At this outcrop, there is an anticline developed within the Salona-Coburn Format

Question

At this outcrop, there is an anticline developed within the Salona-Coburn Formation. At the northern end of the outcrop si the contact with the Antes shale that marks the end of carbonate depostition. You will be expected to answer the following series of questions.

Please answering the following: (Please answer all parts precisely and asap!!!)

-Describe each facies (composition, texture, form, association, fossils, sedimentary structure).

-What depositional environments do the facies represent?

Explanation / Answer

The Salona and Coburn Formations of the Middle Ordovician Trenton Group comprise the uppermost part of the 8,000-foot sequence of Cambro-Ordovician carbonate rocks in central Pennsylvania and reach a maximum thickness of about 600 feet. They crop out extensively in linear belts along the margins of the eroded anticlinal valleys of the Nittany Arch, as well as southeastward in the smaller areas of Kishocoquillas, Black Log, and Path Valleys. As many as 20 exposures of the Salona and Coburn Formations are reported from central Pennsylvania.

The location of this field photograph is needed to specifically comment on this outcrop. Facies varies from section to section laterally as well as vertically (Walthers Law)

However, a general idea of the Salona-Coburn formation and the Antes shale can be given. The facies reported from various outcrops can be clubbed into a more generalized overview.

Salona and Coburn Formation:

The Salona and Coburn Formations each have been divided into two members. In ascending order, the Salona includes the New Enterprise Member, composed almost entirely of black, argillaceous calcilutites, and the Roaring Spring Member, characterized by interlayers of laminated and cross-laminated, fine-grained calcarenite. The Cobum includes the Milesburg Member, with interlayers of Sowerbyella and Sowerbyella -trilobite bioskeletal calcirudites in alternation with other rock types, and the Coleville Member with interlayers of Dalmanella and Dalmanella -crinoidal bioskeletal calcirudites. Zones of contemporaneity provided by metabentonite layers and fossil zones, some newly described, furnish a basis for discussion of the time-space relationships of the rock bodies. The Salona Formation is wedge-shaped and thickens toward the southerly and easterly parts of central Pennsylvania as its upper beds replace the lower beds of the Coburn Formation at gradually higher levels. Concurrently, the Coburn thins by interfingering with the overlying Antes shale as well as the underlying Salona Formation. The New Enterprise Member is a blanket-like deposit over most of the region, whereas other members thicken and thin as do the parent formations. Southward in Franklin County, most parts of the Salona and Coburn Formations are replaced by the calcareous shales and argillaceous limestones of the Martinsburg Formation. The lower part of the Salona Formation is represented there by the thin-bedded calcilutites of the Oranda Formation. The rocks of the Salona and Coburn Formations were classed into six litho types, namely, bioskeletal calcirudites, calcarenites, calcilutites, calcareous shales, metabentonites and dolomitic limestones. The three first-mentioned lithotypes respectively were subdivided into nine sublithotypes on the basis of fossil content, presence or absence of sedimentary structures and percentage of argillaceous material.

In thin sections, the bioskeletal calcirudites consist dominantly of fossil remains cemented by megasparry calcite, and contain subordinate percentages of carbonate pellets, quartz crystals and fragments of lime mud as grains, as well as smaller percentages of lime mud as matrix. The calcarenites are composed principally of carbonate pellets which are cemented by microsparry calcite and are distributed in laminae and cross-laminae. Like the calcirudites, the calcarenites contain subordinate percentages of other mineral grains as well as of matrix. The calcilutites are largely composed of argillaceous lime mud with only a few obvious grains. The dolomitic limestones contain appreciable percentages of xenomorphic dolomite which are concentrated in laminae or bands.

The bioskeletal calcirudites and calcarenites probably formed in shallow water in areas of currents of sufficient strength to move the grains for at least short distances before their final deposition. In contrast, the calcilutites probably accumulated in areas with little current circulation, such as might be expected in the relatively deep water below wave base or in shallow, protected areas. The dolomitic limestones either represent replacements of earlier limestones or concentrations of dolomite crystals that were transported by currents before deposition. The metabentonites seem to be products of altered volcanic ashes and the shales represent influxes of detrital material.

