Conglomerates

1. Conglomerates geology.about.com/.../lagunaconglom.htm San Onofre Breccia, Laguna Beach

2. Introduction Rocks mostly of > 2mm clasts (gravel) Also called rudites Common, not abundant Important for tectonics, provenance, depositional environments, economics Grains = mostly rk fragments May contain matrix--clay or sand matrix is finer material which infills the spaces between grains

3. Example of Matrix Matrix is not cement It is deposited with rest of sediment Miocene San Onofre Breccia, Aliso Beach County Park, www.geoscapesphotography.com/ca-alisobeach.htm

4. Study mostly in field--hand sample not truly representative of outcrop Need to see large scale variations in composition & textures

5. Definition Varies Folk - 30% gravel; less = gravelly mud/sand Gilbert - 50% gravel < 50% = conglomeratic ss/mudstone Also called pebbly ss/mudstone Gilbert uses this term at <25% Is pebbly correct term? Confusion led to term “diamictite” Poorly sorted, large particles in mud matrix So, have conglomerates with/without matrix

6. Diamictite Boulder of diamictite of the PrecambrianMineral Fork Formation. It is lithified glacial till. Along the Elephant Head Trail, Antelope Island, Utah. Scale bar on notebook is 10 cm. Taken 2002.

7. Conglomerates Defined Using Fabric Clast-supported (framework supported) Matrix supported-diamictite memoirs.gsapubs.org/content/198/105/F7.large.jpg

8. Breccias Angular, gravel-sized particles

9. Origin of Breccias Sedimentary--mass flows--triggering mechanism Non-sedimentary--tectonic (faults, folds); volcanic When is rk a breccia?--is any fragmented rk a breccia Miocene San Onofre Breccia, Aliso Beach County Park, www.geoscapesphotography.com/ca-alisobeach.htm

10. Intraformational Conglomerate/Breccia Intraformational= redeposited fragments. of weakly consolidated beds Thin, short term events e.g. landslides Rounded to angular fragments. common in shales & l.s. http://www.ing.pan.pl/Sediment_2009/03-B1_Trips.htm http://www.depauw.edu/acad/geosciences/fsoster/Mudcracks.htm

11. Epiclastic (extraformational) Conglomerates Generated outside basin thru breakdown of older rks Framework clasts Oligomictic (e.g. qtz, arenite-stable fragments.) Polymictic (many dif types of fragments) Polymictic conglomerate. w/-unstable clasts-shale, L.S., basalt -Petromict conglomerate

12. Epiclastic Conglomerate Continues Recycling leads to oligmictic conglomerate, but depends on source of rock (qtzite, qtz arenite, chert etc.) Composition may reflect sorting shale breaks down, metaqtzite= coarser durable clasts volcanic –bigger clasts survive; finer clasts may not survive transport

13. Matrix Supported, Polymictic Conglomerate, New Zealand http://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/conglomerate.html

14. Oligomictic Breccia, New Zealand

15. Matrix of Epiclastic Conglomerate Clay & sand-size material May have qtz, calcite, hematite, clay cement

16. Conglomerate Textures Matrix content & fabric support High energy transport process=gravels w/little sand Glaciers=matrix supported=diamictites Typically, 2 size modes gravel and mud to sand

17. 1998 Debris flow deposits in Chalone Creek, Pinnnacles National Monument, San Benito County, California USGS

18. Debris flow deposits partly cover a road in Stevens Creek Canyon (Santa Clara County). The debris flow occurred during a storm in late January 2003

19. Structures Massive—lacks internal stratification Crude to good planar horizontal or inclined stratification Tabular & trough x-beds , less common than S.S May have normal & inverse grading

20. Fluvial Conglomerate, France Conglomerates and sandstones representing bars deposited in the channel of a braided river. Note the lenticular bed geometry and crude stratification. The northward dip (to the left) is a result of folding during Pyrenean earth movements. http://www.virtual-geology.info/sedshots/sedshot023.html

21. Crudely stratified Conglomerate/breccia,Protestant Cathedral, IrelandNote alignment of the clasts

22. Classification Lack adequate system use informal names--qtzite cong (comp.); fluvial conglomerate (dep env.); pebble conglomerate. (clast size probably because cong. not easily class; don’t have 3 principle clast types Classify Clast Stability ultra stable clasts (>90) = qtzose cong/diamictite---multicycle meta/unstable clasts (> 90)= petromict conglomerate/diamictite; First cycle

23. Classification Continued Classify Clast Lithology clast stability class--little info of comp meta-clast, ig-clast, sed clast, polymict conglomerate tells about provenance also used for diamictite

24. Classification Continued Clast Size cobble-rich qtzose rudite;boulder-rich petromict diamictite

25. Qtzose Conglomerate Metaqtzite, vein qtz, chert--sed, meta, ig rk least stable lithology destroyed by weathering, erosion, transportation vein quartz=erosion of ig/meta rk implies intense weathering or vigorous transport mostly fluvial magicmomma.smugmug.com/.../1/790534557_A6f9J Massanutten Formation ,Silurian, Virginia www.nvcc.edu/.../massanutten_conglomerate.jpg

26. Petromict Conglomerate have metastable clast, various source rks plutonic not as common--maybe due to weather. first cycle can be extensive imply rapid erosion, elevated highlands limited transport--fluvial to deep marine

27. Depositional Environments textures & structures related to environmental conditions reconstruct ancient environments genetic class. desirable but difficult fluvial, glacial, marine gravel process-related sheetflood, streamflow, wave-worked--this is favored

28. Genetic ClassificationPaleocene-Eocene Submarine Fan Conglomerate & Turbidites, German Rancho Fm, Salt Point, San Francisco http://eqdoc.home.netcom.com/salt2.html

29. Conglomerate Textures Shape and orientation Gravel rounded in short distances Therefore, fluvial conglomerate. may have rounded gravel in angular sand

30. Conglomerate Textures Preferred orientation to elongate clasts Transverse to flow, imbricated Pebble Imbrication,Quaternary Aluvial Fan Deposit, Spain

31. More Imbrication, Truckee River Nevada (c) 2006 Andrew Alden

32. Form (sphericity) related to parent rock- fissile/schistose fabric=tabular, massive rocks more equant