Dear Xianghui,

Common features is that pebbles are all rounded, so the pebbles seems to have origin from a more proximal part of the fan or alluvial channel. Another hypothesis would be that the pebbles where embeded in other floating material than ice, I would suggest tree trunks debris. Is there evidence floating woods in the material?

Dear Jérôme Juilleret,

Thank you for replying the question promptly! Unfortunately, I have not found any tree trunks debris yet. Plant fossils are rare. Some invertebrete fossils such as ostrocodes, conchostraca, bivalves, etc. are common within mudrocks. By the way, isolated pebbles are common within mudrocks.

Best regards.

Xianghui

Xianghui,

very interesting set of observations and hypotheses. What is the composition of the subrounded, somewhat elongate cobbles? One appears to be quartz/quartzite. They may be re-worked from a very mature siliciclastic deposit. Is a source of cobble-pebble-sized gravels nearby and is a basement rock source of the 'quartzite?' nearby?  What was the paleolatitude at time of deposition? I agree with Jérôme Juilleret that these pebbles & cobbles may have been entangled with plant material, dropped as the raft floated, and then the plant debris deposited elsewhere and rotted. Bill Hay and Xumian have recently published papers on Cretaceous climate of China.

Good luck.

hi,

made similar observations in the Phu Kradung formation (J-K) of Thailand (Khorat group). Floated tree stumps commonly carry some gravels, and are not fossilized in very oxic environment. But statistically the gravel they may bear is nothing at the scale of a geological section. I have questionned sedimentologists, but never get a clear answer (sorry I am a botanist). However I also saw modern beaches like that, sandy-silty, with only a few much-rounded centimetric gravel on it, it is not uncommon.

Hi, Bob,

Nice to hear you!  

Yes, you are right, most of the "dropstones" are quartz/quartzite. There are lots of pebble and copple sized gravels in the whole succession. The gravels should be derived from the Paleozoic, a decade kmaway in west. Wood fossils are not found at the section I observed. Please note, wood fossils are rare in the Cretaceous terrestrial strata in South China as semiarid and arid were predominant. I will search wood fossils during the field work next time. 

Bill Hay does great work in the Cretaceous climate. Xiumian has been focusing the sedimentary geology in Tibet. 

Thanks for the comment!

Xianghui

Dear Marc Philippe,

Thank you for the comment! I agree with you that woods can not be fossilzed in an oxic environment. This may be a reason I can not find tree trunks. The fact that few arbor plants developed in (semi-) arid climate during the Cretaceous, South China, could be the other clue.  

For the 'drop stones' I observed, two depostions (gravity debris flowing or raft floating)  are possible, so that I ask the suggestion. 

I have also seen gravels within sands/silts on modern lake/sea beach, which are easy to explain, i.e. can be mainly attributed to the erosion and winnowing of wave and/or stormto coast. 

By the way, can we share the similar observation you saw in Thailand?

Many thanks!

Xianghui

But.. in the similar oxic environment the inveretebrates are preserved. If these inveretebrates are marine in origine that it can be estimated that it is the part of turbidite deposites.

Hi, Susheel Kumar,

The Cretaceous is absolutely the terrestrial facies in mainland China except for in Tethys Himalaya, southern Tibet and Xinjing as well as northeastern corner of China. There are lots of terrestrial late Mesozoic invertebrete fossils preseved in the lacustrine fine sediments in China, which are definitely not related to turbidite. 

Have you dug out and measured the orientation of the pebbles? Dropstones have a random orientation/fabric, whereas the results of subwater landslides/mudslides or deposition in moving water should result in the formation of a fabric with the pebbles showing a primary orientation parallel to the direction of movement during the formation of the deposit. A secondary mode at right angles to the other should also be present. If it shows a distinct mode using 30-50 pebbles at one site, try repeating the measurements at other locations to attempt to get a spacial pattern.  Remember that the long axis of the pebble should show a dip as well as an orientation in the horizontal direction.

Hi, Harris,

Thank you for providing the methodoloy to measure the orientation of pebbles !

I have not done yet. Because the isolated gravels are not abundant. One pebble may be found on a meter square. Any way, I will do measurement if I can see more.

Thank you again!

Ni-hao Li xianghui

for sure you can. This was observed in Thailand, Kalasin Province, Phu Phan range, Khorat Group, Phu Kradung Fm. The Thai geologist who was with me, Dr. Varavudh Sutheethorn, explained me that (if I well remember) in a stream thin gravel layers can be interbedded within a globally silty-clayey sequence, and that if this is reworked then  can occur sediments like those you describe. Gravels were typically small, 1-2 cm in diameter, which fits your illustrations.

Hi, Marc Philippe

Thank you for the sharing description! A new question is why the Thai geologist describe "gravel layers can be interbedded within a globally silty-clayey sequence"? Why the clayey seqeunce is global? 

The answer to the last question is probably because of the cycle of low water flow, storm flood and then waning flow as it returns to normal. At high flow, the stones would be carried downstream and the channel widened and deepened. During reduced flow, some stones would be stranded along the edge of the channel with the fastest current and become buried as sediment fills the over-deepened channel. the clay is the result of deposition during times of low flow when the stones cannot be eroded or carried in the slow-moving current..

Hi, Harris,

Thank you for the detailed interpretation on hydrology! 

Dear Xianghui,

looking again to the pictures with pedologic eyes, it seems that the pictures where taken not so far from the soil surface. Obviously in the picture 1 and 2, I  can see many wedge shaped aggregates and fractures (cracks) where roots are visible. So , I will interpret this as a vertic horizon, diagnsotic for vertisol.

Vertisols (from Latin verto, "to turn") are clay-rich soils that shrink and swell with changes in moisture content. During dry periods, the soil volume shrinks, and deep wide cracks form. The soil volume then expands as it wets up.

So, gravels can have been incorported by pedoturbation, the gravels could have fallen into the cracks and coming from an above layers.

Can you provide pictures at other scale?, do you have information on paleosol. Indeed the proposed vertisol can be paleovertisol.

Be carefull, it i s just a new  hypothesis taht yshould be tested, I am not telling that my view is the truth.

Best regards

Hi, Jérôme Juilleret,

Thank you for the new observation and interpretation!

It is sure there are more than 100 layers of paleosols within the 3 km thick cross-section, which are dominated by calcisols. A few gravels occur within either vertisol or calcisol, 

There are not distinct root fossils preserved. Cracks are also not clear. Shrink fractures are abundant, may not be responsible for the isolated gravels.

Please see the other picture.

If the region was under arid, subaerial conditions, you have to consider the possibility of an accumulation of mudflows over a long period of time. What is the tectonic setting? In California, mudflows and sheet wash move sediment downslope across the margins of inland drainage basins bounded by mountains. This also occurs today  in Egypt. The basin floor is often subsiding, allowing great thicknesses of sediment to accumulate in this way, e.g., the Qaidam Basin in North east China where over 4 km of sediment has infilled the sinking basin over a period of 30 Ma.

Hi, Harris,

Yes, it had been in a semiarid - arid climate. In southeastern China, lots of small basins were developed in the Cretaceous-Paleogene  under an exenstional (intracontinent) tectonic setting. The Hengyang basin of Hunan province is also a small basin, which was probably formed by faulting and/or deformation aside the deformed Paleozoic orogen. I have an preferance of mudflow interpretstion for those paraconglomerates and gravel-bearing/sandy/silty mudrocks at right now.

Thank you for the message!