Boston Basin Research Articles

Regional-scale site characterization mapping in high impedance environments using soil fundamental resonance (f0): New England, USA

In this work, we develop regional-scale site characterization maps of New England, a glaciated region in the Eastern United States, using site fundamental frequency (f0). Due to the strong impedance contrast that creates strongly resonant site response behavior in New England, f0 is the preferred site response proxy for the region and best characterizes spatial variability in soil amplification. We first develop a database of 1577 f0 values collected from the literature (1313) and picked from HVSR curves collected during an additional field campaign (487). Using the surficial geologic units from the conterminous US surficial geology map, we compute distributions of f0 for each of the surficial geologic units mapped in New England. We find that the thick glaciofluvial ice-contact sediments and thick proglacial sediments of Cape Cod and Long Island are characterized by f0 distributions with the lowest medians (1.06 and 1.03 Hz respectively) and narrowest interquartile ranges (0.23 and 0.28 Hz respectively) in New England, which we interpret as being the thickest sediments in the region. The f0 distribution of the thin, proglacial sediments, mostly fine-grained in the Boston Basin, the coast of Lake Champlain and the Maine coast has a relatively low median (2.70 Hz), however it has high variability (3.65 Hz interquartile range) since the sediment thickness varies widely in this geologic unit. The f0 distribution of the thin alluvial sediments of the Connecticut River Valley also has a low f0 median (1.83 Hz) however, it has less variability than the proglacial sediments, mostly fine-grained thin (1.59 Hz). We present maps of the median and interquartile range of the f0 distributions and their corresponding Vs30 distribution approximations. Vs30 distributions are developed for each of the surficial geologic units by assuming a layer-over-halfspace model for site response and establishing estimates of sediment average shear-wave velocity (Vsavg) for each of the units based on a limited number of shear wave velocity profiles from the region. We compare our results against two existing site characterization maps for the region, Wald and Allen (2007) and Becker et al. (2011) and find that our updated maps result in median Vs30 values that tend to be higher than Becker et al. (2011) except in geologies with consistently deep profiles, and lower than Wald and Allen (2007) median values, with the exception of coastal zone sediments.

Avalonian arc-to-platform transition in southeastern New England: U-Pb geochronology and stratigraphy of Ediacaran Cambridge “argillite,” Boston Basin, Massachusetts, USA

High-precision CA-TIMS 206Pb/238U zircon dates clarify the age and tectonic significance of the Cambridge Formation which is poorly exposed in the Boston Basin, eastern Massachusetts, but transected by ∼50 km of tunnels beneath the mainland and Boston Harbor. The youngest detrital zircon in a sample from the northern Braintree Weymouth Tunnel establishes a maximum depositional age of 584.09 ± 1.98 Ma, consistent with sources in sills of that age in underlying Roxbury Conglomerate. A 551.22 ± 0.20 Ma ash bed from the Mystic Quarry in Somerville, Massachusetts lies near the top of an approximately 5350 m thick, dominantly argillaceous section measured in subsurface cross sections. These were constructed from attitudes reported in pre-1960 tunnels and from mapping logs obtained from tunnels completed decades later during the federally ordered clean-up of Boston Harbor. A 488.58 ± 0.16 Ma aplite sill intruding argillite ∼800 m above the ash bed sets the minimum depositional age on the north side of the Basin. A tighter constraint comes from trilobite-bearing strata of the lower Cambrian Weymouth Formation located south of Boston that overlies the Cambridge Formation without obvious break in the Braintree Weymouth Tunnel. If Cambridge deposition was continuous after 584 Ma, the depositional interval would exceed 40 million years. An estimated ∼20 Ma depositional hiatus seems more likely because the base of the Cambridge Formation appears to define a regional unconformity above which argillite rests variously on magmatic arc-related units of both the 595 to 584 Ma Roxbury Conglomerate and the 597 to 593 Ma Lynn-Mattapan Volcanic Complex. Cambridge deposition set in once arc activity in more northerly “West” Avalonian terranes extending through Atlantic Canada to the Avalon Peninsula, Newfoundland had given way to wrench faulting and bimodal magmatism. This regime is manifested structurally in Boston-area tunnels by later-reactivated normal faults in which hanging wall blocks of Cambridge argillite were originally downthrown relative to older footwall units. Pyroclastic volcanic textures and thin basaltic flows with soft sediment contacts are present in argillite of the City Tunnel Extension, and whole rock major element and REE compositions reveal mixed terrigenous and volcanic components deposited under marine conditions throughout the Basin. Proposed sources for the latter are voluminous eruptions recorded in the 560 to 550 Ma Coldbrook Group in New Brunswick9s Caledonia terrane.

