Deltas have long been recognized as important surficial geologic features in Vermont, as for example the works of Chapman(1937) and Wagner 1972. 1 ibid , “kame deltas,” or “ice marginal deltas” as they sometimes are called, were not part of the repertoire of features used by Wagner and others in Vermont in the 1970s. As this present study progressed, it became increasingly apparent that deltaic deposits which formed along the ice margin are very common. Such deposits are identified with the aid of LiDAR, topography, and the State surficial geology map, which shows tracts of flat terrain in sandy soil in conjunction with kamic topography in close proximity to stagnant ice deposits and closely associated fine grained lacustrine or marine silts and clays. However, in many places the kamic or ice contact nature of such deposits is inferred based on the sloping flat topographic nature of the deposits, commonly but not always with nearby evidence of stagnant ice such as kettle and associated fluvial deposits, generally along valley margins, immediately downgradient from or in close proximity to typical hummocky stagnant ice deposits, often as part of a step-down sequence of deposits at multiple terraced levels of stagnant ice which step downward to kame deltas. These deposits include typical “Gilbert” type delta deposits and as well broad, shoaling type deltaic deposits.
Where kame delta deposits lack kettles, hummocky terrain, or other stagnant ice expression, their identification as being ice contact generally is based on inference, which carries an element of uncertainty. Their inferred identification as “kame deltas” as opposed to “normal deltas” is supported by Chapman’s observation that in the Champlain Basin Lake Vermont deltaic deposits tended to form in close association with and along the ice margin of the receding Champlain lobe. But obviously not all deltas in this basin formed at the ice margin.
As discussed further below, numerous, recent articles in the literature recognize stepped, terrace-like deposits, progressing downward in elevation from and in close association with stagnant ice deposits, leading to kame deltas, especially in reverse gradient settings. In the present VCGI mapping reported here, these stepped type deposits are so common and prevalent as to suggest that they are part of a standard sequence along the receding ice margin in which ice marginal stagnant ice gave way to proglacial ponding environments with associated kame deltas closely confined by nearby ice. Such deposits may represent deposition associated with drainage from the ice but Wagner’s mapping of deltaic deposits in the 1970s only rarely suggested drainage away from an ice margin. In the vicinity of the “Middlebury Bench,” however, LiDAR imagery shows shoaling kame deltas associated with the development of a calving ice margin, with the evidence suggesting drainage from remnant stagnant ice masses related to the associated lowering of Lake Vermont from the Coveville to the Fort Ann level; these are referred to as “Headless Deltas,” as discussed below.
In the beginning of this VCGI mapping, the label “kame delta” was applied very cautiously, specifically limited to probable delta features which exhibited definite kamic topography, usually on LiDAR imagery, including for example closed kettle-like basins. Whereas flat topography as is typically found in deltas is easily identifiable on VCGI, and the granular nature of such features often can be identified by sewage soil, aggregate, and the State surficial geology tabs, VCGI itself gives no subsurface stratigraphic or structural data, whereby it is impossible to independently verify the identification of these features as deltas, kamic or otherwise. However, again as the study progressed it became increasingly apparent that flat, gently sloping sandy deposits immediately abutting or closely associated with ice margins marked by other features are exceedingly common, leading to a more liberal interpretation. Many of these deposits thus were inferred to be “kame deltas” even where and when kamic topography and closed basins were absent. Closer study of LiDAR imagery of such features in many cases showed features consistent with expectable ice contact deposits, as for example feint traces of lobate patterns or other subtle expressions of the ice margin. Further field study of such features is warranted.
As noted above, recent literature includes many research reports of such features associated with both Pleistocene and modern day glaciers and ice sheets. Articles by Parent and Occhietti 2ibid refer to kame deltas as an important type of stagnant ice margin feature in Quebec. A very substantial report by Winsemann et a, 2018 3 www://search.app/nYXeaxqy2FxcR7pX9 , in its introduction states:
“The stair-stepped profiles of the delta systems reflect the progressive basin-ward lobe deposition during forced regression when the lakes successively drained. Depending on the rate and magnitude of lake-level fall, fan-shaped, lobate or more digitate tongue-like delta morphologies developed. Deposits of the stair-stepped transgressive delta bodies are buried, down lapped and onlapped by the younger forced regressive deposits. The delta styles comprise both Gilbert-type deltas and shoal-water deltas.”
