The answer is very simple and in two words only: Hydrothermal pressures.

I think that highly reducing environment and rapid sediments accumulation are the controlling factors 

Jai and Sobhi:

Thank you so much for your response.We wish that your answer was as simple as you claim. Scientists with meagre geological background would vouch for the formation of coal in a couple of hours primarily based on the well known principle of Hydrothermal Carbonization or Pressure Cooker device. In normal deep sea Flysch sedimentation scenario, when terrestrial vegetal matter is periodically flushed down through turbidity channels to settle at the bottom, the pressure is not a problem but prevailing low temperatures would pose a problem for the genesis of coal. Rapid sedimentation is acceptable, but again, highly reducing environment is ruled out as evidenced by common presence of trace fossils at the time of vegetal matter settling. However, later when the vegetal matter bearing bed is deeply buried and sealed by impervious lithologies, the carbonization is possible, provided appreciable temperature and pressure is achieved. How does this happen and how fast is the question?

Best

Syed 

KMT:

Coalification requires, Pressure, Heat and microbial decay of Plant debris periodically flushed with high energy coarse clastics by turbidity currents in deep sea to produce typical turbidites with less than a milimeter thin carbonized layers to about 50 cm thick coal seams (high grade Bituminous and Anthracite varieties with absence of Peat or Lignite). Plant debris includes Pollen, Spores, leaves and twigs which can be taxonomically determined. Owing to high influx of sand, silt and clay the Planktonic Foraminifera and Calcareos Nannoplankton are much diluted and leached away in hot humid climate outcrop of tropical sections, however both the calcareous walled plankton are well preserved and can be found in mud exuded from the Mud Volcanoes.

Coalification does occur in deep-sea. The question is how sufficient temperature and microbial activity(suspected anaerobic bacteria) is attained to permit coal formation?

Best

Syed 

KMT:

Let me explain. We are talking about an Island-arc system (Andaman-Nicobar Islands for example) where deep sea turbidites are raised due to subduction of Indian Plate to create series of islands exposing deep sea flysch sediments. What to talk of Chalk or Marls, which we expect in normal deep sea basins, it would be difficult to find any trace of carbonate rocks in Flysch siliciclastics. Flux of terrigenous sediments is so enorm that vertical rain of calcareous plankton are terribly diluted or leached away in hot humid climate (in outcrops).

I would be glad to answer any other query.

Best

Syed

No "coal" in deep-sea conditions!

Obviously,  you have received for the "coals - the black shales.

Ya. E. Yudovich:

Coal beds of small thickness were mined in Middle Andaman Islands, India. These are fully marine deep sea Flysch deposits like in several other parts of the world and the coal beds and streaks could act as source rock for Gas Hydrocarbons, like Coal Bed Methane. Kindly see the link:

http://archives.datapages.com/data/sepm_sp/SP22/Flysch_Deposits_of_Antler_Foreland.htm

Best

Syed

KMT:

In Andaman-Nicobar Islands, which is type area of Island-Arc basin of the world, we do get Turbidites including Greywacke Sandstones. We get frequent coal streaks, carbonized wood and tiny twigs and coal seams attaining upto about 50cm thickness. The age of the Flysch is Early Campanian to Oligocene. Similar deep sea Flysch containing autochthonous coal are known from European Flysch including one reported from Polish Carapathian Flysch, where temperature is explained by intense tectonic pressures (Please see link here):

https://www.researchgate.net/publication/232346563_Petrologic_studies_of_terrestrial_organic_matter_in_Carpathian_flysch_sediments_southern_Poland

Best

Syed

Article Petrologic studies of terrestrial organic matter in Carpathi...

KMT:

A lot of reworked stuff can be present in Flysch sediments including remains of plant debris- only autochthonous layers of lustrous coal (Bituminous-Anthracite) lying parallel to bedding are relevant for discussion here. Do you buy the hypothesis that the tectonic pressure could create sufficiently high temperature to induce coalification? 

Best

Syed

Dear all, please enlighten me: it is obvious that reworked coal fragments and plant remains can be entrained in turbiditic deep water sediments, but autochthonous coal seams are another matter: plant debris is at least initially less dense than water, and therefore cannot form gravity-driven sediment flows to the sea bottom. Plant remains can become water-logged and then sink to the bottom, but even then it seems to me an improbable proposition that turbidity flows can create coal seams. Furthermore, you speak of of "autochthonous coals", which would rule out a turbiditic origin.. What type of plants are the coals you speak of composed of?? From what have been said in this discussion, I would suggest your Flysch was deposited in a shallow marine, near-coastal environment, with authochthonous coal seams formed during episodes of emergence in coastal plains. To explain the degree of coalification, you should reconstruct the burial history and model the heat flow.

