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Quaternary Geologic Map of Connecticut and Long Island Sound Basin
DESCRIPTION OF MAP UNITS
Map units include surficial materials, more than 3 ft (1 m) thick, that overlie bedrock. A discontinuous veneer of eolian fine sand and thin colluvium on slopes is not mapped. Because all materials are of Quaternary age, Q is not included as a prefix to unit symbols.
GLACIAL MELTWATER DEPOSITS
GLACIAL MELTWATER DEPOSITS- All sorted and stratified sediments composed of gravel, sand, silt, and clay laid down by flowing glacial meltwater, and including minor lenses of flowtill and other diamict sediment. Mineralogy of sediments is highly variable across the State, but in general is closely similar to subjacent and northerly adjacent bedrock (see bedrock lithologic map, figure 3). Gravel clasts and sand grains are generally fresh and nonweathered.
Sedimentary Facies and Morphosequences
Figure 1 describes the various sedimentary facies that are recognized within the meltwater deposits of the region. These facies are defined on the basis of lithic characteristics of texture and sedimentary structure and are related to specific environments of deposition along the path of meltwater flow: fluvial sediments were deposited in meltwater streams; deltaic sediments were deposited where the meltwater stream entered a glacial lake; and lake-bottom sediments were deposited on the glacial-lake bottom. Glacial sedimentary facies are combined in facies assemblages or are present singly in mappable bodies of sediment known as morphosequences in the region (Koteff and Pessl, 1981). Types of morphosequences and the sedimentary facies that comprise them are described in Figure 1. In general, a morphosequence is coarse-grained at the glacier-proximal head and occurs in collapsed, ice-contact landforms; grain-size decreases and landforms are less collapsed to noncollapsed in distal parts of the morphosequence. Morphosequences were laid down in close association with the ice margin; the surface altitude of each morphosequence was controlled by a specific base level, either a glacial lake plane or a valley knickpoint. Stratigraphic relationships between morphosequences in individual valleys provide ubiquitous evidence that these ice-marginal deposits are systematically younger from south to north. Morphosequences are the basic mappable units of meltwater deposits at 1:24,000 scale, but they are too small and too numerous to be shown as individual units at the scale of this map. The ice-margin positions at the heads of many morphosequences are shown on the map by the solid-ticked-line symbol.
Map Units
Each of the 204 correlated map units of meltwater deposits on this map is a group of morphosequences laid down along the same or related paths of meltwater flow. Deposits in a map unit were laid down in a single glacial lake, a related series of lakes, or along meltwater streams in a valley where no ponding occurred. The position of groups of morphosequences (map units) in the landscape further indicates systematic northward retreat of the ice margin. Where drainage divides were parallel or oblique to the trend of the ice margin, groups of high-level deltaic deposits were laid down when paths of meltwater escape were first held to higher positions against or through uplands, and then gradually lowered as lower paths were uncovered in valleys. On the basis of stratigraphic relationships between deposits, successive retreatal ice margin positions, and changes in glacial lake levels and in paths of meltwater flow, morphosequences are grouped into map units that are chronostratigraphic in character and which define a relative chronology of ice retreat across the State (see figure 5).
Depositional Systems
Six depositional systems of meltwater deposits have been identified in Connecticut as a result of regional synthesis. They are shown on the map by color-groups of units and identified by (mnemonic) capital letter symbols: IL, Major Ice-dammed Lakes -greens and blues; SL, Major Sediment-dammed Lakes - greens and blues; IP, Related Series of Ice-dammed Ponds - purples; SP, Related Series of Sediment-dammed Ponds - browns; FP, Proximal Meltwater Streams - light oranges, FD, Distal Meltwater Streams - dark oranges. Each depositional system is defined by lithostratigraphic principles, characterized by particular morphosequence types, particular spatial arrangements of sedimentary facies, and typical stratigraphic relationships between individual deposits. The six depositional systems represent meltwater deposition in six paleogeographic settings which formed repeatedly in time and space, consequent to the interaction between the ice margin and the landscape over which it retreated.
Ponding of meltwater occurred in nearly every valley in the State during deglaciation, and as a result, most meltwater deposits were laid down in or graded to glacial lakes. Four of the six depositional systems formed in paleogeographic settings in which lakes controlled the distribution and altitude of fluvial, deltaic, and lake-bottom sediments. Lakes controlled deposition in northerly-draining valleys which sloped toward the retreating ice margin; in these valleys the ice margin impounded meltwater against opposing topography and spillways were across the lowest points of drainage divides. Glacial lakes and ponds in northerly sloping valleys are referred to as ice-dammed. Lakes also controlled deposition in most southerly-draining valleys, which sloped away from the ice margin. In these valleys lakes were impounded behind thick, valley-filling bodies of sediment that were constructed during preceding and successive meltwater deposition in each valley. Each sediment dam was itself an ice-marginal meltwater deposit, graded to a slightly older lake in the valley. Commonly spillways for succeeding lakes were over these dams. Lakes and ponds in southerly-sloping valleys are referred to as sediment-dammed.