This description applies to this present study in Vermont as reported here, with both Gilbert type and shoaling deltas mapped on VCGI in many locations along the ice margins, especially the T5 – T8 ice margins where and when meltwater and proglacial lakes were as much a part of the glacial landscape as the ice itself.
Another notable report, by Dietrich et al, 2017, 4www.//nature.com/articles/s41598-017-16763-x and www://search.app/b7B6FNDGETFKMSby7 opens with the first paragraph of their abstract:
The paleogeographic reconstruction of the successive inland positions of a retreating ice sheet is generally constrained by mapping moraines. However, deltaic complexes constructed by sediment-charged meltwater can also provide a record of the retreating ice-margin positions. Here, we examine a serie of ice-contact, ice-distal glaciofluvial and paraglacial depositional systems that developed along the Québec North Shore(eastern Canada) in the context of falling relative sea level during the northward retreat of the Laurentide Ice Sheet(LIS). Ice-contact depositional systems formed when the LIS was stillstanding along the Québec North Shore. Subsequent inland retreat of the ice margin generated glacial meltwaters feeding sediment to glaciofluvial deltas, leading to their rapid progradation. The retreat of the ice margin from drainage basins was marked by the onset of paraglacial processes such as the shutdown of delta progradation, severe fluvial entrenchment, and deposition of shallow-marine strata. Four end-member scenarios describe the spatial and stratigraphic distribution of these three depositional systems(ice-contact deposits, ice-distal glaciofluvial deltas, and paraglacial suites). They reflect both the inherited drainage basin physiography and the retreat pattern of the ice margin. Applied to twenty deltaic complexes, these end-members allowed us to refine the model of LIS-margin retreat over southeastern Québec.
These and other articles make it clear that ice contact or kame deltas in physiographic settings such as Vermont where proglacial water bodies were nearly universal are an expectable ice margin feature, again in a reverse gradient setting. Such features take on particular importance for the Bath Tub Model because they formed along the physiographic margin of the “Bath Tub”, and along with Drainage Lines are part of the chronologic linkage between basins, but unlike the ice sheet itself which had surface gradients, proglacial water bodies along ice margins were flat and horizontal. Thus, ice margins associated with standing water were directly comparable to a Bath Tub Model analogy, without the complication of ice sheet gradients.
Whereas Drainage Lines, deltas, and kame deltas are an exceedingly important part of the study reported on here, their usage requires caution and care, with multiple caveats:
- As noted above, remote mapping using VCGI does not provide any subsurface stratigraphic or structural information, which is important for positive identification of these features.
- Ice margin and proglacial water bodies represent two related but separate chronologies which overlap but are not entirely synchronous. Ice margin and water level times were not instant times in the past but instead represented ranges of time.
- Ice margin retreat from one position to another took place in some cases during the times of the same water levels, and conversely in some cases water levels changed during the time period represented by a single ice margin T level (T levels as defined here span ranges of elevations and times).
- As mentioned above, a) the flat surfaces of deltaic and kame deltaic deposits are gently sloping, usually with only remnants remaining owing to subsequent erosion, and b) contour lines on topographic maps provide the only elevation control. As a consequence, the elevations representing water bodies and ice margins associated with such features are uncertain, imprecise, and variable, commonly leading to kame delta and associated ice margin elevations reported as ranges which reflect substantial vertical intervals. Scatter reflects the inaccuracies and imprecision as just described, even in Chapman’s report, despite the fact that his elevations were determined by survey leveling.