U-Pb zircon geochronology of Roxbury Conglomerate, Boston Basin, Massachusetts: Tectono-stratigraphic implications for Avalonia in and beyond SE New England

High-precision CA-TIMS ²⁰⁶Pb/²³⁸U zircon dates from sandstone and Brighton igneous rocks associated with Roxbury Conglomerate in the Boston Basin, eastern Massachusetts provide constraints on the age and tectonic significance of these deposits. Detrital zircon suites from Roxbury-related sandstones representing, in ascending order, the Franklin Park Member (proposed name), the Brookline Member and the Squantum Member establish closely comparable maximum depositional ages of 595.14 ± 0.90, 598.87 ± 0.71 and 596.39 ± 0.79 Ma, respectively. The youngest of these is the best maximum age estimate of the conglomerate. Brighton dacite near the base of the Brookline Member and amygdaloidal andesite near the top yield respective crystallization ages of 584.19 ± 0.70 Ma and 585.37 ± 0.72 Ma. These virtually identical dates support previous interpretations of these particular units as shallow intrusions and thus represent minimum ages of associated conglomerate. The Roxbury-Brighton sequence is traditionally shown as inter-fingering northward with ≤570 Ma mudstone of the Cambridge “Argillite,” but the age range of the conglomerate makes this impossible. Conglomerate lacking quartzite clasts typical of the Roxbury proper and associated with 593.19 ± 0.73 Ma rhyolite on the south side of the basin is re-assigned to the Lynn-Mattapan Volcanic Complex which rests unconformably on Dedham Granite in basement exposed west and north of the basin as well. Calc-alkaline geochemistry and ages of the 609 to 584 Ma Dedham-Lynn-Mattapan-Brighton assemblage link it with 630 to 580 Ma arc sequences in other northern Appalachian Avalonian terranes. Roxbury Conglomerate accumulated in fault-bounded intra-arc basins near the end of this magmatic cycle.

To date or not to date - Does luminescence dating work in the European Alps and their foreland?

The Squantum ‘Tillite’ (c. 593–570 Ma) consists of thick (up to 215 m) massive and crudely-stratified diamictites conformably interbedded with subaqueously-deposited conglomerates and sandstones within a thick (~ 7 km) Boston Basin fill which is dominated by argillite turbidites. The Squantum Tillite was first interpreted as being glacigenic in origin in 1914 because of the presence of diamictites; argillites were interpreted as glaciolacustrine ‘varves’ with rare ice-rafted debris, and conglomerates as glaciofluvial outwash. More recently these have been shown to be the product of deep marine mass flow processes with no glacial influence, yet because of its age equivalence with the deep marine, glacially-influenced Gaskiers Formation, the Squantum Tillite is still seen by some as supporting evidence for a widespread ‘Snowball Earth’ event at c. 580 Ma. New sedimentological work confirms that conglomerate and sandstone facies are deep marine sediment gravity flows genetically related to massive (homogeneous) and crudely-stratified (heterogeneous) diamictites produced subaqueously by downslope mixing of gravel and cobbles with muddy facies. Rare horizons of ‘ice rafted debris’ in thin-bedded and laminated turbidite facies interbedded with thick debrites show a weak but positive correlation of lamina thickness with grain size, suggesting these facies are non-glacial co-genetic ‘debrite–turbidite’ couplets. A significant volcanic influence on sedimentation is identified from reworked lapilli tuff beds and reworked ash in turbidites. The depositional setting of the Squantum ‘Tillite’ appears to be that of a submarine slope/fan setting in an open marine volcanic arc basin receiving large volumes of poorly-sorted sediment on the mid-latitude active margin of Gondwana. No direct glacial influence is apparent.