The rhythmic deposits of the Roaring Spring Member and of the Coburn Formation appear to constitute a single basic type. A rhythm commonly begins at the base with an abrupt and, in some cases, irregular contact. This contact is overlain by a relatively coarser limestone which grades upward into finer limestones, and these in turn into calcareous shale. The upward decrease in grain size is accompanied by an increase in the percentage of argillaceous material and lime mud and a darkening of the color of the rocks. The rhythmic deposits apparently formed in shallow water in which the sediments apparently were stirred up periodically, possibly by storm waves or turbidity currents.

The Salona and Cobum Formations are composed of three major lithofacies bodies, namely, the black calcilutite facies, the nonbioskeletal rhythmic facies, and the bioskeletal rhythmic facies, corresponding respectively to the New Enterprise Member, the Roaring Spring Member, and to the Cobum Formation as a whole. In accordance with the general characteristics and time-space relationships of the facies, it seems plausible that the black caldlutite facies was deposited in relatively deep water where the sediments were little disturbed by bottom circulation. The non-bioskeletal and bioskeletal rhythmic facies, respectively, seem to reflect progressively shallower conditions and possibly were deposited on banks or shoals where wave or current action was comparatively strong. The three major lithofacies bodies apparently accumulated side by side and acted together as a transgressive sequence during Salona and early Coburn time and may have been a regressive sequence during later Cobum and early Antes time, although the regressive sequence is poorly developed.

Antes Shales:

The Middle to Upper Ordovician, Antes Shale is a Utica-Shale like facies that crops out in the Valley and Ridge of Pennsylvania, overlying the Middle Ordovician carbonate platform deposits of the Trenton Limestone. The Antes Shale is a diachronous unit; it is oldest towards the southeast and youngest towards the northwest.

he Antes shale is generally a finely-laminated, calcareous, black shale; the lower few meters of the Antes Shale are particularly carbonaceous and pyritic. Skeletal debris of the trilobite Triarthrus is common within a limited number of horizons; graptolites occur abundantly on many bedding surfaces, although their preservation is generally poor. Upsection, the Antes Shale becomes more calcareous and often fractures conchoidally. Lenticular pyrite layers, several centimeters thick, occur throughout with numerous exposures and outcrops are sheared where cleavage is faint to moderately well-developed.

The dark color, low diversity of benthic fauna, and lack of bioturbation indicate that the Antes records an oxygen-stressed seafloor. The dominant fossils found throughout the Antes, mainly graptolites and trilobites, provide clues to the depositional environment of specific stratigraphic intervals and localities. The trilobite Triarthrus is present in relatively large numbers at all locations with the exception of the southernmost exposure, at which the oldest shale (C. bicorniszone possibly to C. americanus zone) is present (McConnellsburg, PA). Articulated, complete specimens and molts of Triarthrus indicate lack of transport and relatively rapid burial. In this low oxygen setting, Triarthrus was the only shelled, benthic organism that thrived. In fact, in one bentonite-rich, stratigraphically condensed interval, Triarthrus is present even though graptolites are not preserved, presumably lost to decomposition. This seems to indicate that the dysoxic setting had few inhabitants in the Middle Ordovician. The lack of Triarthrus in the oldest shale may indicate that these strata predate the first appearance of the trilobite in this part of the Appalachian basin. The westernmost and stratigraphically youngest exposure (Martinsburg, PA) is divided into two distinct horizons. The first is a thin (less than 1 meter) shelly interval just above the Trenton Limestone containing articulate brachiopods (Oniella multisecta and Rafinesquina alternata in particular) and the trilobite, Cryptolithus. This fauna seems to record a dysaerobic to aerobic muddy substrate community. The shelly interval is interesting in that it records a facies that is intermediate between the Trenton Limestone and the typical Antes Shale and is not seen in any other part of the basin. The second, younger horizon lacks diverse benthic fauna but includes abundant C. spiniferus zone graptolites and Triarthrus, indicating a deeper, more oxygen-stressed setting.

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