Lithostratigraphic Analysis of Trenton Falls:
Brief statement regarding the historical lithologic analsysis of the Trenton Falls Limestone
The first stratigraphic analyses of the present day Trenton Group were performed and documented by Amos Eaton. His classification disregarded fossil zonations, over-simplified facies, and lumped together major distinctive rock types. The lack of a detailed terminology prevented the delineation of distinct rock and facies types within the Trenton gorge. The Trenton Limestones were referred to as the "Metalifferous Limerock", and without diagnosing key differences, Eaton's classification provided little information regarding lithology.
The first stratigraphic analyses which delineated vertical differences in lithologies were those of Ebenezer Emmons and Lardner Vanuxem as part of their contributions to the first geological surveys of New York . These authors had begun to use fossils in their stratigraphic synthesis along with lithologic differences within the limestones. As a result of their surveys, the term Trenton Limestone was coined, and recognized for the first time as being composed of multiple lithologies. In Emmons' report (1842; p. 112), he states:
"the Trenton, which I may justly remark, is one of the best characterized rocks in the Transition system, both in its fossils and lithological characters. In all localities of this rock, we find it more or less a shaly limestone. It sometimes occurs as a black thick-bedded rock, with argillaceous matter diffused through it; or in thin beds of limestone, alternating with those of a thin shivery shale. In addition to the masses of limestone and shale arranged as here described, there is sometimes another important one in the form of a grey crystalline rock, occupying sometimes a position inferior, and at others superior, to the black shaly limestone."
Emmons presented in his analysis one of the most critical lithologic differences within the section, the differentiation of coarse-crystalline limestone and dark shaly fine to medium grained limestone. In outcrop, his "inferior" and "superior" coarse grained lithologies are known today to be represented by the Kings Falls , the Steuben formations. Aside from describing faunal differences between strata, and a few other unique lithologic characters, these early workers lacked a system for classifying carbonate rocks. In contrast, today we have at least two major classification systems for the description of carbonate rocks, as well as a set of physical properties with which to further distinguish rock characters.
By the turn of last century, Trenton Falls had become world famous for its geology and its paleontology. Early workers such as Vanuxem, White, Prosser and Cumings had delineated several major sedimentologic differences within these strata. Building on these earliest studies, Percy E. Raymond, William J. Miller and Marshall Kay championed the effort to investigate and map the Trenton Limestone and its equivalents elsewhere. By 1937, faunal zonations had been established (see Raymond, 1903), and Marshall Kay had begun to develop a stratigraphic nomenclature for the Trenton Group. He published many articles, abstracts and bulletins on the subject. >>Back to Top
COMPARISON OF STRATIGRAPHIC SECTIONS: A CENTURY APART
Major advances in carbonate sedimentology have been made since the time of Eaton. With the establishment of several classification schemes, geologists look at finer-scales (sub-centimeter) for compositional differences in rocks. Consequently they are able to make more accurate predictions regarding the depositional history of the rock.
The following diagram (below) demonstrates this concept very clearly. On the left is a schematic stratigraphic section of the Trenton Falls Group as exposed in the Trenton Falls type locality. It was published by Brett and Baird (2002) in a paper titled "Revised Stratigraphy of the Trenton Group in its type area, central New York State: sedimentology and tectonics of a Middle Ordovician shelf-to-basin succession". The column on the right is just over one hundred years older and was published in 1896, by Prosser and Cumings. Their manuscript, entitled "Sections and Thickness of the Lower Silurian Formations on West Canada Creek and in the Mohawk Valley", represents the first stratigraphic section ever published for Trenton Falls.
Although these stratigraphic columns are drawn to slightly different scales, they do illustrate several key points:
1) "Before Folk and Dunham", Prosser and Cumings designated two main lithologies: a lower dominantly thick-bedded argillaceous limestone from the base of the Narrows through near the base of the railroad bridge (this lithology was their dominant rock type); and an upper crystalline limestone from near the base of the railroad to the top of the gorge section. This assessment is simple compared to the lithology diagnosis of Brett and Baird. In their study, several repeating rock types are found. Athough similar in color, the subtle textural, bedding, and faunal compositional differences help to differentiate the facies of each of the stratigraphic units.
2) "Dilemma of the full cascade": Diagrammatically, the Prosser and Cumings stratigraphic column shows the relative position of each of the four main waterfalls and establishes the total thickness of strata between. The same features are labeled on the Baird and Brett column. Given the scale difference, the relative thickness of strata between each successive set of falls is similar for the Sherman Fall-to-Lower High Falls interval and for the Upper High Falls-to-Mill Dam interval in both diagrams. However, the Lower High Falls-to-Upper High Falls interval is substantially greater in the Prosser and Cumings figure. Given that both diagrams were drawn to scale (albeit different than each other), the anomalous stratigraphic thickness difference within the region of Lower-to-High Falls region is not consistent with the other interval measurements.
This "sticking point" is perhaps a puzzling but it is explainable. Since the time when Prosser and Cumings measured their stratigraphic sections, the flow of water through the gorge has been substantially less due to the construction of the power dam. With high discharge rates nearly year round, Prosser and Cumings had to measure sections from disparate sides of the gorge in order to construct a composite, reference column. They traced individual beds across the face of Upper High falls and continued their measurements. Because water flow rates were high, their bed tracing deceived them.
Lower water discharge rates flow through the gorge today, and occasionally is completely shut down. Professors Carlton Brett and Gordon Baird made an important observation with respect to the lateral continuity of beds from one side of the falls face to the other. Especially in this falls, the beds cannot be traced laterally. There is a roughly north-south striking high angle normal fault that is hidden in the face of the Upper High Falls waterfall. It is most often concealed with even the slightest amount of run-off. This single observation is most likely the source of the discrepancy between the two stratigraphic columns. Prosser and Cumings' measurements were off because they mistakenly duplicated section.
It is more than likely that this same fault extends some distance up and downstream and could potentially have further repercussions for other measurements. >>Back to Top
MODERN LITHOSTRATIGRAPHY OF THE TRENTON GROUP
Through his intuitive observations and keen eye for subtle facies variations and well-correlated cross-sections, Marshall Kay's studies have paved the way for our modern studies of the Trenton Group. His nomenclatural system, although modified slightly, is the primary stratigraphic column used today. The discussion here on the stratigraphy of the Trenton Group is based on the most recent up-to-date stratigraphic analysis of these rocks. Two recent papers by Brett and Baird (2002), and Baird and Brett (2002), looked at the Trenton Group and its lateral downslope equivalents, the Flat Creek (or Canajoharie Shales) and the Indian Castle Shales (Utica Group). The discussion covers nomenclatural history, lithologic details, distinctive facies components and other key descriptive characteristics that help to delineate the stratigraphic character of each unit. >>Back to Top