Meltwater deposition in streams that were not tributary to glacial lakes was relatively uncommon in Connecticut; glaciofluvial deposits were laid down in positions both proximal and distal to the ice margin.
Glaciolacustrine Systems
Deposits of four depositional systems (IL, SL, IP, and SP) were laid down in or graded to glacial lakes. Sediments deposited in lakes include delta foreset and bottomset beds, lake-bottom sediments and local lacustrine fan sediments. Lake-bottom sediments in all map units are shown by horizontal-line pattern. Sediments graded to glacial lakes include fluvial delta topset beds, delta-tributary fluvial sediments, and local ice-channel sediments. Delta-tributary fluvial sediments in all map units are shown by dot pattern. Deposits of glaciolacustrine systems are predominantly deltaic. Altitudes of topset-foreset contacts in deltas record the paleo-water-plane altitudes of the glacial lake into which they were built. Deltas of all lakes in Connecticut indicate paleo-water-plane slopes of 0.9 m/km up to the north-northwest. This slope is due to glacio-isostatic tilt of the earth's crust.
Large Glacial Lakes include Major Ice-dammed Lakes (IL), and Major Sediment-dammed Lakes (SL); map units include all sediments graded to or deposited in single, relatively large, specifically named glacial lakes, some of which had several stages. These lakes existed in the wider valleys and large basins of the State. Deposits of major glacial lake systems are distinguished by several morphologic and stratigraphic characteristics: 1) deltas in each glacial lake (or lake stage) are at similar altitudes (when adjusted for glacioisostatic tilt), 2) deltas have free fronts, 3) lake-bottom deposits occur in front of deltas, and 4) delta-tributary fluvial deposits occur in side valleys.
Small Glacial Lakes include Related Series of Ice-dammed Ponds (IP) and Related Series of Sediment-Dammed Ponds (SP); map units include all sediments graded to or deposited in sequentially ponded and chronologically related series of small lakes (ponds). These small lake series existed in the narrower valleys and small upland basins of the State. Deposits of small glacial lake systems are distinguished by several morphologic and stratigraphic characteristics: 1) deltas in each map unit are at divergent altitudes, 2) deltas commonly do not have free fronts, but rather are contiguous with each other, 3) lake-bottom sediments occur only beneath the deltas, not at the surface, and 4) fluvial deposits (only in SP depositional system) occur in steeper sections of main valley and sometimes overlying deltaic deposits.
Glaciofluvial Systems
Deposits of two depositional systems (FP and FD) were laid down in meltwater streams that were not tributary to any glacial lake. Meltwater streams deposited ice-marginal and near-ice-marginal glaciofluvial sediments in the steeper sections of some southerly draining valleys and in front of end moraines; these are deposits of Proximal Meltwater Streams (FP). Distal meltwater streams flowed in other valleys after glacial lakes in those valleys had drained; these are deposits of Distal Meltwater Streams (FD)
u UNDIFFERENTIATED MELTWATER DEPOSITS- deposits of sand, gravel, silt, and clay laid down in unidentified depositional systems. Most deposits appear to be ice-contact and were probably laid down entirely on glacial ice and collapsed down to their present positions in the landscape. Unit locally includes lake-bottom deposits (ruled pattern) that can not be associated with any particular glacial lake..
IL Depositional System
(Units ln, lcw, lh, lds, ldp, ldh, lpt, lw, lnp, lbr, lcd, lcm, lma, lsb, lrb, lc, lex, lon, lvo, lp)
Paleogeographic Setting: Gently sloping, relatively wide valleys and basins with drainage outlets to the north; the main valleys were commonly fed by steeper tributary valleys. Lakes were impounded in these valleys and basins when the ice margin blocked the drainage outlet to the north; the lakes spilled through cols floored in till and/or bedrock across drainage divides. Some lakes had two or three stages as northerly retreat uncovered lower spillways out of the basin.
