Transcript stratigraphy14

Stratigraphy
Francis, 2014
Stratigraphy
Stratigraphy is the study of successions of stratified
(layered) rocks in time and space. In its classical days,
stratigraphy involved simply the study of lithostratigraphy, that is the succession of rock types in
stratigraphic sections. Strata were grouped according to
lithologic affinity into the following litho-stratigraphic
hierarchy:
Supergroup
Group
Formation
Member
Bed
Formations are the basic building block of lithostratigraphy, in effect the unit that can be mapped in the
field. They are vaguely defined as any unit that can be
recognized according to its lithologic character. Over
short distances, lithologic formations can commonly be
correlated between stratigraphic sections. Distinctive
units that occur over wide distances, such as
isochronous volcanic ash beds, provide particularly
useful correlations.
Correlation Problems
Lithologic correlations work well, however, only over relatively short distances. When
attempts are made to correlate spatially distant stratigraphic sections, it becomes apparent that
lithologic beds are typically diachronous, and lithologic boundaries do not represent constant
time lines.
There are a number of resolutions to the correlation problem, including biostratigraphy,
which uses fossils to correlate between sections, or the recognition of isochronous marker
horizons such as bentonite (altered volcanic ash) layers. In recent years, however,
attention has focused on sequence stratigraphy, which uses the cyclic nature of stratgraphic
successions to correlate between sections.
Sequence Stratigraphy
Sequence stratigraphy uses the cyclic nature of stratgraphic successions to correlate between sections.
The recognition of cycles, and cycles within cycles, has now become quite an art form, and today it is
done largely using reflection seismic sections, and is becoming increasingly divorced from direct
connection with actual rocks. Individual bands in these images do not correlate to specific rock types,
but rather parasequences, thought to represent individual shallowing upward sequences.
Parasequence:
The parasequence is the basic unit of sequence stratigraphy. A
parasequence is an outcrop scale (meters to 10’s of meters)
conformable succession of sedimentary rocks that typically
represents a single shallowing upward cycle, bounded by marine
flooding surfaces. A parasequence thus represents a single
episode of sediment progradation (the seaward movement of
shoreline), typically lasting 10’s to 100’s of thousands of years.
Lithologic facies within a given parasequence may vary from
section to section, and each section need not contain a full range
of the lithologies of the shallowing upwards cycle, but the
upward succession of lithologies is always the same and strictly
obeys Walther’s Rule.
Walther’s Rule of Facies Succession:
In a conformable sequence of sedimentary facies, vertically succeeding facies must
laterally coexist in time with the facies they succeed.
As an example, examine the schematic representation below of an idealized prograding
continental margin, comparing what is observed in a single stratigraphic coarseningupwards sequence, with what is actually happening on a regional scale.
Parasequence
Parasequences are terminated by marine flooding
events possibly associated with fluctuations in
glaciation driven by external Milankovich cycles,
but they could also reflect tectonic subsidence or
autocyclic processes such as avulsion or simply
lobe switching in an active depositional regime.
Milankovich cycles:
~100,000 & 400,000 yrs - cycle of orbital eccentricity
~100,000 yrs
- cycle of tilt of orbital plane
to the ecliptic
41,000 yrs
- cycle of tilt of rotation axis
21,000 yrs
- chandler wobble of rotation
axis
Parasequences occur in “sets” with consistent staking vectors.
Prograding basin margins have a consistent geometric
architecture consisting of:
• Topsets beds with very low dips (< 0.1o) comprised
of relatively shallow water sediments transported by
fluvial, tidal, and storm currents, as well as wave
action.
• Clinoforms beds with steeper dips (> 1.0o)
comprised of deeper water facies transported by
gravity flows such as debris flows and turbidity currents.
The transition between these two is termed the offlap break
that is not equivalent to the present continental shelf edge
off eastern North America, which is a relict failure, not a
depositional feature.
The rate of progradation is a balance between sediment supply
and the creation of accommodation space:
accomodaton space = water depth + deposited sediment
= Δ eustasy + Δ tectonic subsidence
There are endless arguments about the relative important of
eustasy and tectonics in sedimentary basins, which arise from
the observation that great thicknesses of sediments appear to
have been deposited over time in very shallow water depths and
the basic postulate that you cannot sink a rowboat by filling it
with sawdust.
Sequences are stratigraphic successions bounded by surfaces of significant sub-aerial
erosion, representing a major cycle of sedimentation lasting from ~ 5 to 15 Mys. They reflect
sea level changes in response to major tectonic activity, such as changes in the volume of
oceanic ridges and/or sense of sea floor spreading. They are typically comprised of a number
of different parasequence sets that can be grouped into one of three “tracts”:
low stand tract
-
sea level below offlap break, down and up
transgressive systems tract
-
topset accommodation vol. > sediment supply
maximum flooding surface
high stand systems tract
-
topset accommodation < sediment supply
Sequence Boundary
deep water sands and muds - turbidites
shallow water
carbonate
Role of Tectonic Subsidence
169 Ma
17 Ma
12 Ma
6 Ma
Development of the Queen Charlotte Basin
now
Sea Level
Variations
The complete melting of the Antarctic icecap will cause sea level to rise ~ 70 meters;
Greenland’s icecap would cause an additional 7 meters of rise. The fact that sea level
has been more than 200 meters higher than today requires higher temperatures and
shallower ocean basins in the past.
Sea Level Variations
Late-Tertiary : 12 Mys
Phanerozoic : 500+ Mys
sequence
boundary
Stratigraphic Records of Climate Change
Paleo-temperature variations can be obtained using the stratigraphic variation of
oxygen isotopes 16O and 18O.
Fractionation of 16O from 18O occurs when water evaporates or condenses, with
liquid water always being more 18O-rich than coexisting water vapour. Relatively
16O-rich water vapour migrating from warm equatorial regions towards the colder
poles becomes increasingly 16O enriched because of the distillation of 18O into rain.
As a result, the polar icecaps are relatively enriched in 16O compared to the oceans.
At times of thick ice caps, the oceans are relatively 18O rich because 16O is stored in
the ice, while at times of low ice volumes, the oceans are relatively 18O poor
because of added 16O from melted ice.
Delta 18O = δ18O = ((18O/16O)sample / (18O/16O)standard – 1) × 1000%o
Analyses of the climate variation over
the last few 100 thousand years
reveals a strong inverse correlation
between the concentration of CO2 in
the atmosphere, δ18O, and the volume
of polar ice.
Present CO2 concentration ~ 387 ppm
PETM
Marine Methane Clathrate
De-Stabilisation?
Initial Temperature
increase due to CO2 released
by the eruption of the Slave
Provonce Kimberlite Field?
Patterson & Francis, 2013
The Rise of Oxygen
global glaciations
The Rise of Oxygen
The future of
Sedimentology :
Mars
Gale Crater
Curiosity
Upper Mound Unit or Formation
Aeolian Dust Rock?
Boulders
Lower Mound Unit
or Formation
Martian Climate Change
Lower Mound Unit
or Formation