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Lost Branch Formation

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Depositional history

Having correlated the marine horizons represented by the Lenapah Limestone and the Lost Branch Formation, I present a brief outline of the depositional history of this part of the sequence based on the phases of deposition outlined by Heckel (1980).

Lenapah transgressions

The marine transgression that produced the lower, most widely traceable part of the Lenapah Limestone (Eleventh Street and Norfleet limestones) inundated only the lower to intermediate parts of the broad northern shelf and extended only a short distance into Iowa (Greenberg, 1986). The coal near the middle of the Nowata-Memorial sequence at the Elko section (outcrop 2 in fig. 8) in Putnam County, Missouri, may represent a swamp just landward of the extent of this transgression. Even over the lowest parts of the shelf, the water was not deep enough to lose vertical circulation and produce a black shale facies. Among the conodonts, no Gondolella, Idioprioniodus, or many species of Idiognathodus found this shallower water favorable. Species of Neognathodus dominated the faunas of the deeper offshore parts of the lower Lenapah sea, and Adetognathus dominated the faunas of the shallower nearshore parts, which included northern Missouri and parts of Iowa.

After a minor regression exposed Iowa, Missouri, and parts of Kansas to subaerial soil-forming processes and brought Perry Farm sands into transitional environments in east-central Kansas, Perry Farm mud to the Oklahoma border, and lower Memorial mud into the Tulsa area, a minor transgression established Idenbro carbonate production from northern Oklahoma to southern Linn County, Kansas (Greenberg, 1986). A local swamp that developed just shoreward of the maximum extent of this transgression allowed coal to form at the Idenbro horizon in the Trading Post section (outcrop 11) in eastern Linn County.

Upper Memorial regression

During the regression that closed Lenapah deposition, the sea withdrew into the Arkoma-Anadarko basin of east-central Oklahoma and the shales and thin sandstones that form the upper Memorial Shale were deposited in alluvial environments across Missouri into Kansas. The Jenks sandstone and associated shales were deposited in alluvial to deltaic environments in the Tulsa region while their southern lithic equivalent, the upper Holdenville sandstone interval of Dott and Bennison (1981, p. 17), accumulated in fluvial environments along the southern shoreline. During the low sea-level stand at maximum regression, most of the present outcrop belt and the Forest City basin were exposed. Intense soil-forming processes, which had continued uninterrupted since the Altamont inundation beyond the extent of the Lenapah transgression in western Iowa and Nebraska, turned much of the earlier alluvial deposits and perhaps some of the highest Idenbro carbonates into the blocky mudstone that characterizes most of the upper Memorial Shale [see Schutter and Heckel (1985)]. Across parts of Nebraska, Iowa, and Missouri and along the alluvial southern shoreline in Oklahoma, the soils were drained well enough for iron to oxidize and dehydrate to produce a red color.

Early Lost Branch transgression

The earliest phase of the Lost Branch transgression raised sea level at a rate that was sufficiently balanced by freshwater inflow to form a great swamp on lower areas of the broad northern shelf. This swamp migrated toward higher areas to form the Dawson coal in northeastern Oklahoma and adjacent Kansas. The Laredo coal may have formed at this time in a poorly drained area between the Saline County arch and the Kirksville-Mendota anticline [see Price (1981, p. 30)] in north-central Missouri. The rate of transgression then probably increased and inundated the originally better drained, higher areas of the shelf too rapidly for swamp or peat formation.

Late Lost Branch transgression

During the later stages of the Lost Branch transgression, an area from western to north-central Missouri and adjacent parts of west-central Iowa and eastern Nebraska and an area in southern east-central Oklahoma remained in the photic zone over a peat-free bottom long enough for predominantly algal carbonate muds to produce the thin Sni Mills and Homer School limestones. Elsewhere invertebrates flourished locally in a generally thin layer of argillaceous mud that settled slowly onto the old soil and alluvial surface. Eventually the water became too deep for algal muds to be produced, and the shoreline lay far away from the midcontinent. Thus little detrital material was transported onto the shelf, where conditions of sediment starvation produced a diastem.

