Conglomerates Flashcards
Siliciclastic sedimentary rocks made up of
gravel-size (>2 mm) clasts.
Conglomerates A.K.A.:??
Rudites
Sedimentary rock composed of Gravel size sediments or
30% clast composition
Common in stratigraphic sequences of all ages but **less than ** of sedimentary rock mass
1 % of the total weight
Useful due to:
- tectonic and provenance analysis
- has specialized depositional environments
- reservoir rocks of oil and gas
Folk
- ???? = <30 % Gravel
-????= >30% Gravel
- gravelly mud/gravelly sand
Gravel
-
Gilbert
25-50% gravel-size clasts
>50% gravel size clasts
<25% gravel size clasts
Conglomeratic Sandstones or Conglomeratic Mudstones;
Conglomerate ;
Pebbly SST/Pebbly MST;
Non sorted - Poorly Sorted w/ larger particles of any size in a muddy matrix
Diamictite
poorly sorted sedimentary rock made up of dispersed pebbles in an abundant MST matrix
Pebbly SST
- Mud/Sand Matrix abundant
- Clasts does not form a supporting framework
Matrix Supported
- Little Mud/Sand Matrix
- gravel sized framework in contact, forming a supporting framework
Clast Supported
- Aggregates of Angular, gravel size fragments
- distinguished from breccias as sharp edged and unworn cornered clasts
- Can also be non sedimentary in origin (ie. volcanic and tectonic breccias)
Breccias
- Extraformational : Clasts originating from outside the formation
- Epiclastic : Generated by breakdown of older rocks via processes of weathering and erosion
Extraformational, Epiclastic CGL + BRC
- formed by penecontemporaneous fragmentation of weakly consolidated beds and subsequent redeposition of fragments w/in the same depositional unit
- processes usually involve short term events (few hours or days) such as storm waves, mass flows which bring clast fragmentation and redeposition
- occur as thin units and generally localized
- well rounded to angular - depends on amount of transpo + rework
- Intraformational CGL + BRC
- flat/disc-shaped clasts
Flat Pebble CGL
flat/disc-shaped clasts stacked virtually on edge
Edgewise CGL
- Formed by Primary Volcanic Processes
- Explosive Volcanism
- Autobrecciation of partially congealed lavas
- Quench Fragmentation of Hot Magmas
Volcanic Breccia
- formed via cataclasis/collapse
Cataclastic Breccias
soluble rock dissolution, leaving behind insoluble gravel size residues
Solution Breccia
- stable CGL
- single clast type
-composed of clasts from a single source/mineralogy/petrology
- Oligomict Conglomerate
assortment of clasts
Polymict Conglomerate
a type of polymict that is made up of unstable/metastable clasts
Petromict Conglomerates
- Not Recycled from an older gen of CGL
- Some are enriched from quartzose due to source rocks being quartzite, qtz, arenite or chert nodule LST
- Other came from mixed parent-rock sources w/ less stable rock types
- Continued Recycling can cause selective destruction of unstable clasts and concentration of stable ones
- Petromict and Oligomict composed of weak clasts, both of which are more likely to be first-cycle deposits
First Cycle Deposits
For CGL
can remove fine size detritus and deposit gravels w/ little sand/mud matrix
High Energy processes (fluvial or beach)
Glacial transport and sediment-gravity flow yields ????