Deposits: Deltaic, fluvial, and lake-bottom sedimentary facies comprise these deposits. Delta-tributary fluvial sediments are shown by dot pattern; lake-bottom sediments are shown by line pattern. The most prevalent morphosequences are ice-marginal deltas, but ice-marginal and near ice-marginal fluviodeltaic sequences also occur; locally, ice-marginal lacustrine fans are found. Lake-bottom deposits associated with multiple deltaic morphosequences comprise large areas of these deposits.
Stratigraphic arrangement of deposits: Deltas in this depositional system generally have free fronts with lake-bottom sediments in front that separate later deltas from earlier ones. Delta altitudes reflect stable lake levels within each identified lake stage; delta surfaces rise to the north only in the amount that they have been tilted by postglacial rebound. Lower groups of deltas graded to lower spillways occur in those lakes which have two or more stages identified. Fluvial facies within fluviodeltaic morphosequences occur, predominantly in tributary valleys to the main basin or wide valley that contained these glacial lakes.
Map units of the IP depositional system: shown in blue colors on the map; unit descriptions (below) include location of the particular glacial lake and altitude of associated spillway, location and altitude of lower lake stages and associated spillways, altitudes of deltaic deposits, location of fluvial sediments in tributary valleys (if present), indication of buried lake-bottom sediments and their thicknesses if known and of importance. Each map unit description may also include discussion of damming mechanism and ice-margin positions requisite to formation of the glacial lake; and in some cases, interesting points of geologic history.
SL Depositional System
(Units lg, ll, lk, lb, lpg, lt, lf, ls, lq, H, M, lso, le, lu, lqb, C)
Paleogeographic Setting: Gently sloping, relatively wide valleys and basins which drained southerly, away from the ice margin. Relatively large glacial lakes formed in these valleys and basins behind thick, sediment dams that filled narrower sections of the valleys. Dams most commonly were composed of ice-marginal meltwater deposits (usually deltaic) laid down at slightly earlier ice-margin positions in the valleys. Lakes developed in wider sections of valleys or basins within valleys and were commonly fed by streams in tributary valleys to the lakes. Spillways for some of these lakes were over their sediment dams; in which case, the lake-level lowered continuously during the life of the lake because the spillway was over easily erodable sand and gravel deposits. Others of these lakes had spillways floored in bedrock across basin divides that were lower in altitude than the surface of the valley-blocking sediment dam; lake levels were stable through the life of these lakes.
Deposits: Deltaic, fluvial and lake-bottom sedimentary facies comprise these deposits. Delta-tributary fluvial sediments are shown by dot pattern; lake-bottom sediments are shown by line pattern. Morphosequence types include ice-marginal deltaic, ice-marginal fluviodeltaic, and near-ice-marginal fluviodeltaic deposits; locally, ice-marginal lacustrine fans occur; the Lake Hitchcock deposits include meteoric deltas. Lake-bottom deposits associated with multiple deltaic morphosequences comprise large areas of these deposits.
Stratigraphic arrangement of deposits: Deltas in this depositional system commonly have free fronts with lake-bottom sediments beyond that separate lower deltas from earlier ones. Deltas altitudes in most of these deposits rise to the northwest at a rate less than the slope of postglacial tilt, a result of lowering spillway levels; Exceptions are deltas of glacial lake Quinebaug, and stable-phase deltas of glacial Lake Hitchcock; these deltas rise to the northwest in the amount of postglacial tilt and reflect stable, bedrock-floored spillways. Fluvial facies within fluviodeltaic morphosequences occur in valleys tributary to the main basin or wide valley that contained the glacial lake.
Map units of the SL depositional system: shown in green colors on the map; unit descriptions (below) include location of particular glacial lake and location and type of spillway (that is whether it was a lowering spillway floored in sand and gravel across the sediment dam, a lowering spillway floored in till across a basin divide, or a stable spillway floored on bedrock). Several of these lakes had more than one type of spillway during their existence. Descriptions also indicate altitudes of deltas, location of fluvial deposits in tributary valleys, presence of buried lake-bottom sediments. Each map unit description also includes discussion of damming mechanism and ice marginal deposition requisite to the formation of the glacial lake; and in some cased, interesting points of geologic history.
IP Depositional System
(Units co, cg, ba, whd, ml, mop, wd, jg, su, cl, sd, bv, pd, mp, to, cr, emt, cs, be, fv, wbs, gl, om, led, bt, nth, wv, epg, qm, wb, msr, ms, so, wr, sh, mb, wa, bd, vl, bm, cd, hh)
Paleogeographic Setting: Steeper, small valleys that slope northerly toward the ice margin. Series of small lakes or ponds, impounded to the north by the ice margin in one or several northerly-sloping valleys. Multiple spillways cut into till or bedrock across divides are at successively lower altitudes to the north. A few of the units formed in this depositional setting were built in a single north-draining valley, into only one small lake; most, however, were a series of lakes which lowered successively to the north in several valleys descending from particular major or minor divides. Each group of ponds was formed during retreat of the ice margin from impingement against the divide, and before uncovering of lower drainage outlets.
