3. Siliciclastic Sedimentary Rocks
โ Definition: Siliciclastic rocks are clastic non-carbonate sedimentary rocks that are
composed primarily of silicate minerals, such as quartz or clay minerals.
โ Siliciclastic rock types include mudrock, sandstone, and conglomerate.
โ They make up roughly 3/4th of all sedimentary rocks.
โ e[
4. Carbonate Sedimentary Rocks
โ Defination: Carbonate rocks are a class of sedimentary rocks composed
primarily of carbonate minerals. Carbonate rocks form in shallow marine
environments.
โ The two major types are limestone, which is composed of calcite or aragonite (different
crystal forms of CaCO3
), and dolomite rock (also known as dolostone), which is composed
of mineral dolomite (CaMg(CO3
)2
).
Fig: Limestone
5. Carbonates V/S Clastics
Most of the carbonate deposits are in situ, whereas clastics are transported.
โ Most of the carbonates are of biogenic.
โ Therefore, carbonates are not affected by external processes.
But, in some cases siliciclastics and carbonate rocks are deposited together.
7. Siliciclastic dominant continental shelf
โ The sediments accumulating on the continental margins vary between siliciclastics and
carbonates with most areas accumulating siliciclastics.
โ These siliciclastics of continental shelf are sourced from the shoreline.
โ The grain size tends to decrease with distance from shore.
Various types of siliciclastic dominant shelf are:
1. Storm dominated shelf
2. Tide dominated
3. Shelf dominated by Ocean Currents.
8. Carbonate dominant continental shelf
โ Carbonates are basically deposited in shallow marine environment called
carbonate platforms
โ Two main conditions for formation of carbonate platforms:
1) isolation from clastic supply.
2) shallow marine waters.
โ Change in pH, salinity & temperature can affect carbonate productivity.
Types of carbonate platforms are:
โ Carbonate Shelf:
a)Non-rimmed shelf
b)Rimmed shelf
โ Ramp
โ Epeiric platform
โ Isolated Platform
Fig: cross section of various carbonate shelfs
10. Mixed Carbonate-Clastic Environments
Conditions for mixed siliciclastic-carbonate deposits:
โ in stratigraphic record
โ climate
โ alteration of bed of limestone with mudstone
โ other mechanism
โ with increasing water depth, most of the shelf areas will be starved of mud and sand,
allowing carbonate sedimentation to occur in place of clastic deposits
โ Fluctuation in sea level results in alteration between limestone and mudstone deposit.
Two types of siliciclastic-carbonate environments are:
1)Spatial variations
2)Lateral variations
11. Fig:Stratigraphic occurrence of mixed siliciclastic/carbonate examples
These examples are from Phanerozoic successions in North America
Kutch basin of western India
12. SPATIAL VARIABILITY (LATERAL FACIES MIXING)
โ It is the most common type of mixed system.
โ Differences in trace-fossil distributions between carbonate and clastic components are a direct
function of grain size and shape and thus commonly reflect the influences of depositional energy
and environmental segregation of grain type.
Case study: Western Canada and the western United States
In both of these regions, the Permian was dominated by carbonate deposition The demise of many of
the carbonate-producing taxa coincides in many areas with a shift from carbonate dominance to clastic
dominance at the Permian/Triassic boundary.
Some examples are included in this section (e.g., Montney Formation, Alberta; Liard Formation,
British Columbia; Moenkopi Formation, Nevada and Utah).
13. Trace fossils in the Limestone Member of the Moenkopi
Formation (Early Triassic), Nevada and Utah, USA.
(A)Asteriacites lumbricalis on the sole of a sandy storm
bed, lower shoreface, Ute, Nevada.
(B) Bedding-plane view of branching Thalassinoides
within a bioclastic packstone bed, proximal offshore,
Beaver Dam Mountains, Utah. The burrow fill comprises
disarticulated and loosely packed ossicles of the crinoid
Holocrinus smithi.
(C) Rhizocorallium within amalgamated hummocky
cross-stratified sandstone, lower shoreface, Ute, Nevada.
(D) Bedding-plane view of branching Thalassinoides
within a bivalve wackestone, proximal offshore, Lost Cabin
Spring, Nevada. The burrow fill consists of densely packed
bioclastic debris.
(E) Firmground horizon with sharp-walled Thalassinoides
penetrating the underlying bivalve-wackestone storm bed,
offshore transition, Ute, Nevada. The burrow was passively
filled with bioclastic debris from overlying bed.
14. TEMPORAL VARIABILITY (STRATIGRAPHIC MIXING)
โ Many mixed systems are produced by stratigraphic controls; changing relative sea-level results
in changes to sediment sources
Case studies: Chase Group (Early Permian), Oklahoma; Schei Point Group (Late Triassic), Sverdrup
Basin; Baldonnel Formation (Late Triassic), British Columbia; Baldonnel Formation (Late Triassic),
British Columbia
15. Conclusion
โ Mixed siliciclastic/carbonate successions are common in the rock record.
โ In many cases, the mineralogical mixing appears not to have affected trace-fossil assemblages.
with few exceptions, organisms do not care whether the sediment they burrow in is composed of
carbonate grains, quartz grains, olivine grains, or any other grain type.
โ In many of the mixed systems discussed herein (e.g., Montney Formation, Moenkopi
Formation, Liard Formation, modern Craig Bay intertidal), siliciclastic sand/sandstone
successions occur interstratified with variably abundant bioclastic detritus. The grain sizes
within the bioclastic component are highly variable but are generally significantly coarser than
in the siliciclastic component.
โ Trace fossils being inherently more difficult to recognize in coarser bioclastic intervals than in
fine-grained siliciclastic intervals.
16. Reference
โ PhD scholar Dr. Suraj Bhosale
โ Zonneveld 2012 (Chapter26)
โ Introduction to sedimentology by Supriya Mohan Sengupta
โ Principles of Sedimentology & Stratigraphy by Sam Boggs.
โ Sedimentology and Stratigraphy by Gary NIchols.
โ Wikipedia
โ Google
