Stratigraphyx

clankflaxMechanics

Feb 22, 2014 (3 years and 5 months ago)

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Stratigraphy

Notes


1.13.2011


Structure:

Principles of stratigraphy (description

identification)

Sedimentary petrology

Interpretation

sedimentology and environment of deposition


Observation and common sense is necessary to stratigraphy, the oldest of
the geologic sciences

Started with
Nicolas Steno
, a Catholic priest who lived form 1638
-
1686. He observed nature,
wrote in Latin, was a medical doctor, was Danish, N. Italy, Germany, found by observing rocks I
n N. Italy 4 principles “The Firm and the Fir
m enclosed” (seashell fossils)


1.

Law of Superposition

youngest layers on top, as you go lower in a series of bed, age
increases, as long as no disruption has take place

2.

Law of inclusions

inclusions formed before the rock they are encased in; fossils are
old
er than surrounding rock

3.

Law of (near) original horizontality

sediments are not stable on steep slopes, over large
distances, there is only a very slight deviation from horizontal deposition

4.

Principle of original continuity

beds were once continuous, erosi
on occurred later

5.

Cross
-
cutting relationships

sedimentary layers present before volcanic intrusions


*in a dry climate, limestone weathers as granite does in a wet climate

*remote sensing, landscape

underlying rocks, grain size, etc based on a 80mx80m pix
el, you
can determine underlying grain size down to the mm


~History of the earth handout


Precambrian rocks generally missing


We usually deal with the Phanerozoic


Look for best exposed examples

*Quaternary v. Neogene
---
deposition is never truly complete
ly continuous, find best area for
example, use that area’s name, n.b. Paleogene + Neogene = Tertiary


Geologic time periods est. 1790
-
1860, all the major periods were recognized and named


Cambrian

Latin word for Wales


Ordovician

Latin word for the people

of Wales


Silurian

Celtic tribe of Wales


Devonian

Devonshire, England


Carboniferous

coal bearing/industrial revolution repercussions


Triassic

central and western Europe, 3 distinct rock layers


Jurassic


Jural/Ural Mtns. In Switzerland, grey/green colo
r


Cretaceous

chalk bearing, Dover, light grey limestone


Tertiary

primary, secondary, tertiary (and quaternary) beds defined in N. Italy


Quaternary

see above

*other smaller time chunks named for grapes, regions, etc.

How do we approach stratigraphy? Don
’t jump to conclusions!


1)

Lithostratigraphy

describe the rocks

a.

Bed

single bed in a formation, e.g. coal seam

b.

Member

subdivision of a formation

c.

Formation

a mapable unit thick enough to show on a map

d.

Group

several formations

e.g. Mata Vaca formation

dead cow
valley, use good names and give good reasons

*you can’t correlate between different basins,
local

*Lithostratigraphy has nothing to do with time other than the principle of superposition


2)

Biostratigraphy

major basis of all further work, use of fossils to c
reate zones, fossils
change through geological time, more recent fossils are more similar to modern life than
more ancient fossils

a.

Zone

basic unit


3)

Chronostratigraphy

emphasize rock aspect

a.

System


4)

Geochronology

emphasize time aspect, numerical data

a.

Period
e.g. Devonian, etc.


1.18.2011


Things that can happen in 1 basin, original continuity is not always mandatory, see handout p.
221, fig 9.3, boundaries, intertonguing of facies


*book figure 12.1, pinch
-
out, intertonguing, lateral gradation, contacts

abru
pt
/progressive

Hiatus

a gap in time, no deposition, or very little deposition, nice sharp bedding planes are
marked by hiatuses


e.g. ripple marks in siltstone

1)

Silty clay deposits

2)

Deposition stopped

3)

Water on top created pattern

4)

Lithification


Some events
very short/quick, e.g. 2 tides a day, looks continuous on grand scale, but up close
you see hiatus, rate, time frame, what’s going on?


Continuous sedimentation

is relative, doesn’t really exist

except for deep oceans, where 1 mm
accumulates over a million

years, continuous “rain”


Sedimentation usually only occurs with extreme events, e.g. freak convergent T
-
storms in
Appalachia account for strange coarse pebble deposits, highly discontinuous


How doe we describe lateral facies changes?