- Thus, this same imprecision and inaccuracy likewise can be seen in the strandline profiles of Chapman(1937) 5 Chapman, D.H., 1937, Late-glacial and postglacial history of the Champlain Valley. American Journal of Sciences 5th Series, 34: 89-124. Also, appearing as a report of the Vermont State Geologist. Wagner(1972), both of which include substantial scatter. A close examination of Chapman’s strandline profiles shows first that individual strandlines are represented by a relatively small number of data points which do not exactly fit on his strandlines. The same is the case for Wagner(1972), which in fact, for example, identifies the Coveville strandline as questionable, meaning uncertain, and suggests a more complex multi-leveled Fort Ann. Added to this, as Franzi recently reported, as discussed elsewhere herein, his recent study shows that in fact the water levels for major water bodies in the Champlain basin tened to fluctuate and were gradational in nature.
- Further, Chapman presents two separate profiles, one for Vermont and a second for New York. Whereas these profiles show strandlines which are generally the same for Coveville and Champlain Sea marine limit strandlines, the Fort Ann strandlines differ significantly. The explanation for this was at first attributed to the possibility that that isobases for isostatic rebound may not be oriented east-west(as suggested by Chapman), causing shifts in the Vermont versus New York profiles. However, the fact that Champlain Sea and Coveville strandlines for New York and Vermont are similar refutes that explanation. Apparently, the difference is attributable to the imprecision and scatter of strandline features, or the variability identified by Franzi. Again, the profiles drawn by Wagner(1972) likewise show substantial scatter.
As a general guide, the elevations of Wagner’s major strandlines in the Champlain Basin projected to the Quebec border were used to identify Coveville, Fort Ann, and Champlain Sea water bodies, as likewise all ice margin feature elevations were adjusted for isostatic rebound, giving the reference for elevations at the Quebec border.
An additional complicating factor is that at the higher elevation range of these strandlines, specifically at and above Coveville, is the fact that proglacial water bodies may be local upland lakes and not part of the regional Champlain Basin water bodies, meaning that many ice margins are attributed to kame deltas which are strictly local. A close examination of the data used by both Wagner and Chapman for Coveville in Vermont shows that three deltas reportedly representing Coveville are relatively substantial deposits and thus carry more weight in terms of documenting the presence of a regional water body at the so-called “Coveville” level. These are the Brandon, Bristol, and South Hinesburg Coveville deltas. The Brandon delta is a sprawling shoaling type delta which by its nature and the manner by which the Coveville level is estimated gives a very imprecise fix on the Coveville level. As discussed elsewhere herein the evidence suggests that this delta formed in close proximity to an ice margin. Likewise, as stated elsewhere herein the South Hinesburg delta actually comprises four distinctive levels, the third level down taken to be Coveville, but this judgment carries some uncertainty. This leaves only the Bristol delta as giving a reasonably good control on the Coveville strandline, which of course does not establish the slope of this strandline. Evidence likewise exists suggesting that this delta likewise formed in close proximity to an ice margin. In fact, Calkin and Stewart and MacClintock interpreted this feature as a kame terrace based on the abundance of boulders. However, the stratification is deltaic with no collapse structures.
Suffice it to say, taking all of the above observations into account, it is concluded that the information regarding the Coveville level is at best imprecise, but in any case likely was associated with the ice margin in Vermont as part of the evidence supporting the Bath Tub Model. Further, the evidence, again as discussed below, indicates that the ice margin likewise was closely associated with Fort Ann and Champlain Sea marine limit levels, again strengthening the argument for the Bath Tub Model.
As discussed below, kame deltas were very common along the Champlain lobe ice margin as part of a step-down sequence with the ice margin fronted by local proglacial water bodies, giving way to Coveville, Fort Ann and Champlain Sea regional water bodies in long narrow corridors which extended progressively northward along the ice margin. These long, narrow corridors were important in the deglacial history as discussed below. Again, the close relationship between the ice margin and water bodies serves to strengthen the case for usage of a “Bath Tub Model,” but as well the complexities as just described indicate that deciphering deglacial ice and water body history is a challenging proposition.
Footnotes: