24.2: The Tippecanoe Sequence - Transgressing Seas, Taconian Mountains

The story of the Tippecanoe is the story of the beginning of the end of the Iapetus Ocean. During the late Proterozoic, a supercontinent referred to as Rodinia would break up. The breakup of this supercontinent would create a rift basin into which seawater would flow, eventually widening into a very large ocean. The event left its mark across the Laurentian craton, in the form of a basal sequence boundary. Known as the Great Unconformity, this erosional boundary would mark the start of deposition of sediments along the margin of the Sauk Sea that existed from the late Precambrian through the early Ordovician. The end of the transgression of the Sauk Sea (Highstand Systems Tract) marked a second important sequence boundary (Falling Stage Systems Tract). Known as the Knox Unconformity, it marks the base of the sequence of rocks that record the Tippecanoe Sea’s interactions with North America from the middle Ordovician Period through the middle Devonian. (See stratigraphic column below). Before it was all over, it be interrupted by the Taconian Orogeny, a major orogenic event. Laurentia at this time sat mostly south of the paleoequator with the modern eastern United States along its southern flank. Eventually, the Tippecanoe Sea reaches its penultimate highstand in the middle Devonian and, as the Falling Stage Systems Tract commences the formation of the Wallbridge Unconformity, deposition of Tippecanoe sediments comes to an end.

Eventually, the Taconian Mountains formed off of the coast an arc of Japan-like volcanic islands. Evidence for this is contained within the lithostratigraphic record as widespread and in many cases very thick volcanic ash deposits. While the entire sequence is transgressive across the craton, tectonic change and sedimentation changes taking place due to the process of orogeny along the southern portion of the craton would lead to more localized changes. Relative sea level, sedimentation, and tectonics would lead to lower orders of sea level sequences (3rd and higher). Embedded within the Tippecanoe as a whole then are successively smaller sequences of shorter duration that record events taking place throughout this time period.

Middle Ordovician Tippecanoe Sea. Note the position of the Paleoequator. The Taconian Island arc is still off of the southern coast of Laurentia (modern eastern North America) and the Tippecanoe Sea lies to its north (forearc basin). Sediments deposited in the forearc basin include the early Tippecanoe Carbonates into the Martinsburg Formation.

Figure \(\PageIndex{1}\): Generalized stratigraphic column for the central and northern Shenandoah Valley of Virginia and eastern West Virginia,
encompassing the lithologic units visited on Day 2 of the field trip. After Fichter and Diecchio (1993).

Tippecanoe Sequence 1

We can divide the Tippecanoe into two smaller sequences. The first records deposition directly following the Knox Unconformity and ended with the onset of coarse sandstones. This 3rd order sequence includes the carbonate bank sediments of the lower Tippecanoe, the New Market Limestone through the early Martinsburg Formation. It also includes the very distal upper Martinsburg Formation to the base of the Oswego Formation, where an unconformity existed across some of the area. The Oswego Formation has places in it that are rich in river cobbles, a good indication that this represents sediment that was being deposited very near land. Below is an accommodation space plot for this 3rd order sequence:

Figure \(\PageIndex{2}\): Subsidence and Accommodation Plot for the first half of the Tippecanoe Sequence in the Mid-Atlantic of North America (Whitmeyer et al., 2015).

This plot might be a bit confusing at first. But, you are looking at the cross section of the Taconian forearc basin over time during this portion of the Ordovician Period (left to right). As time passed the shelf basin was deepened and accommodation space increased (hachured area). Carbonate deposition was still the norm, as it was during the latest portion (uppermost) of the Sauk Sequence.

Figure \(\PageIndex{3}\): The contact between the lower Ordovician New Market Formation and Lincolnshire Formation. These carbonate deposits are typical of rocks deposited at a part of the “Great American Carbonate Bank”. They also represent shallower-water conditions. This is because of their carbonate lithology, fossil content, and structure.

The sequence began with the lowstand systems tract (LST), as recorded by the New Market Limestone. The sequence boundary already existed (Knox Unconformity) and the bulk of sediment being deposited was being formed in place by organisms secreting carbonate for their shells, tests, and some perhaps through evaporation. The seas at this time were conducive to calcite, rather than aragonite, being the primary shell-forming mineral. Abundant seafloor spreading is keeping magnesium levels stable and a warm global climate favored widespread biochemical carbonate production.

This sequence began to deepen as the onset of the Transgressive Systems Tract (TST) approached, marking the progression of deposition into the Edinburg Formation. This deepening, or subsidence, was the result of the downward flexure of the forearc basin. The transgression of relative sea level was due to the approaching Taconian island arc. Subsidence continued until about 468 million years ago, when deposition of the lower Martinsburg Formation began. Terrestrial sediments took over, eventually drowning out carbonate production.

Figure \(\PageIndex{4}\): Martinsburg formation, Page County, VA. The Martinsburg is composed of cyclical packages of turbidite sequences. These “fining-upward” sequences are interpreted as representing offshore mixed carbonate-siliciclastic conditions below wave-base. The dominant lithologies are siltstones and shales. Carbonate-rich materials are richest at the base of the formation, where the transition from the Edinburg Formation below is gradational in many places (Stickley Run Member, Martinsburg Fm.).

Figure \(\PageIndex{5}\): Graptolite fossils in the Martinsburg Formation, Page County, VA. Graptolites are fossilized here as carbon films. They break into small pieces very easily. Preservation in such detail is indicative of rapid burial, which would be typical in turbidite deposits.

