**5. Discussion**

#### *5.1. Occurrence and Classification*

The development of ORCs in coal-bearing sandstone is an important geological phenomenon which cannot be ignored. The occurrence of ORCs is not only influenced by the transportation of water flow, but also related to their own composition [2,12,30]. The hydrodynamic conditions of the water flow control the transportation process of ORCs (including the transportation ways, the transportation distance and the reworking intensity), which determines the final sedimentary style of ORCs, specific performances: (1) Some rolling transported ORCs have a short transportation distance and low degree of re-working within the dense flow, chaotically accumulating above the erosion surface with low texture maturity (Figure 9J–L). (2) The jumping transported ORCs are subjected to the strong hydrodynamic forces. Some large and intact ORCs are caught in the rapidly deposited sandy sediments leading to the severe plastic deformation (Figure 9G–I); some ORCs that "survived" from the actions of water flow and the collisions of grains (mostly sandy sediments) are small-sized but numerous with good sorting and roundness (Figure 9D–F); but the others are completely disintegrated during the transportation. (3) Some suspended and lightweight ORCs are transported over long distance with relatively weak reworking degree, and deposited as thin interlayers together with fine-grained sediments (mainly silts and clays) under weak hydrodynamic conditions (Figure 9A–C). The composition of ORCs also has an important influence on their susceptibility to mechanical disintegration [30]. The ORCs are mainly composed of higher plant debris Under the action of water flow, some muddy ORCs can be completely disintegrated into clay particles after undergoing mechanical attrition and disintegration; while higher plant fragments only change from large to small particles.

**Figure 9.** Typical core photos and 3D sketches of 4 types of the organic-rich clasts in the coal-bearing sandstone strata. Abbreviations: ORC-1 = Diamictic organic-rich clasts (**J**-**K**), ORC-2 = Floating organic-rich clasts ( **G**-**I**), ORC-3 = Loaded lamellar organic-rich clasts ( **D**-**F**), ORC-4 = Thin interlayer organic-rich clasts ( **A**-**C**).

The classification of ORCs proposed here is based on considerations of their occurrence and the characteristics of hosting sediments. The characteristics given in Figure 8 and Table 2 are designed to aid in identification and interpretation with di fferent types of ORCs. The energy of water flow changes dynamically, resulting in its di fferent transportation mechanism at di fferent stages [5,12,69]. Under a range of its processes, the vertical distribution characteristic of ORCs in coal-bearing sandstone confirms to the law of mechanical sedimentary differentiation. As the energy and transporting capacity of single water flow changes from strong to weak, the sedimentary sequences of ORCs can be concluded from the bottom up as follows: diamictic organic-rich clasts (ORC-1), floating organic-rich clasts (ORC-2), loaded lamellar organic-rich clasts (ORC-3) and thin interlayer organic-rich clasts (ORC-4, listed in Table 2, Figure 9). From ORC-1 to ORC-4, the grain size of their hosting sandy sediments shows a change from coarse to fine (Figure 9), which also confirms a vertical differentiation of ORCs. However, these four types of ORCs can occur differently in different facies associations or not developed originally in part, depending on the actual conditions in a special geological environment.


**Table 2.** Classification of the organic-rich clasts in the studied coal-bearing tight sandstone.

## *5.2. Formation Mechanism*

The Upper Paleozoic sedimentary period in the Ordos Basin is in the background of transgression, and ORC is generated under the water action from rivers, tides, waves, and episodically floods and debris flows (Figure 10). The formation process of ORC underwater flow can be distinguished into

four stages in sequence: (1) Original sedimentation of organic matter and fine-grained sediments to form source; (2) detachment of organic-rich sediments under hydrodynamism, which includes two main mechanisms: one is the basal erosion resulting in high-energy water flow, and the other is the instability slope failure triggered by water flow vibration; (3) transportation and reworking by water flow and water-transported debris; among them, the reworking is a dynamic process, including multiple crushing, deformation (such as squeezing squashing folding pressing), sorting, rounding; (4) allogenic re-deposition [1,2]. The initial failure products of organic-rich sediments are irregular ORC blocks, while transportation by water flow is the determinant of the difference in ORC deposition characteristics [1,2,30]. Based on this, we propose that the formation mechanisms of ORC could be classified into: long-term altering of continuous water flow (such as fluvial water, tide, etc.) and short-term water flow acting triggered by sudden events (such as flood, debris flow, etc.).

**Figure 10.** Schematic sedimentary model of the organic-rich clasts in the coal-bearing sandstone strata in the northeastern margin of the Ordos Basin.

Regarding the first instance, the sedimentary characteristics of the residual ORC in the coal-bearing sandstone are the results of modification by the continuous or periodic water flow within a long period. In the process of transportation, ORCs undergo a series of dynamic processes including liquefying, crushing, squeezing, squashing, winnowing, folding and pressing in the long-term and repeatedly, resulting in broken-up of most pre-formed ORC into smaller fragments. The vertical differentiation between sandy sediments and organic-rich debris is obvious, which suggests that the evolution of water flow from high-density, high-energy to low-density; low-energy turbulence is a slow but gradual process. In the lower flow regime, the suspended ORC often deposits as thin interlayers with fine-grained muddy sediments, usually neglecting the role of ORC previously.

Interestingly, the modification degree of transport on ORC in the short-term water flow is less than the first type, typically low textual maturity. The temporary water flow is often triggered by a sudden event, which may be flood, earthquake, storm or tsunami. This water flow tends to be high-energy, high-density and dissipates energy rapidly (often lasting only a few hours) for breaking through the confined state, resulting in the weak re-working on ORC during transportation. In this study, the thick-bedded diamictic ORC deposited in the early stage of the high-energy water flow energy weakening. Where after, some ORCs were captured during the deposition of sandy sediments, transported in skipping. Finally, the ORCs contained in the suspension transport formed thin interlayers in a lower flow regime, but it is seldom recognized. There are two major reasons: the basal erosion by the new sudden water flow or continuous deposition with fine-grained deposits in the absence of new sudden water flow.