Facies

“face,”
general aspect of the rocks, e.g. sandstone facies v. shale facies, marine facies v.
terrestrial facies


Walter’s Law

w/i 1 system, facies migrate over time if you move laterally in a facies, they
migrate as they do vertically, depositional environments mi
grated over time b/c sea level changes




Transgression

ocean moves over land


Regression

ocean retreats from land

*hole drilled for city hall construction 150 ya revealed cretaceous beach sands, a major
regression of the ocean has occurred to the jersey s
hore


Thru deep geologic time, sea levels vary greatly


*during last interglacial, 100,000 ya, sea level was 6m higher

*in Jurassic and Triassic, there was a mid North American shallow sea dividing east and west,
“ocean of Kansas”


*when drawing beds, alwa
ys indicate their relative resistances to weathering by making more
resistant beds protrude more than beds that are more easily eroded, coarse grained materials like
sandstones are more resistant than fine grained materials like shales


Datum

a layer you c
an correlate, find best, use it first, line up 2 columns, a general, not perfect
fit, bottom and tops of columns not as important


Section

what you can see or what you are interested in


*ultimately, it’s all a hypothesis, eventually clarified by later wor
k, but it’s the best you can do at
a certain time


Craton

consolidated continental Precambrian crust, N.A., roughly delineated by the Rockies
and the Appalachians, these mtns formed later when other land masses smashed into the craton
curst


*spot where Ph
ila. Is did not exist before the Ordovician


1.18.2011.lab


Drilling techniques

rotary drill, see handout for illustration, drilling fluid, sampling of debris,
cores, wild cat wells 1/10 productive, geophysical measurements needed to find oil/gas,
radioactivity indicates shaliness of rocks as clay has high levels o
f K40 unlike sandstones


Sonde

French probe using sound waves, Schlumberger company,


4 Long Hiatuses


1)

Angular uncomformity

deposition, tilt, erosion, horizontal deposition

2)

Disconformity

deposition, uneven erosion, deposition

3)

Paraconformity

deposition, completely even erosion removing an entire bed,
deposition, no trace time is missing unless radiometrically analyzed, looks normal

4)

Nonconformity

igneous uplift, erosion, down sinking, sedimentation


1.20.2011


Microtidal


1 m, mesotidal


2 m,
macrotidal


+2m, e.g. Bay of Fundy up to 17 m in Nova
Scotia


Eustatic

world wide sea level change, not local change


Biostratigraphy

we use marine invertebrates and terrestrial plant/animal fossils to describe and
identify formations

*Kingdom

灨pl畭

捬ass

o牤敲

family

g敮es

sp散e敳

underline or italicize the Genus and species e.g.
Neuropterus ovata

or
Neuropterus
ovata


fossil collection

superposition, observation and logic, time periods,
marine records are more
complete, 85% preservation in oceans v. 15%
preservation on land (erosion)


epicontinental seep

shelf by coast, oceans of Kansas, shallow ocean deposits


index fossils

widespread, dominant, specific to time periods, prolific, common, extinct, e.g.
trilobites, brachiopods


restrictions: no preservat
ion, not discovered, destroyed, never occupied that area,


*not finding something doesn’t mean anything, only relationships to area where we do find it


Fig. 14.12
--
# of genera v. time, mass extinctions in the Permian and Triassic,

increase in
proliferation and diversity


Trilobites

Paleozoic, wiped out at Permian mass extinction

Ammonoids

Mesozoic, extinct at K
-
T boundary


*consider Monoplacophora, thought long extinct, discovered living in the 1950s


Polyplacophora, 7 hinged shell


*splitters and lumpers


Ecostratigraphy

ecology of facies, environmental change, abundance, biozone, niche,


LAD

last appearance datum, less reliable than FAD,

extinction
--
worldwide disappearance


exterplation

only local disappearance, organism survive
s elsewhere

FAD

first appearance datum, when organism became prolific, fossils are only a natural
sampling, species either evolved from something else via natural selection or took on novel
characteristics,

broke out of isolation or migrated, Hawaii, Aust
ralia,


new species arise in small isolated populations where beneficial mutations can be easily passed
on, geographical barriers such as mountains or water are helpful