Deposits: Predominantly deltaic sedimentary facies comprise these units. Lake-bottom facies occurs locally beneath the deltas, but is not exposed at the surface. Ice-marginal deltas are the only type of morphosequences present. Some ice-marginal deltas have fluvial feeder eskers (shown by chevron pattern).
Stratigraphic arrangement of deposits: These ice-marginal deltas generally do not have free fronts; the delta commonly completely filled the small body of water into which it was built and distal sand extends into the spillway entrance. Lake-bottom sediment consists mostly of bottomset beds (rather than extensive varved silts and clays) and occurs only beneath delta sediments. In any one valley, where deltas were built into a lowering series of ponds, deltas are commonly contiguous; each lower delta surface laps against the ice-contact proximal slope of the earlier delta.
Map units of the IP depositional system: shown in purple colors on the map; unit descriptions (below) include location of valley or valleys in which small glacial lake or series of lakes existed; spillway altitude, or range of altitudes of multiple spillways and particular divide or divides that the spillways cross. Delta surface altitudes (not usually given) are typically 5-10 ft (2-3 m) higher than the altitude of the related spillway due to the thickness of fluvial topset beds. Some map unit descriptions include important points concerning the history of ice margin retreat. Unit ip (p) -uncorrelated deposits of ice-dammed ponds.
ip uncorrelated deposits of ice-dammed ponds.
SP Depositional System
(Units sy, sa, hrn, bl, hrs, mr, sr, bg, eb, sk, pf, hp, sas, mf, nu, na, rn, wru, nrt, wrl, nrn, wil, mdr, lru, nrd, lrl, bk, mbb, ss, str, fu, yr, sl, tc, ws, ox, un, el, mu, fr, sb, wt, fm, ew, wu, pt, db, da, hu, qb, hr, sm, qn, mm, bfr, ib, wo, cw, lce, sn, lcc, pu, lct, an, ol, pl, eg, sp)
Paleogeographic Setting: Many of the valleys of Connecticut sloped southerly away from the ice margin. In the narrower sections of these southerly sloping valleys, series of small lakes developed sequentially as a result of northward ice retreat. Each pond was dammed behind (northerly of) the valley-blocking body of sediment that filled the next previous pond. In steeper sections of these valleys, meltwater streams fed a small lake down valley. Spillways for each small lake were over the sediment dams; commonly these spillways no longer exist because much of these deposits were removed by distal meltwater and the ancestral stream in each valley. Degradation and entrenchment of the ice-marginal deposits in these narrow south-sloping valleys was aided by a lowering base-level (glacial Lake Connecticut) in Long Island Sound.
Deposits: Predominantly deltaic and fluvial sediments; lake-bottom sediments occur locally beneath deltaic sediments but are not exposed at the surface. Ice-marginal deltaic, fluviodeltaic, and deltaic-fluvial morphosequences are present; near-ice-marginal fluviodeltaic deposits occur rarely.
Stratigraphic arrangement of deposits: Deltas of this depositional system generally do not have free frontal slopes; deltaic sediments commonly filled small ponds and are contiguous with the ice-marginal heads of previous morphosequences in the valley. Lacustrine sediments consist mostly of bottomset beds and are present only beneath coarser deltaic sediments. Deltaic surfaces and topset-foreset contacts within a map unit commonly rise up valley at a rate greater than the slope of postglacial tilt. Because of thick fluvial-topset aggradation in each deltaic sediment dam, the levels of ponding were successively higher up valley. Fluvial sediments within fluviodeltaic morphosequences occur in the steeper sections of some valleys where the till/bedrock floor is at shallow depths. Deltaic-fluvial morphosequences occur in sections of gentler gradient in some valleys; in these places fluvial beds overlie deltaic beds and may overlap from one morphosequence to overlie other ice-marginal deltas down valley. In many valleys only remnant-deposits of the SP depositional system remain. This is especially true in the longer river valleys of the Eastern and Western Highlands such as the Thames, lower Connecticut, Naugatuck and Housatonic.