Dietrich:

Your concerns about occurrence of autochthonous thin coal seams in deep water turbiditic Flysch facies is genuine, because it is not much known, hence this question. However, several flysch facies around the world including Andaman-Nicobar Flysch and Carapathian Flysch (Pl. see link above) do contain submillimeter to several Cm thick coal seams lying parallel to turbidite beds. In contrast, reworked coal fragments of varying sizes may be found lying in erratic fashion (Carapathian Flysch). Flysch sediments are characterized by extremely thick sequences (several Thousand meters thick) and lateral extent of  several Thousand Km) containing sparse body fossils but abundant deep water "Spurenfossilien" in the form of sole markings at the base of Graywacke Sandstones (for your enlightenment see):

 https://www.google.co.in/search?newwindow=1&biw=1600&bih=775&q=cycles+and+events+in+stratigraphy+gerhard+einsele&oq=Cycles+and+events+in+Stratigraphy&gs_l=serp.1.1.0j0i22i30l3.4758.25076.0.153949.63.29.0.13.13.0.261.2789.2j21j1.24.0....0...1c.1.64.serp..29.34.2540.VNtQVrKdU3s

High energy turbidity currents can bring in vegetal matter along with clastics and settle it in deep sea. Several group of plants have been recognized, including Family Lauraceae and Eucalyptus, spores and Pollens etc. from such deep sea turbidites. Dietrich, where do you get shallow water coastal Flysch sediments?

Magmatism of volcanic arc coupled with overburden of sediments and Tectonic pressure induced by subduction could be evoked to explain necessary heat needed from coalification.

Best

Syed 

KMT:

You are absolutely right. The discussion is a bit leading towards circular reasoning - but this is important to collect divergent views to reinforce confidence and to inch towards actual truth for the genesis of deep-sea coal.

Best

Syed

Thin bedded coals are common in deep water sediments in th Kutai Basin, in the Makassar Straits, and offshore Sabah, in sediments mainly of Late Miocene age and form a major hydrocarbon source rock. These have been described by Rui Lin in Indonesian Petroleum Assn (IPA) proceedings and summarised by Saller et al in AAPG Bull.Also described from Sabah. Most organic matter is from angiosperm leaves. These have been illustrated in a paper by myself in IPA in 2011 which discusses the Neogene climate of Makassar Straits.

For such organic rich beds to occur in such abundance in deep marine sediments, you need a major sediment source from an area exhibiting a perhumid lowland equatorial climate, and most importantly this perhumid lowland climate needs to be maintained during periods of sea level lowstand, during which time extensive peat swamps formed around much of the coast of the island of Borneo (most lowstand tropical climates tended to be distinctly seasonal, and hence not peat forming - the Sunda region is the only low latitude area where lowstand peats are known to have widely formed during the Neogene). Coastal instability resulted in downslope transport of leaf-rich sediments in turbidite currents, with the leaf beds settling out typically just after the sands, and sometimes forming beds with a high organic carbon content, and mainly with liptinitic kerogens. Hyperpycnal processes may also be involved  

A confusing aspect of such successions is that due to the high input of organic matter, typical deep marine calcareous microfossils, such as benthonic forams, are missing due to dissolution. Nannofossils and planktonics may also be missing, with only agglutinated forams present. Dating is therefore difficult 

I am very interested to hear of any occurrences of leaf-rich deep marine thin bedded coals from any other areas

The first essential factor to contribute to the formation of thin coal beds in deep-sea sedimentary settings is a depositional process, for which Robert Morley provided a reasonable model. Such coals, however, are allochthonous, not autochthonous, and I doubt that 50 cm thick beds can be generated by this process. I wonder whether coastal swamp deposits have here lumped together with a flysch formation. Anyway, this cannot be meaningfully discussed without knowledge of the sedimentology of the 50 cm coal and the immediately over-and underlying beds, and their position in the sedimentary succession. As to the degree of coalification, why not trying the standard tools of burial history and heat flow modelling?

Dietrich:

You need to go through the Chapter 2 of the book provided in the link cited above. I have my own doubts if you ever worked in a Flysch terrain to understand the basics and simplistic model of Turbiditic succession. We are talking about essentially Autochthonous thin coal beds. The link provided here would give you some idea of the Flysch succession containing coal.

https://www.researchgate.net/publication/264500108_Late_Triassic_Palynomorphs_from_the_Andaman-Nicobar_basin_Andaman_Sea_India

Best

Syed

Article Late Triassic Palynomorphs from the Andaman-Nicobar basin, A...