Maximum Lost Branch transgression

Finally, during the higher sea-level stands through maximum transgression, the water was deep enough to establish a thermocline that cut off vertical circulation and oxygen replenishment to the sea bottom across the lower to intermediate parts of the shelf. There, a large amount of incompletely decomposed organic matter accumulated under anoxic conditions to form the black fissile Nuyaka Creek black shale bed from the basinal areas of east-central Oklahoma across Kansas into west-central Missouri and over the Laredo coal in north-central Missouri. Around the edges of these lower areas, in most of the remainder of Missouri and parts of Oklahoma and Iowa, the organic content was still high enough to color the shale black to dark-gray but apparently not high enough to bind the clay particles into the fissile shale that is difficult to disaggregate. Farther away, in Iowa and Nebraska, organic matter was sparse enough to color the shale only green-gray. Throughout the entire area, however, upwelling, engendered at times by quasi-estuarine circulation of the two-layer stratified water mass, produced conspicuous amounts of nonskeletal phosphorite within the sediment as nodules toward the south and peloidal laminae toward the north [see Kidder (1985)].

During the higher stands of sea level, Idiognathodus and Neognathodus swarmed in the surface water, feeding on the plentiful organisms nourished by the upwelling. As the cooler, deeper water spread over the shelf, Idioprioniodus returned and Gondolella made its first appearance since the Excello shale was deposited, after missing several cycles of inundation. Adetognathus, which had lived across the shelf in shallower water during earlier stages of transgression, was confined to similar nearshore environments away from the presently preserved limits of this horizon. Sedimentation at this time virtually ceased in the northern midcontinent, where an inch or two of sediment yields thousands of conodonts per kilogram. In contrast, sedimentation remained sufficient in the basinal area nearer the southern Oklahoma detrital source to produce up to 8 ft (2.4 m) of this facies with the same conodont fauna but in a much lesser concentration.

Early Lost Branch regression

After sea level started to drop, the thermocline disappeared from the shelf, oxygen returned to the sediment-water interface, and benthic invertebrates returned in great number. These invertebrates are particularly well represented in southern Kansas and Oklahoma, where the greater rate of sedimentation ensured preservation through rapid burial [see Boardman et al. (1984)]. To the north, however, even where the bottom had been oxygenated throughout maximum transgression, sedimentation was so slow in water deep enough to be below the aragonite compensation depth and within the calcite lysocline (of this Pennsylvanian sea) that all the mollusks and perhaps many thin-shelled calcitic organisms were dissolved before burial (Malinky, 1984).

Late Lost Branch regression

Soon sea level dropped to a point at which much of the northern shelf in Iowa, Nebraska, and northwesternmost Missouri was brought back into the effective photic zone for carbonate-mud-producing algae, and deposition of the Cooper Creek Limestone Member ensued across that area. Southward, closer to the detrital sources in Missouri, Kansas, and Oklahoma, prograding detrital sediments, locally in the form of small deltas, reached the area, clouded the water, and destabilized the substrate before carbonates had a chance to form. These sediments produced some sandstone facies of the Hepler unit in Missouri, the marine to transitional shales in Kansas, and the thin-bedded sandstones below the Glenpool limestone bed south of Tulsa. Later, after the sea had withdrawn from the surface of the Cooper Creek Limestone Member, the water became shallow enough in parts of Oklahoma and southern Kansas that sufficient invertebrates and locally algae proliferated (although in places with quartz sand) to produce the thin, discontinuous, locally sandy Glenpool limestone bed. The conglomeratic limestone at the base of the Glenpool near Uniontown in Bourbon County, Kansas, indicates local erosion before Glenpool deposition and suggests the possibility that the Glenpool might represent a minor transgression (during this generally regressive phase) that stymied the coarse clastic influx in east-central Oklahoma and established marine deposition above an erosion surface in east-central Kansas.

