Drowned and buried valleys on the southern New England continental shelf

Marine Geology, 15 (1973): 249-268. © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

DROWNED AND BURIED VALLEYS ON THE SOUTHERN NEW ENGLAND CONTINENTAL SHELF

ROBERT L. McMASTER and ASAF ASHRAF

Graduate School of Oceanography, University of Rhode Island, Kingston, R.L (U.S.A.) (Accepted for publication August 7, 1973)

ABSTRACT McMaster, R. L. and Ashraf, A., 1973. Drowned and buried valleys on the southern New England continental shell Mar. Geol., 15: 249-268. Bathymetry and seismic reflection profiling have revealed a sequence of seven post-Jurassic drowned or buried drainage systems on the southern New England shelf. The basement and younger stream patterns have a dominant southward trend with preferred drainage avenues from the mainland to the middle shelf indicated by superposed valley ground positions from unconformity to unconformity over time. Fluvial action under stable tectonic conditions is inferred by low valley height/width ratios with higher ratios related to ice modification of inner shelf pre-glacial river valleys. Fluvial processes responding to sea4evel withdrawals have greatly influenced the shelf's later development. Periodically during post-Paleocene time, sediment from subaerial erosion has been transported to the shelf edge by streams. Deltaic deposition on a subsiding base has controlled outbuilding on the outer shelf where the frequent presence of overlying drainage networks is the result of numerous sea-level regressions. Since Eocene time, sediment has been channelled to the deep sea via Block Canyon and its progenitor. Locally structures created by erosion and glacial deposition have governed drainage direction. On the inner shelf, late Tertiary - early Pleistocene streams were diverted southeastward and southwestward by the magnitude of Long Island's Coastal Plain escarpment and by secondary cuestas between eastern Long Island and Block Island. The probable eastern reach of Dana's southern Sound River valley can be traced from northeastern Long Island across Block Island Sound. An early Woodfordian end moraine of the Wisconsin stage impounded melt waters in Block Island and Rhode Island Sounds. Where the moraine was breached near Block Island, fans were formed adjacent to the water gaps. In Rhode Island Sound the earlier and later Woodfordian end moraines deflected some mainland drainage toward the southwest. INTRODUCTION T h e p r e s e n c e o f d r o w n e d valleys on t h e c o n t i n e n t a l s h e l f o f f s o u t h e r n N e w E n g l a n d is well k n o w n ( V e a t c h a n d S m i t h , 1 9 3 9 , p . 4 3 ; G a r r i s o n a n d M c M a s t e r , 1 9 6 6 , p . 2 8 1 ) . A l o n g the C o n n e c t i c u t a n d M a s s a c h u s e t t s s h o r e l i n e , F l i n t ( 1 9 4 7 , p . 4 4 8 ) s u m m a r i z e d t h e l o c a t i o n s a n d d e p t h s o f b u r i e d b e d r o c k valleys a n d suggested t h e m o d i f y i n g role o f glacial e r o s i o n in t h e d e v e l o p m e n t o f these valleys. U p s o n a n d S p e n c e r ( 1 9 6 4 ) p r e s e n t e d d e t a i l e d d a t a on

250

ROBERT L. McMASTER AND ASAF ASHRAF

the dimensions of southern New England bedrock valleys and the nature and age of the contained sediment fill and discussed the origin of these valleys in reference to sea-level fluctuations. Based upon recent seismic reflection surveys of the southern New England continental shelf, McMaster et al. (1968, p.471) and McMaster and Ashraf (1973a, b) traced the extent and probable evolution of these deeply buried valleys and defined the basement drainage pattern beneath the Coastal Plain strata on the inner shelf. To compile information concerning relationships of Pleistocene and pre-Pleistocene valleys systems of southern New England's continental shelf, 40 seismic reflection profiles were made off eastern Connecticut, Long Island, New York, Rhode Island, and southeastern Massachusetts in 1968-1969 from the inner limits of Block Island and Rhode Island Sounds to the shelf edge (Fig. 1). These profiles were positioned to intersect major drainage trends with longitudinal lines, spaced 4 - 9 km apart, dominating the survey. The specific objectives of the investigation were: (1) to identify and trace additional shelf drainage networks; (2) to compare the drowned and buried systems in terms of position and origin; and (3) to ascertain the role of these valley networks in the evolution of the continental shelf. Specifics regarding the seismic reflection technique were reported previously (McMaster and Ashraf, 1973a). Navigation control for the profiling was by Loran A, accuracy 1 km.

