at 2.5 km depth
Calcareous oozes
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
AMorphous silica and chert (flint)
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
Flint and chert
Bands of flint are typical of the Upper Chalk (Upper Cretaceous)
amorphous silica collects along bedding planes and in sediment altered by bioturbation
Replacememnt chert
diagenesis and lithificatin
replacement chert
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
Marl and Marlstone
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
Dolomitisation
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
Dolomite
commonly brown (compared to unaltered limestone) harder than limestone does not react with acid unless powdered porosity increases fossils are destroyed