Maximum Hepler regression

Shortly after the Lost Branch regression, the sea withdrew completely from the shelf and from the part of the basin exposed along the outcrop. Sand moved in alluvial channels across parts of Missouri and Kansas to produce channel sandstone facies of the Hepler unit; intense soil formation altered the top of the Cooper Creek Limestone Member and any overlying alluvial deposits to a blocky mudstone that extended downward into fissures and vugs in the limestone that were left unfilled by sparry calcite. This process accounts for the fragmental, shale-penetrated appearance of Cooper Creek limestone, in which the floating nodules in the green shale at the top are part of the C horizon of the soil profile. Similar soil-forming processes affected the Glenpool limestone bed and overlying material, but the Glenpool was less intensely weathered because it was lower on the shelf and therefore exposed only later during the regression, leaving less time to develop conspicuous weathering features. There was sufficient time, however, for the top of the thick gray shale sequence around Sasakwa, Oklahoma, to be oxidized and dehydrated to a red color and for channel sands and gravels of the type Seminole Formation to cut deeply into the sequence in places.

Possible cause of terminal Desmoinesian extinctions

The maximum regression of the sea that closed the Lost Branch marine cycle coincides with the changeover from Desmoinesian to Missourian biota in both the terrestrial and marine regimes and so marks the Desmoinesian-Missourian Stage boundary. The changeover involved mainly extinctions of Desmoinesian forms, at least in the midcontinent, for example, the conodont Neognathodus, the brachiopod Mesolobus, several ammonoid taxa, and the fusulinid Fusulina (Beedeina) eximia (Thompson et al., 1956) in the marine regime and the swamp-dwelling arborescent lycopods in the terrestrial regime. Consequently, Schutter and Heckel (1985) reasoned that this sea-level drop may have been sufficiently greater and longer than those that closed previous marine cycles to have actually caused the extinctions. It can be readily seen that a greater than usual drop of sea level would crowd both the shelf-dwelling and deeper-living marine forms into less living space for a longer time so that, as the shallow shelf area shrank and the basins shallowed, more organisms than usual would not survive. The same principle also applies to the terrestrial extinctions, because they were mainly among the swamp-dwelling arborescent lycopods (Phillips and Peppers, 1984), which required freshwater for reproduction (Phillips, 1979). Thus, if a sea-level drop greater than usual brought the shoreline off the broad, gently sloping shelf everywhere onto the steeper slopes of the basins, it would greatly reduce the areas of freshwater swamps that could form. If sea level remained this low long enough for the reproduction of these lycopods to be severely inhibited, it would have caused their extinction.

Comparison of the Lost Branch Formation with other marine formations

The Lost Branch inundation was much more widespread than those of the previous Lenapah transgressions. It reached depths great enough to produce over most of the seafloor the anoxic black fissile facies that is similar to those in other black- shale-bearing but much more limestone-rich marine formations. Only in those cores from Iowa and Nebraska where both the Sni Mills and Cooper Creek Limestone Members are present with an intervening thin shale does the Lost Branch Formation display the same transgressive limestone-offshore shale-regressive limestone sequence that characterizes the more limestone-rich marine formations along most of the outcrop belt. In contrast, the Lost Branch Formation almost everywhere is dominated by shale and has much thinner carbonate members than nearly all other widespread cycles. This explains why it had long been overlooked or at least not well understood in Pennsylvanian stratigraphic work. Heckel (1984) explained this different nature of the Lost Branch by greater rates of both transgression and particularly regression. The more rapid regression allowed much less time for the sea bottom to remain in the shallow sunlit depths optimal for carbonate production before the bottom was exposed or before shoreline detrital material moved in and swamped the carbonate-producing organisms.

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Kansas Geological Survey, Geology
Placed on web Nov. 2, 2010; originally published 1991.
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