Dear Mr. Mcclure

it is an interesting issue but a never-ending story to discriminate depositional environments by grain-size variation only. It does not work. There are too many overlaps. I have only recently published a great many papers in applied sedimentology and geomorphology and used what I call the GMS tool. GMS is representative of (1) granulometry , (2) Morphology of grains. (3) Situmetry ( = orientation of grains). There is a GMS tool plus . It means a combination of the mineralogical/petrographic composition of the sediment (matrix + clasts) and the CMS tool.

(1) overall grain variation in a cumulative frequency distribution + sorting, median, skewness (lab-based)

(2) sphericity of grains , angularity (lab-based)

(3) orientation of the longest axis in plan view (+ vertical) (field-based)

Only by a multiple-tool approach you are able to constrain the depositional environments.

The bimodal grain size variation ( matrix vs. clast association) is a common one from fine sand to gravel and from pyroclastics to siliciclastics.

DILL, H.G., BUZATU A., and BALABAN S.-I. (2021) Straight to Low-Sinuosity Drainage Systems in a Variscan-Type Orogen—Constraints from Tectonics, Lithology and Climate.- Minerals (in print)

DILL, H.G., BUZATU A., and BALABAN S.-I. (2021) Coastal morphology and heavy mineral accumulation in an upper-macrotidal environment– A geological-mineralogical approach from source to trap site in a natural placer laboratory (Channel Islands, Great Britain).- Ore Geology Reviews 138: 104311

DILL, H.G., KUS J., BUZATU A., BALABAN S.-I. ,KAUFHOLD S., and BORREGO A. G. (2021) Organic debris and allochthonous coal in Quaternary landforms within a periglacial setting (Longyearbyen Mining District, Norway) - A multi-disciplinary study (coal geology-geomorphology-sedimentology).- International Journal of Coal Geology 233: 103625

DILL, H.G., BUZATU A., BALABAN S.-I. , UFER K., TECHMER A. SCHEDLINSKY, W., and FÜSSL M., (2020) The transition of very coarse-grained meandering to straight fluvial drainage systems in a tectonized foreland-basement landscape during the Holocene (SE Germany) – A joint geomorphological-geological study.- Geomorphology 370: 107364.

DILL, H.G., BUZATU A., BALABAN S.-I. , UFER K., GÓMEZ TAPIAS J., BÎRGĂOANU D. and CRAMER T. (2020) The “badland trilogy” of the Desierto de la Tatacoa, Upper Magdalena Valley, Colombia, a result of geodynamics and climate: With a review of badland landscapes.- Catena 194: 1-20.

DILL H.G., BUZATU A., GOLDMANN S., KAUFHOLD S., and BÎRGĂOANU D. (2020) Coastal landforms of “Meso-Afro-American” and “Neo-American” landscapes in the periglacial South Atlantic Ocean: With special reference to the clast orientation, morphology, and granulometry of continental and marine sediments.- Journal of South American Earth Sciences 98: 102385

DILL, H.G. , KAUFHOLD, S., TECHMER, A. , BARITZ, R., and MOUSSADEK, R. (2019) A joint study in geomorphology, pedology and sedimentology of a Mesoeuropean landscape in the Meseta and Atlas Foreland (NW Morocco). A function of parent lithology, geodynamics and climate.- Journal of African Earth Sciences 158: 103531.

DILL, H.G., UFER, K., BORNEMANN, A., TECHMER, A. , and BUZATU, A. (2019) From the strand plain to the reef: A sedimentological–geomorphological study of a Holocene coast affected by mud diapirism (Archipélago Rosario- Barú, Colombia).-Marine Geology 415: 105953

Available on the RG server for download upon request

Kind regards H.G.Dill

The exoscopy of quartz grains is a powerful tool. It allows to recognize characteristics related to the action and transport by the glaciers such as: shape, character of surface and micro textures.

Dear Dr. Dill,

Thank you for a helpful introduction to your GMS and CMS classification schemes. These approaches would be best suited to appraisal of sediments deposited in uniformitarian regimes that imply a stable crust. However, processes occurring in both environmental subjects of this discussion imply transient phases of crustal instability.

Mudslides as distinct from Bogslides tend to occur most frequently in subtropical and warm temperate areas both potentially subject to late Tertiary uplift or related seismic activity. Many of these areas have been subject to deep tropical weathering during the Cenozoic, which affects both inherent stability and chemical composition. Therefore CMS may be of more value than GMS as an indicator of a mudslide. Can you suggest possible chemical signatures that might indicate a subtropical weathered origin in redeposited sediments?

The grain sphericity and angularity you mention, together with the shape, character of surface and micro texture characteristics suggested by Dr. Hamoumi, could help distinguish mudslides from till, but again grain roundness can be an inherited feature in both cases; many tills are known to be marine deposits, which may obscure further their primary (or secondary) erosive origin and initial passage through a glacial environment.