GENERAL DESCRIPTIONOF SUB-BOTTOMRECORDS

Narragansett Bay-Block Canyon transection Profiles W and 3 7 (Fig.l) represent a cross-section of the shelf. Inshore Profile W (Fig.2) reveals an irregular, seaward-dipping, acoustic basement with a slope increase at 40 kin, a lowland-cuesta feature, seaward-dipping reflectors of late Cretaceous age lying south of the cuesta or escarpment, and uneven Pleistocene erosional surfaces immediately below the bottom (McMaster et al., 1968, p.468). Profile 3 7 (Fig.2) shows the acoustic basement reflector continuing offshore with an increasing dip which is slightly steeper than the bottom. Uppermost Coastal Plain deposits are truncated by an unconformity that parallels the shelf surface. Beneath the outer shelf near the 40 km mark, deltaic structures appear with accompanying unconformities which increase in number and complexity toward the shelf edge.

Inner shelf On the inner shelf, numerous valleys of varying size and extent are present in Block Island and Rhode Island Sounds (Fig.l, 2). Some are associated with the acoustic basement; others lie in Cretaceous or post-Cretaceous deposits, and some occur on near-

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Middle and outer shelf Deeply incised channels are present on Profiles 6 and 12 (Fig.l-4), but only two appear on Profile 13. However, valleys are present on various erosion surfaces including the basement on each of these profiles. Profiles 18 and 20 (Fig.l, 3, 4) also contain channels but none of these are deeply cut into the sections. Because the number of depositional-erosional surfaces increases beneath the outer shelf (Profdes 20 and 24), more channels are present. Profile 24, across the head of Block Canyon, indicates the complex development of the canyon, although the reflectors farther shoreward are continuous and generally parallel. A transverse fault can be observed on Profiles 6, 12, 13 and 18 (McMaster, 1971, p.2001). The reflectors on Profiles 20 and 24 reveal overlapping and coalescing deltas and associated erosional surfaces. Profile 24 clearly shows the configuration of the uppermost delta adjacent to Block Canyon. The general absence of deltaic structures below reflecting horizons T1 (Profile 20) and T2 (Profile 24) may be real or have resulted from the loss of resolution. DRAINAGE PATTERNS AND THEIR GEOLOGICSETTING The authors have not attempted to piece together all the drainage networks suggested by the seismic profiles but have limited their coverage to seven systems which are readily observed and are related to sea-level fluctuations encompassing the segment of geologic time represented by the profiles. As dated stratigraphic units are unavailable for this shelf area, the tentative ages of the various drainage systems have been based on stratigraphic sequences used by other workers cited below. Each system is presented on a structure map (Fig.5, 6) and its valley dimensions are summarized in Table I.

Acoustic basement beneath the Coastal Plain deposits The oldest drainage mapped occurs on the acoustic basement (Fig.2, Prof'fles W and 37) and has been discussed elsewhere (McMaster and Ashraf, 1973a). The channel pattern is outlined in Figure 5A.

Reflector M surface M lies some 300 m above the acoustic basemen t (Fig.2, Profiles W and 3 7), essentially paralleling this surface but much smoother. M reflector's inshore subcrop occurs at an unconformity approximately 25 km from the mainland. From this point, the reflector, showing an increasing offshore dip, can be traced approximately 90 km seaward but resolution on the longitudinal profiles is lost south of Profile 19 (Fig.l, 7). No direct correlation of this reflector with the New Jersey and Long Island stratigraphic sections can be made although extrapolation from Long Island indicates that M occurs in the undifferentiated Magothy Formation and Matawan Group of late Cretaceous age (Maher,

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1971, P1.9). Possibly the Morgan beds of New Jersey's Magothy Formation may be correlative with this reflector. These beds, considered to be subaerial plain deposits, characteristically contain what has been interpreted as overbank deposits in the form of horizontally stratified interbedded sequences of thin dark silts and light sands (Ownes and Sohl, 1969, p.255). Field studies have shown that these thin-bedded sequences grade into cross-stratified point or channel-bar deposits. Structural contours on M reveal no gross irregularities (Fig.5B). Directed toward the outer shelf, the valley pattern consists of Simple dendritic channels.

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TABLE I Summary of valley dimensions on each erosion surface Surface

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(56%) (53%) (56%) (53%) (30%) (40%)

0.6-8 0.94.9 0.7-2.5 0.5-4 0.7-3

1-2 (59%) 2-3 (54%) 1-2(80%) 1-2 (64%) 1-2(54%)

1/12-1/62 1/16-1/40 1/59-l/250 1/59-1/250 1/59-1/250 1/33-1/120

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T2 T1 M Basement landward of cuesta seaward ofcuesta

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32-86 ( 5 6 % ) 17-30 (55%)

*Poor resolution conceals true valley height (see text p. 260).

Reflector T1 surface Overlying M is reflector T1. It is truncated near the shelf surface some 75 km from the coastline (Fig.2, Profile 37) but continues beyond the shelf edge. This reflector also has an increasing dip offshore and its down-dip surface appears to be more uneven than that of M. Its stratigraphic position is considered to be near the top o f the Vincentown Formation o f Paleocene age and it probably marks the Paleocene-Eocene boundary (Garrison, 1970, p.115). Structural contours on T1 bend slightly landward on the east and west sides but the primary feature is a valley system composed of some six channels that trend toward the shelf edge (Fig.5C). Some concentration o f valleys in the vicinity o f Block Canyon is indicated.

Reflector T2 surface Extensive deltaic deposits occur on and above the T1 surface (Fig.2, Profile 37). A major unconformity (T2) denotes the top of this sequence and extends from its subcrop, 110 km from shore to the upper continental slope. A middle Tertiary age has been suggested for this reflector (Garrison, 1970, p.116). The gross characteristics o f the T2 surface are poorly defined because of survey limitations and the position o f the first multiple reflector on Profile 26 (Fig.5D). The channels are directed seaward with notable convergence at Block Canyon.

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surface, and is in turn truncated by P2, a younger erosion surface after only 2 0 - 3 0 kin. The seaward extent of P1 is considered to reflect a pre-Woodfordian, glacially eroded surface of Wisconsin age (McMaster and Ashraf, 1973b). As the position, intensity and duration of the older Pleistocene ice advances and accompanying glacial processes varied over time, an attempt to piece together the stratigraphic and age relationships or the erosional-depositional effects of these events would prove fruitless. Therefore, a summation of these variations is presented in composite form as the glaciated P surface of the inner shelf (Fig.6A). The seaward edge of the P surface lies between the eastern end of Long Island and Martha's Vineyard (McMaster and Ashraf, 1973b). Longitudinal profiles of certain valleys on P surface are compared with those of M and P2 surfaces in Fig.7. Apparently the termini of the P surface valleys occur where the P1 reflector is truncated by the P2 erosional surface (Fig.2, Profiles W and 37).

Reflector P2 surface The generally rough P2 reflector extends almost across the shelf and lies 17-34 m below the shelf surface with its dip paralleling the present surface gradient (Fig.2, Profiles W and 37). It is considered to be Wisconsin in age, possibly Farmdalian as described by Schafer and Hartshorn (1965, p.118), and correlates with Garrison's (1970, p.116) Pleistocene unconformity. Structural contours on the P2 surface indicate an inner shelf irregularity which reflects previous glaciation, although on the middle and outer shelf relief is more subdued (Fig.6B). The network of dendritic valleys has a pronounced southward trend with the inshore channels aligned in the same general directions as those of the older P surface (Fig.6A). Northeast of Block Island uncertainty exists as to the actual course of the valley extending from Narragansett Bay. Perhaps each course was active at different times during this erosional interval. On the outer shelf deposition from the mouths of these valleys continued to outbuild Block Delta with Block Canyon active in dispersing sediment to the deep sea (Fig.6B).

Present shelf surface The inner shelf shows a variety of forms. Elongated topographic highs extend southwest of the Elizabeth Islands and Martha's Vineyard, southeast of Pt. Judith, north of Block Island, and northeast of the tip of Long Island. Also distinct highs occur around Block Island. Between these features broad valleys and troughs appear. Troughs, trending southeastward, are common in Block Island Sound whereas in Rhode Island Sound northsouth troughs and a southwestward-oriented trough are present. A shallow channel appears to connect the two sounds north of Block Island. At the seaward extent of the inner shelf two small coalescing fans or deltas occur each of which is crossed by a deep trough. These fans are located on either side of Block Island.