- May contain more muddy matrix than framework clasts
gravel with abundant matrix
CGL w/o matric (unfilled voids among pebbles)
- Uncommon compared to SST w/ open pores
- Two Size Modes
- Gravel size range (framework)
- Mud size range (matrix)
Openwork Conglomerate
- Shallow, Braided Streams
- Steam Energy high
- Episodic Discharge
- Dominantly Clast supported
- Non graded beds but with vertical facies having a fining upward trend
- Clasts
- upstream dipping imbrication
- Transverse long axis orientation
Sheetflood (braided stream)
- Deeper Fluvial Channels
- Clast Supported
- Silts/Sand Matrix rare to abundant
- Clasts
- unimodal orientation
- dominant upstream imbrication dips
- transverse long-axis orientation
- fair size sorting
Streamflow
Nearshore environment
- Sufficient wave energy to transport and rework gravels supplied via fluvial or coastal erosion
- Constant reworking in surf zone produces well sorted + rounded gravel deposits
- Clast supported
- Clasts
- may appear as thin gravel deposits in sandy beaches
- disc shape, good sorting
- well developed
- seaward dipping imbrication
- seaward stratification
Wave Worked (beach face)
- deposited in shoreface and shelf
- poor to moderate sorting
- fabric ranges from clast to matrix supported
- Structures:
- sharp-based
- beds more than 1 m thickness
- cross beds, normal grading or imbricated in lower parts
- Clasts
- bimodal dip directions
- Shoreface deposits gravel dominated or;
- Consists of sand w/ interbedding gravelly layers
Wave-, storm-, and current worked (shoreface and shelf)
not well represented in the ancient sed record
- Matrix supported but some are clast supported
- Clasts
- moderate-poorly sorted
- rounded to well rounded
- Grading
- variable
- Tabular to trough cross bedding
- Fining upward trend
- diminishing upward in size + abundance
Tide-Worked
- Deposited Subareally
- Materials dropped by melting of grounded glaciers
- poorly sorted
- matrix rich gravelly deposits; called till/glacial diamictite
- Clast
- meter size boulders
- faceted or striated
- long dimension parallel to ice-flow direction
- little or no imbrication
- massive but has lenses of better sorted material
Meltout/Lodgement
- AKA aquatillites
- lacustrine/marine environment
- due to deposition from melting, gravel-charged ice
- Deposition occur as traction carpet
- from dense meltwater underflows from subglacial tunnels
- or meltout of ice rafted material from floating ice
- Matrix supported
- Poorly sorted
- Clasts
- angular to well rounded
- striated, polished and faceted
- Random Dropstone orientation unless reworked by bottom currents
- Traction underflow deposits
- parallel-to-current-flow long axis trend
- up current imbrication dips
Subaqueous meltout
from land debris flows
- alluvial dans and proglacial outwash fans
- sediment gravity flows
- gravel sized particles
- presence of cohesive matrix of clay and sand particles
- capable of tranpo mats of varying sizes
- deposits are matrix supported
- poorly sorted
- Non graded
- show no preferred orientation
- no preferred internal stratification
- Cgl units maybe capped by a thin later of gravel overlain by sts
Subaerial Debris Flow
Retransportation of previously fluvial, lacustrine, coastal, or shelf to deep water environments
- Can occur via massflow process
- subaqueous debris flows,
- marine and lacustrine, also seen in glacially influenced envi
- density-modified grain flows, and
- high-density turbidity currents.
- Both subaqueous debris flows and grain flows can likely evolve into fully turbulent turbidity currents with downslope mixing and dilution
Resedimented
Pyroclastics @ 2-64 mm
Lapili
Pyroclastics > 64 mm =
Blocks (angular); bombs (round)
- lithified deposits from coarse pyroclastic particles are called
lapillistone, pyroclastic breccia, and agglomerate (bombs).
Angular fragments from lava-water induced granulation or lava shattering
Hyaloclastites
- Thick sequences of cgl implies that
- preserved and accumulated metastable clasts
- rapid erosion of sharply elevated highlands or areas with active volcanism
- Alternatively,
- Clasts may form from glacial processes such as LST conglo
- consists dominantly of metaquartzite, vein-qtz or chert clasts
- derived from metaseds, seds, and some igni rocks
- residuum concentrated by destruction of a larger rock volume
Quartzose conglomerates
Less Stable Metaseds
-like argillite, slate and schist
- from weathered, eroded and sed transpo
- Concentration of Vein-Qtz clasts in qtzose cgl
- implies destruction of large igni/met bodies
- Concentration of chert clasts
- destruction of large chert nodules in a lst
- may also be derived from erosion of bedded cherts
Since Qtz are a small part of source rocks, an enrichment of these in cgl implies that
extreme chemical weathering or vigorous transpo
- More than one cycle is involved
- Source rocks likely come from orogen or continental block provenances
Two Groups of Clasts by stabilityy
- Ultrastable Clasts
- Metastable Clasts
- Two CGL based on stability
- Quartzose CGL
- Petromict CGL
framework grains with more than 90% ultrastable clasts
Quartzose CGL
- less than 90% ultrastable clasts; more metastables
- common use for cgl with abundant un/metastable clasts
Petromict CGL
Elongated Gravel size clasts orientation
- Orients transverse to current flow
Sand Size clast orientation
oriented parallel to current flow
Tabular and Elongated ones
can be imbricated/shingled under unidirectional currents
- Clasts in a fluvial CGL is
well rounded
Fissile fabric are likely to release
tabular or disc shaped fragments
Massive rocks tend to be
more equant
Debris flow type of deposition may have matrix supported, with rapid erosion
Fanglomerate