Roundness and orientation in the pebble and boulder content can be supplemented by boulder splitting of dropstones as a signature of marine till. However many glacial erratics are well rounded and mudslide boulders can be both angular or rounded as below depending on their transport history.

Malcolm McClure

Can we know their ages since the older rocks have weathered than not so old ones.

Dear Dr. Mcclure,

in the current situation I see a composite mass wasting process which during the final stage may be called simply rock fall coming to rest on the tarmac of the country road. At an earlier stage the situation is certainly different an can most suitably be depicted by the following photographs from the Central African savannahs.

Fig. 12a shows the disintegration of K gneiss boulders strewn across a flat-topped hill. The boulders are rounded by the wet-dry topical climate giving rise to thick regolith and geomorphologically to peneplains. The weathering mantle is almost completely washed away . And the boulders still show the initial rock unit converting through tors (woolsack) into a boulder slide

Fig. 12b shows a boulder-strewn etch plain. The slope angle of the plain is as much as 3°. The syenitic gneiss is broken into pieces which still fit together and thereby attest that these boulders have not been subject to gravitationally forced mass movements. Below the slope angle increases to as much as 9°

Fig. 21a , however, is indicative of talus creep which has been emplaced on top of a strongly altered saprock on K gneiss. Angular gneiss boulders shaped like blades and discs are floating in a gritty matrix (grus) and were aligned subparallel to the slope. Under the influence of the gravitational forces and related mass movement the foliation in the saprock became bent downslope and cubes of gneiss were rotated.

Fig. 21b A large boulder came to rest on top of a dark loamy substrate after the gravity-driven mass movement decelerated. The loamy substrate reacted as some sort of a lubricant. It contains clasts aligned parallel to the hillslope and lensoid bodies of argillaceous material squeezed out downslope. The features attest to a gravity sliding of boulders on well-defined discrete shear planes (white arrow heads).

The facies zones show different stage from in-situ, to parautochthonous to splitting of the various components. Only Fig. 21a can spark a mud slide to form, 21b has already deprived of its matrix and therefore isolate block sliding resulted from that.

Further reading: DILL H.G., LUDWIG R.-R., KATHEWERA A. and MWENELUPEMBE J. (2005) A lithofacies terrain model for the Blantyre Region: Implications for the interpretation of palaeo-savanna depositional systems and for environmental geology and economic geology in southern Malawi.- Journal of African Earth Sciences, 41: 341-393.

H.G.D.

Dear Dr. Kiprotich Kiptum The age of Quaternary tills is usually established using AMS 14C dates from the marine microfauna it contains. The streamlined form of northern Irish tills resulted from flow events during a period 25ka to 14.5ka BP. These forms are usually attributed to outflow from an oscillating ice sheet. In Africa as you are aware, LGM was a moist cooler period 25 to 22 ka BP. The Last Glacial Aridity maximum there followed 22 to 13 ka BP, so of course any soils containing dispersed cobbles etc in Africa do not signify an influx of glaciers or icebergs and must have a different explanation.

In my opinion, the default or customary condition of the Earth’s continental surfaces throughout geological time has been a peneplain that developed as a gently undulating surface within 100m above and below a generally constant sea level. Otherwise, long distance correlation of Palaeozoic and Mesozoic formations would not be possible. These tranquil surfaces were occasionally interrupted by rather localized orogenies and volcanoes that deflected the crust surface and caused outflow of mudslides that spread rapidly (Up to 80 km.p.h) and for considerable distances through shallow seas and lake water.

Finally, uplift of the peneplanes produced the plateaus that are widespread in Africa.

I should appreciate any comments on this hypothesis, Dr Kiptum.

Malcolm McClure

Dear Dr. Dill,

Your photos are a useful reminder of the importance of the thixotropic principle in mudslides. Boulders, pebbles and sand can move together if associated mud forms a mobile thixotropic paste when subjected to earthquake vibration, as in Anchorage, 1964. This movement may be triggered by a crack on a steep slope if diminished frictional resistance is overcome by gravity. The rapidly moving mass will be diluted to a slurry if it falls in a lake where its momentum will cause a ‘wave of translation’ (Scott Russell, 1834), a feature now called a ‘Soliton’ by physicists. This remarkable phenomenon may be a factor in several geological processes. (See Section 2 in ‘Soliton Nature’ Book by Prof. Sergei Eremenko, 2020.) It also plays a role in other physical processes, including fibre optics and possibly quantum effects. Murchison realised in 1846 that 'Solitons' were likely responsible for the transport and deposition at considerable distances of material in Swedish osar. He also suspected that they had a role in the deposition and shaping of Quaternary sediments throughout northwest Europe, erroneously (he felt) attributed by Agassiz and his followers to sub-glacial processes.

Malcolm McClure