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The valley pattern on the present shelf surface can be seen in more detail than those previously discussed. From Martha's Vineyard, valleys trend toward the southwest, but swing to the south near Narragansett Bay. Valleys and troughs in Block Island Sound are oriented predominantly toward the southeast and south, exiting the sound through a water gap between Long Island and Block Island. Block Channel acts as the major trunk channel. West of the channel, valleys are oriented toward the east, whereas further inshore toward the northeast, tributaries trend across the shelf to Block Channel. Moreover Block Channel contains several subsidiary valleys, all of which lead to Block Delta, but no distinct valley connects the inner shelf with Block Canyon. East of the channel several valleys trend southward, but apparently are unrelated to the inshore system or to Block Delta. Many distributaries lie on the Block Delta front. Shelf surface valley dimension were taken from the seismic profiles (Table I). Undoubtedly the 5-7.5 m reflected signal width masks the true depths of these valleys.

COMPARISON OF CHANNELED SURFACES

Only two of the surfaces described above extend across the shelf. However, because of the relative locations of these surfaces, the trends, numbers per area or densities, ground positions, and dimensions of the various valley segments can be compared for at least three drainage networks.

DROWNED AND BURIED VALLEYS

261

Inner shelf The Quaternary channel systems show both similarities and dissimilarities relative to each other and to valleys on the acoustic basement. In Rhode Island and Block Island Sounds the P and P2 valleys are generally consistent as to trend, density and ground position (Fig.6A, B). However, the valley height/width ratios are in marked contrast (Table I). Comparison of the characteristics of Quaternary channels with those of the acoustic basement valleys (Fig.5A) indicates that the trends in Rhode Island Sound are southward, except on the eastern side where P surface channels are directed toward the southeast. In Block Island Sound the orientation of Quaternary valleys is generally dissimilar to those of the basement. Valley densities are variable: in Block Island Sound the networks on P and P2 are more dense than that of the basement, whereas in Rhode Island Sound there is no consistency. Furthermore, a comparison of the ground positions of the basement valleys (Fig.5A) with those of the P (Coastal Plain) and P2 surfaces (Fig.6A, B) shows that most basement channels have well-defined counterparts on the overlying surfaces. However, in Block Island Sound only one example of valley superposition between these surfaces is evident (Fig.5A and Fig.6A, B). Valley height/width ratios are distinct for the P2, P and Coastal Plain basement surfaces (Table I). The valleys on the present surface show similar and dissimilar characteristics when compared with those of the underlying surfaces (Fig.6C-A and Fig.5A). In Rhode Island Sound trend and ground position are not consistent with those valleys on the P2 and P surfaces except in the nearshore zone, whereas in Block Island Sound these characteristics correspond closely. The widths of the valleys on the present surface (Table 1) are of similar sizes to those of the P2 surface, but no correlation of valley attributes exists with those of the basement. Middle and outer shelf The pre-Quaternary valleys have the same southward orientation but near the outer extent of the shelf the trends change slightly so as to become perpendicular to the shelf edge (Fig.5A-D). Valley density is variable: basement and T1 channels reveal greater numbers per area than those of the M surface, whereas the T2 valleys are more dense than those of T1. Generally the ground positions of valleys are superimposed from surface to surface although the consistency of this rule lessens seaward. At the shelf edge four T2 valleys directly overlie those of the T1 surface with Block Canyon showing definite valley stacking. Over the middle shelf there is general correlation of the basement valleys with those on the overlying M surface and general correspondence of the valleys on the M and T1 surfaces. Only Block Channel seems to contain valley superposition over the preQuaternary interval. Generally valley ground position offsetting shows no preferred direction. The range of height/width ratios for the basement valleys, is lowest with greater