Map units of the SP depositional system: shown in brown colors on the map; unit descriptions include location of valley or valleys in which small glacial lakes existed; types of morphosequences and their distribution within the valley or valleys; ranges of delta-surface altitudes and fluvial-surface gradients across the groups of morphosequences within the map unit; Most unit descriptions include reference to the cause of initial ponding, and the circumstances under which the particular lake series ended and a new one began. Unit sp - uncorrelated deposits of sediment-dammed ponds.
sp uncorrelated deposits of sediment-dammed ponds
FP Depositional System
(Units nf, by, nwb, me, ww, bu, bf, sbp, jhg, gr, kb, ca, ao, cho, fo)
Paleogeographic Setting: South-draining valleys that had a relatively steep gradient and that were not tributary to any glacial lake. These valleys were steep enough to avoid ponding, but not so steep that the sediment load of the meltwater stream was carried beyond. Outwash plains, broad areas of meltwater-stream deposition in front of end moraines, also are a part of the FI depositional system.
Deposits: Fluvial sediments in ice-marginal and near-ice-marginal fluvial morphosequences. In general, these deposits are coarser-grained (gravel and sand) and only 10 to 30 ft (3 to 10 m) thick (rarely as much as 50 ft (15 m) thick in moraine-proximal outwash deposits).
Stratigraphic arrangement of deposits: Deposits have relatively steep surface gradients, generally on the order of 10 to 40 ft/mi ( 2 to 7.5 m/km). In Connecticut, deposits of this depositional system are relatively uncommon; the best developed ice-marginal fluvial deposits occur as parts of fluviodeltiac morphosequences in valleys tributary to glacial lakes and therefore are included in Glaciolacustrine Systems. Many deposits of the FP Depositional System are small, isolated bodies, not extensive enough to be individually named and correlated map units; these are shown as uncorrelated unit fp (g). Ice-marginal fluvial deposits on outwash plains in front of end moraines are also uncommon; only one small deposit is known to occur in Connecticut (unit wo in the Broad Brook quadrangle), but extensive deposits in front of the Fisher's Island-Charlestown moraine occur in the Rhode Island part of the Watch Hill quadrangle and the New York part of the New London quadrangle and are shown on the map.
Map units of the FP depositional system: Shown in light orange colors on the map; unit descriptions (below) include identification of valley and topographic situation in which unit occurs,range in thickness of deposits, range in thickness of deposits, altitudes of surface slope (gradient), and base-level controls. Unit fp (g)- small, uncorrelated proximal fluvial deposits.
fp small, uncorrelated proximal fluvial deposits
FD Depositional System
(Units bb, fvt, qt, hm, ht, ft)
Paleogeographic Setting: South-draining valleys and basins after ice-marginal lakes had drained, allowing distal meltwater to incise, terrace, and redeposit sediment of the slightly older ice-marginal meltwater deposits. In some cases these distal meltwater streams originated at the glacier margin, which was more than 5 miles (8 km) away; in other cases a glacial lake separated the glacier margin from the site of meltwater-terrace deposition, and the meltwater stream issued from the spillway of a glacial lake.
Deposits: Distal fluvial sediments which are not traceable to an ice-marginal head. Deposits commonly consist of sand and fine gravel only 3 to 10 ft (1 to 3 m) thick; as much as30 ft (10 m) thick in the more extensive map units. Sediment is commonly lithologically distinct from underlying deposits.
Stratigraphic arrangement of deposits: Deposits occur on terraces erosionally inset into older and higher-altitude, ice-marginal (generally glaciolacustrine) deposits; these terraces extend for relatively long distances down valleys, and may be entrenched through several higher and older units. These units are the most extensive glaciofluvial deposits in Connecticut.
Map units of the FD depositional system: Shown in dark orange colors on the map; unit descriptions (below) include location of particular valley and identification of older units into which the fluvial sediments are inset; range in thickness of terrace sediments; specific lithology, if distinct; discussion of requisite timing and position of the ice margin at time of deposition. Unit fd (mt) -uncorrelated meltwater terrace deposits.
fd uncorrelated meltwater terrace deposits
Contents | Credits
Description of Map
Introduction | Map Units | Preglacial Landscape and Bedrock Source Areas | Glaciation | Glacial Ice-laid deposits | Deglaciation | Glacial Meltwater Deposits | Chronology of Ice Retreat and Major Glacial Lakes | Postglacial Conditions | References
Description of Map Units
Postglacial Deposits | Early Postglacial Deposits | Glacial ice-laid deposits | Glacial meltwater deposits | Housatonic - Southwest Coastal Basin | Naugatuck Basin | Farmington - Quinnipiac Basin | Upper Connecticut Basin | Lower Connecticut Basin | Thames Basin | Quinebaug - Southeast Coastal Basin | Long Island Sound Basin | Radiocarbon-dated Localities
