Galatia Channel:Model of Channel Development

From ILSTRAT
Jump to: navigation, search

Model of Channel Development

Our model is presented in 10 illustrated stages (Figures 49 to 58).

Stage 1: Prograding deltas during early regression.

To set the stage, the Houchin Creek Coal, Excello Shale, and Hanover Limestone sediments were laid down on a tectonically stable platform from lowstand (underclay and peat) through rapid marine transgression to highstand (black shale and limestone). During late highstand to early regression, clastic sedimentation resumed. A series of deltas were built into the basin, yielding upward-coarsening sediments of the Delafield Member (Figure 49). This part of the model is similar to past cyclothem models in the Illinois Basin.

  • Figure 49 Deposition of the Delafield Member as a series of coalescing deltas during the onset of a glacial stage as the sea level began to fall. The product is a thick succession of clastic rocks that coarsen upward.

Stage 2: Valley incision and soil development during regression.

Delafield sediments rapidly filled available accommodation space in the basin. With continuing marine regression, the delta platform became exposed. Rivers cut downward to base level, creating entrenched valleys. Given the extremely low gradient and the substrate of recently deposited sediments, these streams actively meandered. Outside of valleys, soil development was under way (Figure 50). Previous models (e.g., Potter 1962, 1963) placed channels in a deltaic setting and mostly did not take account of eustasy, although Hopkins (1958) interpreted channels as being cut subaerially during regression and backfilled during transgression.

  • Figure 50 Channel incision of delta sediments.

Stage 3: Aggradation and gleying during late regression.

Channels developed broad meander belts. As soon as the channel bottoms were cut below sea level, suspended sediment (largely sand) began to drop out. Rivers reworked their own sandy deposits. Valleys aggraded rapidly approaching maximum lowstand, yielding an upward-fining profile (Figure 51). Approaching maximum glaciation, climate in the Illinois Basin became increasingly humid. This fostered vegetation growth, reducing soil erosion and sediment transport. Increased rainfall leached iron from soils, gleying much of the upper soil profile (Rosenau et al. 2013). Because of the lack of time and persistently wet conditions, little or no soil formation took place within the Galatia meander belt. This part of the new model is similar to previous models, although most treated paleosols in cursory fashion.

  • Figure 51 The Galatia channel developed a meander belt.

Stage 4: Peat initiation and paludification at early lowstand.

Approaching glacial maximum or lowstand, climate in the basin became humid. Constant rainfall produced lush, dense vegetation, reducing sediment flux and creating “black water” rivers of mostly plant tannin, clay, and perhaps a small bed load. Peat began to develop earliest in low-lying areas on the landscape, such as abandoned meanders. Vegetation lining the banks of the Galatia channel stabilized the banks, locking meanders into place for the duration of lowstand. Along the channel margins, fine clastics infiltrated the peat swamp, perhaps by flocculation of clay in vegetated areas along channel margins. The result was shaly coal in these channel-margin settings (Figure 52). Previous Carbondale cyclothem models did not incorporate syndepositional channels.

  • Figure 52 The change to a humid climate caused peat to form.

Stage 5: Widespread peat accumulation at lowstand.

Peat development overspread the basin except for active channels and well-drained tectonic highs (Figure 53). Naturally, the thickest peat accumulated in the lowlands flanking the Galatia channel. Periodic floods brought fine silt and clay out into the swamp, creating clastic laminae in the peat close to the channel. This process is taking place today along the Rajang River in Sarawak, Malaysia, where domed peat deposits fringe the river under a tropical, ever-wet climate (Staub and Esterle 1993). Tectonic subsidence and concurrent peat compaction created space for thick (albeit nondomed) peat deposits along the Galatia channel. Placing coal at lowstand is conceptually different from many peat or coal depositional models and is elaborated in the following section.

  • Figure 53 Peat accumulated across a large area of the basin.

Stage 6: Peat rafting and splitting during initial transgression. Glaciation abruptly ended as the polar climate warmed. During the initial stages of sea-level rise, the lower part of the Galatia channel was subjected to increasingly vigorous tidal ebb and flow (Figure 54). Buoyed by trapped air and self-generated methane, large mats of peat floated free of the substrate. At the same time, climate shifted from ever-wet to wet–dry seasonal. This led to less vegetation cover in inland source areas and a consequent increase in runoff and sediment load. Silt carried down the Galatia channel was deposited beneath floating peat layers, creating splits and major seam disruptions.

  • Figure 54 A warming climate brought rapid melting of the glaciers and a sea-level rise. The Galatia channel became an estuary, subject to strong tidal currents.

Stage 7: Estuarine flooding during early transgression.

Melting induced rapid, pulse-like transgression, drowning the Springfield peat swamp first along the Galatia channel, which became an estuary (Archer et al. 2015, 2016). The tropical climate became drier and more seasonal, which reduced the vegetation cover upstream, enhancing runoff, erosion, and fluvial sediment transport. A heavy load of gray silt (Dykersburg Shale) rapidly overwhelmed the peat, entombing tree stumps and other plant remains in place (Figure 55). Some Dykersburg sediment might have been derived from offshore and carried up the estuary by tidal currents, as is happening today in the Kampar River of Sumatra, which is located just north of the equator under perhumid conditions. The upper Kampar has a deep channel and “tea-colored” water that carries practically no bed load or suspended sediment. Cecil et al. (2003, p. 32) wrote, “The lower 85 km [52.8 mi] is exceedingly shallow and difficult to navigate even in small boats of very shallow draft” because of the sediment carried up the estuary by the tides. Placing the Dykersburg in a tidal–estuarine facies puts this relatively new concept into the larger cyclothem model and emphasizes the coastal–estuarine rather than the deltaic nature of this part of the cycle.

  • Figure 55 Peat swamps drowned as the estuary became an embayment. Dykersburg sediments rapidly buried the peat.

Stage 8: Estuary advances inland as regression continues.

Loaded with sediment, peat compacted rapidly, making space for more sediment. Basin subsidence, sea-level rise, and peat compaction worked in concert until more than 98.4 ft (30 m) of Dykersburg Shale was in place (Figure 56). The process ended when the basin area became so deep and far offshore as to be beyond the realm of coastal processes.

  • Figure 56 As the transgression continued apace, the entire basin area was submerged in deep water, which became stratified and anoxic, and black mud (Turner Mine Shale) was deposited.

Stage 9: Black shale of late transgression and highstand.

The Turner Mine Shale records rapidly deepening water. The water column stratified, bottom circulation ceased, and the Illinois Basin became sediment-starved. Fine sediment that filtered down resulted in black, phosphatic shale (Figure 57). Our model is similar to previous ones, such as those of Heckel (1977, 1986, 1994).

  • Figure 57 Normal marine circulation resumed near the apex of an interglacial stage (marine highstand), bringing a brief interlude of carbonate sedimentation (St. David Limestone).

Stage 10: Limestone deposition at highstand to early regression.

At maximum deglaciation, sea level crested and climate was seasonally dry. During highstand, bottom circulation became reestablished. Because climate was seasonally dry, evaporation was high and carbonate deposition took place as the St. David Limestone (Figure 58). Continued clastic influx during highstand led to delta progradation (Canton Shale), ending carbonate production and beginning the next cycle. This process is similar to those in previous models.

  • Figure 58 Marine regression begins the next cycle.

Primary Source

W. John Nelson, Scott D. Elrick, William A. DiMichele, and Philip R. Ames xxxx, Evolution of a Peat-Contemporaneous Channel: The Galatia Channel, Middle Pennsylvanian, of the Illinois Basin FINISH CITATION

References