262

ROBERT L. McMASTER AND ASAF ASHRAF

but equivalent ratios, associated with the T1, and T2 surfaces (Table 1). Ratios for the M surface valleys occur in two significant ranges. The Quaternary valleys have inconsistent characteristics when compared with each other and with valleys on the older surfaces (Fig.6B, C). On the middle shelf the diagonal trend of the present surface valleys is in contrast with that of the P2 valleys. However, over the outer shelf both networks have the same orientation. Valley superposition is limited to Block Channel and the vicinity of the shelf edge, especially near Block Canyon head. On the middle shelf the ground positions of the P2 surface valleys have a general correlation with the underlying T1, M and basement channels. Valley widths are comparable for both the present and P2 surfaces (Table I). In terpreta tion Surprisingly general superposition of valley ground positions occurs from erosional surface to erosional surface. Close scrutiny of longitudinal seismic profiles (Fig.2, 3) reveals that this tendency is also apparent on those reflectors lying between the selected unconformities. Moreover preferred drainage routes from the mainland to the middle shelf have endured over time (Fig.8). Major valleys of the basement, Morgan(?), and preWoodfordian surfaces follow these broad avenues which can be traced southward from Block Island Sound, entrances to Narragansett Bay and Sakonnet River, and coastal valleys of southeastern Massachusetts. Apparently the route followed by Block Channel and its predecessors is the most persistent avenue. Thorn (1946, p.742) reported that progressive compaction of old valley fillings resulted in some of the smaller recent New Jersey Coastal Plain streams re-adopting the lines of the pre-Cretaceous drainage, even though their flow is now reversed from that of the original streams. Almost all New Jersey Coastal Plain formations have maximum known thicknesses in outcrop less than 45 m (Ownes and Sohl, 1969, p.237). If these thicknesses are comparable to those of Coastal Plain formations of the inner to middle shelf off southern New England, it is conceivable that basement modal valley depths of 17-30 m (Table I) could be reflected by the surface of each depositional unit with subsequent stream erosion re-excavating these depressions. Recently the senior author observed 2 0 - 3 0 m of Holocene sediments draped over the irregular basement in the Gulf of Maine. An analysis of valley height/width ratios reveals that these ratios fall into two broad categories (Table I). Ratios less than 1[62 which are the most persistent are the consequence of stream action under stable tectonic conditions, whereas those greater than 1/62 are the result of ice modification of pre-existing river valleys. The significant ratio range 1/33-1/120 associated with the Morgan(?) surface (Magothy age) may indicate some upwarping over the fluvial erosion interval. GEOLOGICHISTORY Garrison (1970, p.117) visualized the late Mesozoic and Cenozoic development of the southern New England shelf to be the result of mild and intermittent subsidence coupled

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with transgression and regression of the sea produced by tectonic movements, fluctuations in sediment supply and eustatic changes of sea-level. He found these influences to have acted at different times and places to construct this prograding shelf. Consideration of the regressional history as portrayed by the drainage patterns will be regarded as an element of shelf evolution. The tentative stratigraphic sequence outlined should be viewed as a working framework since the chronology of the various units is yet to be established.

Pre-upper Cretaceous stream activiO, Marked erosion of the basement has been long recognized (Sharp, 1929, p.35; Johnson, 1931, p.14; and others). For the Fall Zone surface beneath the Coastal Plain prolonged pre-upper Cretaceous stream action under stable tectonic conditions is clearly indicated by the broad valleys, low interfluves and small valley height/width ratios (McMaster and Ashraf, 1973a).

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ROBERT L. McMASTER AND ASAF ASHRAF

Upper Cretaceous Morgan(?) stream activity The Morgan(?) surface has low relief across which sediment from the mainland was carried to the outer shelf by stream systems. Post-Cretaceous erosion truncated the warped surface near the seaward extent of the present inner shelf.

Paleocene-Eocene stream activity Garrison's (1970, p.115) suggestion that this surface may be erosional is supported by the presence of well-developed stream valleys comparable to those of the Morgan(?) surface. However, the occurrence of stream valleys on this surface indicates a major early Tertiary age withdrawal of the sea in this area. If this age is correct, an ancestral Block Canyon was carrying sediment to the deep sea as early as Eocene time. Kelling and Stanley (1970, p.652) believe that Baltimore and Wilmington Canyons, off Delaware, originated during a late Tertiary lowstand. Irregular subsidence of the surface followed the erosional phase. Locations where subse. quent stream sedimentation was rapid are shown in Fig.5D. The inshore portion of the surface was truncated by later erosion.

Middle Tertiary stream activity The Paleocene erosional interval was followed by extensive delta building at river mouths on the outer shelf that continued into middie Tertiary time. Deltaic construction temporarily ceased in the middle Tertiary and the sea once again regressed. Stream action, similar in character to that of Paleocene time, established an erosional surface of low relief. Block Canyon continued to serve as a channel for funneling sediment seaward. Following this erosional interval deltaic deposition resumed.