During the Highstand Systems Tract (HST), submarine turbidite (submarine landslide) deposits became the norm. Accommodation space began to diminish as the Taconian highlands approached. During HST, sediment supply from land typically causes a prograding shoreline situation. This was likely the case for the Martinsburg Formation as its deposition progressed. Beginning as a very distal carbonate/mud in the lower Martinsburg, it eventually prograded into more proximal thick-bedded deposits as terrestrial sediment supply continued to increase and relative sea level dropped. By the end of Martinsburg Formation deposition, over 900m of sediment had been deposited and an erosional boundary formed at the base of the Oswego Formation.

At this point, the Falling Stage Systems Tract (FST) was completed and an entire sequence stratigraphic cycle is complete.

During this time, the craton as a whole experienced the results of an overall rise in global (eustatic) sea level. But, the paleocoastline (of today’s mid-Atlantic) saw a period of transgression (New Market Lm to Edinburg Lm) followed by a period of regression (Lower to Upper Martinsburg Fm). The end of this first 3rd order sequence of the Tippecanoe brought to an end the Ordovician Period and the full Taconian Orogeny was underway.

Tippecanoe Sequence 2

The latter half of the Tippecanoe is marked by another 3rd order sequence.

Beginning with the landward Massanutten Formation, replete with its Skolithos sp. and Arthrophycus sp. burrows marking it as sandy shoreline facies, this entire sequence ends beneath the Wallbridge Unconformity. The Taconian mountains reached their peak elevation during the late Ordovician and very early Silurian and were eroding into the nearby passive margin basin.

Figure \(\PageIndex{6}\): Massanutten Sandstone, Woodstock Tower, Massanutten Mountain, VA. The image below is of a bedding plane surface on a nearly vertical slab of Massanutten Sandstone. Replete with what are very likely Arthrophycus sp. trace fossils, the Massanutten Fm. represents shallow water conditions typical of the base of a sequence.

Figure \(\PageIndex{7}\): Hand sample of the Massanutten Sandstone. Note the coarse-grained nature of the specimen. Unlike the base of the other 3rd order sequence of the Tippecanoe, the Massanutten Sandstone is not a carbonate.

This passive margin (FST to HST) brings with it an eventual break from siliclastic sedimentation, marking the erosion of the Taconian mountains now to a peneplain. With the eventual onset of the Acadian Orogeny coming at the end of the Tippecanoe, there is time for a carbonate bank to form once again in this still very tropical sea, south of the Paleoequator. The transition from the coarse sandy Massanutten Sandstone to the fine-grained intertidal Bloomsburg and Tonoloway Formations and into the Helderberg Limestones that represent this new carbonate bank is very typical of TST conditions. Sea level is rising, even if the rocks represented here are, overall, more proximal to shore. Deepening is occurring.

Figure \(\PageIndex{8}\): Bloomsburg Formation, Fort Valley, VA. This iron-rich sandstone was mined as a source of iron during the 19th century in the Shenandoah Valley of Virginia. Finer-grained than the Massanutten Sandstone, it represents the early onset of TST.

Figure \(\PageIndex{9}\): Tonoloway Formation, hand samples from Corridor H in West Virginia. The left image shows evaporite structures in the form of salt casts. There is a short-term cyclicity evident in the sample also, as the environment alternated from carbonate production to sabkha. The image on the right is of dessication cracks, a cross-cutting feature indicative of very dry conditions.

Figure \(\PageIndex{10}\): The Tonoloway Formation, Corridor H, West Virginia. This outcrop of the Tonoloway formation well represents the higher-order cycles that exist just within this single formation. Carbonate production occurring in marine conditions is alternated with periods of intense evaporation.

Figure \(\PageIndex{11}\): Helderberg Limestone escarpment, New York. These massive limestones represent the deepest waters of the HST.

Figure \(\PageIndex{12}\): Late Silurian (Taconian Orogeny) Laurentia.

The sequence boundary is best placed at the top of the Oswego Formation. LST existed at the time of the deposition of the Massanutten (Tuscarora) Sandstone. The coarse sands and trace fossils of the Massanutten are indicative of shallow water. The Transgressive Systems Tract, TST, began when the first carbonates appear in the stratigraphy, interbedded with the very iron-rich red beds of the Bloomsburg Formation. The seas were beginning to rise and accommodation space was increasing. The transgression continued with the tidal Tonoloway Formation. This very interesting formation, at once full of ostracode fossils, salt casts, and mud cracks, is interpreted as representing a sabkha-like environment. It was a time of relative aridity with intense evaporation. While the salt casts of the Tonoloway Formation were being generated in what is now the Virginias, major salt deposits currently residing under Lake Erie were also forming.

As this new carbonate bank developed, it was deepened as sediment supply from the land diminished and accommodation space increased. This led to the formation of the very fossiliferous Helderberg Group limestones of the late Silurian and early Devonian. Having arrived now at HST, the waters of basin were as deep as they were going to get. As the sequence matured, a new moutain-building event began its onset, the Acadian Orogeny. The first clean quartz arenites show up in later Helderberg deposits, marking the coming of another terrestrial highland, called “Avalon”. These arenites mark the beginning of the prograding deposition so indicative of HST periods. The Acadian Orogeny got its start with the deposition of the Oriskany Sandstone, a fossiliferous and Skolithos sp.-rich sandy shore facies.

This sequence again ends with the formation of a sequence boundary as FST once again begins due to the sedimentation and then tectonic changes that lead to the formation of the new sequence boundary, the Wallbridge Unconformity. After a time, these Acadian mountains eroded to a plain. Following some amount of time and the Wallbridge sequence boundary, the Kaskaskia supersequence progressed.

4th Order and Higher Cycles

Within these two 3rd order sequence that have been described above, there are certainly 4th and perhaps 5th order parasequence sets and parasquences to be found. Identifying such short-term packages of sediments would add much to the story of the Tippecanoe. Any in-depth analysis of that level of detail goes beyond the scope of this case study, however.