Pleistocene, pre-Woodfordian stream and glacial action Late Tertiary-early Pleistocene drainage was well-established on lhe Fall Zone or inner lowland and the Coastal Plain. Early ice advances deeply scoured the pre-glacial valleys with the maximum progression of the ice extending some 50 km off Rhode Island (McMaster and Ashraf, 1973b). P surface channels undoubtedly reflect the courses of late Tertiary drainage on the inner shelf. In Block Island Sound the ground positions of the P channels on the Coastal Plain do not generally overlie those of the basement. From an analysis of soundings in Long Island Sound, Dana (1890, p.426) conceived a Pleistocene southern Sound River flowing eastward, that reached the ocean through Peconic Bay near the tip of Long Island. Grim et al. (1970, p.663) found an east-west channel, with depths of over 200 m below sea level, that extends along the south side of Long Island Sound from about 73 ° 20'W to approximately 72 ° 40'W. They propose that this channel may be part of the glacially

DROWNED AND BURIED VALLEYS

265

deepened channel of the Sound River. The presence of a major valley trending eastward from northeastern Long Island across Block Island Sound suggests that this valley may be the eastern reach of the southern Sound River. Several factors may be relevant in accounting for the divergence of the late Tertiary early Pleistocene valley trends from those of the basement in the inner sounds. For Block Island Sound the presence of the E - W Coastal Plain escarpment and at least one E - W secondary cuesta apparently deflected the stream courses. Near the tip of Long Island the buried surface of the Magothy Formation shows a 30 m ridge paralleling the coastline (Suter et al., 1949, PI.19) with this same formation cropping out on Block Island (Woodworth and Wigglesworth, 1934, p.212). Thus the nature and magnitude of erosional structures appear to have caused the major drainage to be diverted toward the southeast and southwest with these trends being maintained throughout the Pleistocene. On the eastern side of Rhode Island Sound a pre-Woodfordian trunk channel has a southeastward course. This valley may be directed toward the Tertiary trough south of Martha's Vineyard defined by Garrison (1970, p.114). Four unconformities can be observed on the outer shelf between the middle Tertiary boundary (T2) and the present shelf surface (Fig.2, Prof'de 37). However, correlation of these surfaces with specific late Tertiary and Pleistocene sea-level fluctuations is not possible with the present data. Knott and Hoskins (1968, p.12) found a series of five erosional surfaces of Pleistocene age in the vicinity of Hudson Canyon. These surfaces could be correlative with the Block Delta sequence when the T2 unconformity is included.

Pleistocene, Farmdalian stream activity The P2 erosion surface truncates the underlying beds with angular unconformity. Its extensive well-developed valley system marks a major regression on the shelf. During this regression sediment discharge from the mouths of P2 streams near the shelf edge produced a significant growth phase of Block Delta. Inshore most of the P2 valleys lie in the larger P surface channels and probably predate the late Wisconsin glaciation. Pleistocene, Woodfordian-Holocene stream and glacial activities Woodfordian events of the Wisconsin stage had a noticeable effect on the shelf drainage pattern. The first advance of the ice reached a position south of Block Island curving toward Long Island's south shore and Martha's Vineyard (Schafer and Hartshorn, 1965, p.119). This advance produced the Ronkonkoma-Nantucket end moraine (Fig.6A). The inshore P2 valley system appeared to be relatively unaffected by ice action. When the ice began its irregular retreat, the terminal moraine impounded melt water (Lougee, 1953, p.365; Newman and Fairbridge, 1960; and others). In Block Island Sound a non-fossiliferous, concretionary clay, believed to be of glacio-lacustrine origin, has been reported (Frankel and Thomas, 1966, p.240). Ground moraines were also formed on the inner shelf. The melt water finally breached the moraine between Long Island and Block

266

ROBERT L. McMASTER AND ASAF ASHRAF

Island and southeast of Block Island. Adjacent to the water gaps, the flooding discharge produced fans on which primary distributary channels were developed (Lougee, 1953, p.265; Newman and Fairbridge, 1960, p.1936). Drainage rearrangement occurred in Block Island and Rhode Island Sounds. In Block Island Sound major adjustment was limited to its southern part where the old Sound River channel was closed by the morainal filling. In Rhode Island Sound drainage continued to flow southward from Narragansett Bay, but the trunk valley near Block Island was closed and all the drainage from southeastern Massachusetts was diverted by end moraine (Cox Ledge) toward the southwest. On the middle and outer shelf some drainage adjustments occurred. Seaward of the moraine at Cox Ledge the main valleys also turned southwestward, following the base of the moraine toward Block Channel. On the west, adjacent to Block Island Sound, Block Channel persisted as the trunk valley offshore. Valleys of the outer shelf continued to be oriented toward the south. The second Woodfordian ice advance entered western Block Island Sound and inner Rhode Island Sound. The end moraine, known locally as the Harbor Hill (Long Island), Charlestown and Pt. Judith (Rhode Island), and the Buzzards Bay (southeastern Massachusetts), reached its greatest distance offshore adjacent to Narragansett Bay. Drainage rearrangement was limited to Rhode Island Sound: the valley from Narragansett Bay's East Passage was diverted more toward the south by the moraine near Pt. Judith, but continued to follow the deep channel remanent at East Ground. The Sakonnet River southeastern Massachusetts drainage was turned toward the southwest by the Buzzards Bay moraine. The trunk valley for this flow was directed toward the East Ground channel, passed through the moraine south of Sakonnet River, or followed both of these routes at different times. Also some drainage from Buzzards Bay was diverted to the southwest but the younger end moraine was breached southwest of the Elizabeth Islands. Vineyard Sound valleys followed the outer trend of the moraine but joined the trunk valley beyond the water gap. Seaward of the younger end moraine, the drainage apparently followed the older drainage patterns of Rhode Island and Block Island Sounds and those of the middle and outer shelf with Block Channel carrying most of the flow to Block Delta and Block Canyon. On Block Delta a number of distributary channels are present but no uninterrupted valleys are apparent from the middle shelf into the double-headed canyon. CONCLUSIONS (1) During post-Jurassic time the dominant trend of buried and drowned drainage systems on the continental shelf off southern New England has been toward the south. Locally on the inner shelf segments of the Pleistocene pre-Woodfordian glaciated channel system that reflects late Tertiary-eariy Pleistocene fluvial courses and late Pleistocene drainage networks diverge from this trend. The southeastward direction of major Coastal Plain late Tertiary-early Pleistocene valleys across Block Island Sound appears to have

DROWNED AND BURIED VALLEYS

267

been caused by the formidable E - W Coastal Plain escarpment along the northern edge of Long Island and associated well-developed E - W secondary cuestas between Long Island and Block Island. In eastern Rhode Island Sound a late Tertiary-early Pleistocene trunk channel, trending southeastward, may have extended to a Tertiary trough south of Martha's Vineyard. Also in Rhode Island Sound and its approaches Pleistocene Woodfordian end moraines caused some drainage to take a southwestward direction. (2) Valley ground positions tend to be superposed from unconformity to unconformity. Apparently preferred drainage avenues from the mainland to the middle shelf persisted over time. These routes extended southward from Block Island Sound, Narragansett Bay and the Sakonnet River, and southeastern Massachusetts. (3) In general, valley'height/width ratios less than 1/62 are due to stream action under stable tectonic conditions, whereas those greater than 1/62 are the consequence of ice modification of pre-existing river valleys. (4) Views regarding the upbuilding and outbuilding development of the New England shelf and processes responsible for the evolution have been recently presented (Knott and Hoskins, 1968, p.34; Garrison, 1970, p.122). Although net intermittent downwarping has been recognized, this study has disclosed the role of frequent post-Cretaceous subaerial erosion on the outer shelf which has resulted from recurring local to regional upwarps or eustatic regressions. These fluvial erosional intervals not only produced shelf unconformities but also provided detritus that accumulated as shelf edge deltaic deposits thus accelerating outgrowth and creating further regressions. (5) Block Canyon and its ancestor have served to funnel sediment to the deep sea since Eocene time. ACKNOWLEDGEMENTS Support by the Office of Naval Research under contract N00014-68-A-0215-003 is gratefully acknowledged. Special thanks are due A. B. Buddington for his field assistance, Mrs. H. McClennen for preparing the illustrations, and Capt. B. Collinson and crew of R/V "Trident". Sincere appreciation is expressed to Dr. L. E. Garrison, U.S. Geological Survey, and Dr. E. Uchupi, Woods Hole Oceanographic Institution, for critical reading of the manuscript. REFERENCES Dana, J. D., 1890. Long Island Sound in the Quaternary era, with observations on the submarine Hudson River channel. ArrL Sc£ 3d Ser., 40: 425-437. Flint, R. F., 1947. Glacial Geology and the Pleistocene Epoch. John Wiley, New York, N.Y., 589 pp. Flint, R. F., 1963. Altitude, lithology, and Fall Zone in Connecticut. Z Geol., 71: 683-697. Frankel, L. and Thomas, H. F., 1966. Evidence of freshwater lake deposits in Block Island Sound. J. Geol., 74: 240-242. Garrison, L. E., 1970. Development of continental shelf south of New England. Bull. Am. Assoc. Pet. Geol., 54: 109-124.

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Garrison, L. E. and McMaster, R. L., 1966. Sediments and geomorphology of the continental shelf off southern New England. Mar. Geol., 4: 2 7 3 - 2 8 9 . Grim, M. S., Drake, C. L. and Heirtzler, J. R., 1970. Sub-bottom study of Long Island Sound. Geol. Soc. Am. Bull., 81: 6 4 9 - 6 6 5 . Johnson, D. W., 1931. Stream Sculpture on the Atlantic Slope. Columbia Univ., New York, N.Y., 142 pp. Kaye, C. A., 1964, Ulinoian and early Wisconsin moraines of Martha's Vineyard, Massachusetts. U.S. Geol Surv., Prof. Pap.. 50142: C140-C143. KeUing, G. and Stanley, D. J., 1970. Morphology and structure of Wilmington and Baltimore submarine canyons, eastern United States. J. Geol., 78: 6 3 7 - 6 6 0 . Knott, S. T. and Hoskins, H,, 1968. Evidence of Pleistocene events in the structure of the continental shelf off northeastern United States. Mar. Geol., 6: 5 - 4 3 . Lougee, R. J., 1953. A chronology of postglacial time in eastern North America. Sci. Mon., 76: 259-276. Maher, J. C., 1971. Geologic framework and petroleum potential of the Atlantic Coastal Plain and Continental Shelf. U.S. Geol. Surv., Prof. Pap., 6 5 9 : 9 8 pp. McMaster, R. L., 1971. A transverse fault on the continental shelf off Rhode Island. Geol. Soc. Am. Bull., 82: 2001-2004. McMaster, R. L. and Ashraf, A., 1973a. Subbottom basement drainage system of inner continental shelf off southern New England. Geol. Soc. Am. Bull., 8 4 : 1 8 7 - 1 9 0 . McMaster, R. L. and Ashraf, A., 1973b. Extent and formation of deeply buried channels on the continental shelf off southern New England. J. Geol., 81 : 3 7 4 - 3 7 9 . McMaster, R. L., Lachance, T. P. and Garrison, L. E., 1968. Seismic reflection studies in Block Island and Rhode Island Sounds. Bull. Am. Assoc. Pet. Geol., 52: 4 6 5 - 4 7 4 . Newman, W. S. and Fairbridge, R. W., 1960. Glacial lakes in Long Island Sound (Abstract). Geol. Soc. Am. BulL, 71: 1936. Ownes, J. P. and Sohl, N. F., 1969. Shelf and deltaic paleoenvironments in th e Cretaceous-Tertiary formations of the New Jersey Coastal Plain. In: S. Subitzky (Editor), Geology of Selected Areas in New Jersey and Eastern Pennsylvania and Guide Book o f Excursions. (Prepared for Geol. Soc. Am.) Rutgers Univ. Press, New Brunswick, N.J., 382 pp. Schafer, J. P., 1961. Correlation of end moraines in southern Rhode Island. US. Geol. Surv., Prof. Pap., 424-D: D68-D70. Schafer, J. P. and Hartshorn, J. H., 1965. The Quaternary of New England. In: H. E. Wright Jr. and D. G. Frey (Editors), The Quaternary of the United States. Princeton Univ. Press, Princeton, N.J., 922 pp. Sharp, H. S., 1929. The physical history of the Connecticut shoreline. Conn. State Geol. Nat. Hist. Surv. Bull,, 4 6 : 9 7 pp. Suter, R., De Laguna, W. and Peflmutte, N. M., 1949. Mapping of geological formations and aquifers of Long Island, New York. N. Y. Dept. Conserv. Bull., GW-18:212 pp. Thom, Jr., W. T., 1946. Post-Triassic evolution of a part of the Atlantic coastal region (Abstract). Bull. Am. Assoc. Pet. Geol,, 30: 742. Upson, J. E. and Spencer, C. W., 1964. Bedrock valleys of the New England Coast as related to fluctuations of sea level. U.S. Geol. Surv.~ Prof. Pap., 454-M: 44 pp. Veatch, A. C. and Smith, P. A., 1939. Atlantic submarine valleys of the United States and the Congo submarine valley. Geol. Soc. Arrt, Spec. Pap., 7 : 1 0 1 pp. Woodworth, J. B. and Wigglesworth, E., 1934. Geography and geology of the region including Cape Cod, the Elizabeth Islands, Nantucket, Martha's Vineyard, No Mans Land, and Block Island. Harv. Coll. Mus. Comp. Zool., Mere., 5 2 : 3 3 8 pp.