Chapter 43 The Squantum Member of the Boston Basin, Massachusetts, USA

Abstract The Neoproterozoic diamictite-bearing Squantum Member is located in the Boston Basin in eastern Massachusetts, USA. The Boston Basin forms part of the Avalonia island arc terrane ( c. 650 Ma), and appears to have originated as a rift-type basin in an extensional setting along the northern margin of Gondwana, although its exact position is debated. Inferred palaeoenvironmental reconstructions of the Boston Basin have alternated between a fluvial basin where ice played a major role in transporting much of the coarse material and an evolving marine basin dominated by non-glacial subaqueous mass flow, submarine fans and turbidity-current deposition. The age of the Squantum is bracketed between c. 595 and 570 Ma, and is correlated by some to the glaciogenic diamictite succession of the Gaskiers Formation (eastern Newfoundland) as part of the putative global Gaskiers Glaciation c. 582–585 Ma. However, the Squantum Member consists of diamictite, graded sandstone and siltstone units, and fine-grained laminated argillite/mudstone units typical of debris flow and turbidite facies that accumulate in a submarine setting. A glacial influence is not readily identified and revolves around early interpretations of the diamictite as being ‘till-like’, the presence of laminated horizons that resemble glaciolacustrine ‘varvites’ and the disputed recognition of ice-rafted dropstones. There are no associated carbonates and, consequently, no geochemical data are available in connection with the Squantum Member.

Palaeoenvironments and weathering regime of the Neoproterozoic Squantum ‘Tillite’, Boston Basin: no evidence of a snowball Earth

The snowball Earth hypothesis describes episodes of Neoproterozoic global glaciations, when ice sheets reached sea-level, the ocean froze to great depth and biota were decimated, accompanied by a complete shutdown of the hydrological cycle. Recent studies of sedimentary successions and Earth systems modelling, however, have brought the hypothesis under considerable debate. The Squantum ‘Tillite’ (Boston Basin, USA), is one of the best constrained snowball Earth successions with respect to age and palaeogeography, and it is suitable to test the hypothesis for the Gaskiers glaciation. The approach used here was to assess the palaeoenvironmental conditions at the type locality of the Squantum Member through an analysis of sedimentary facies and weathering regime (chemical index of alteration). The stratigraphic succession with a total thickness of ca 330 m documents both glacial and non-glacial depositional environments with a cool-temperate glacial to temperate non-glacial climate weathering regime. The base of the succession is composed of thin diamictites and mudstones that carry evidence of sedimentation from floating glacial ice, interbedded with inner shelf sandstones and mudstones. Thicker diamictites interbedded with thin sandstones mark the onset of gravity flow activity, followed by graded sandstones documenting channellized mass gravity flow events. An upward decrease in terrigenous supply is evident, culminating in deep-water mudstones with a non-glacial chemical weathering signal. Renewed terrigenous supply and iceberg sedimentation is evident at the top of the succession, beyond which exposure is lost. The glacially influenced sedimentary facies at Squantum Head are more consistent with meltwater dominated alpine glaciation or small local ice caps. The chemical index of alteration values of 61 to 75 for the non-volcanic rocks requires significant exposure of land surfaces to allow chemical weathering. Therefore, extreme snowball Earth conditions with a complete shutdown of the hydrological cycle do not seem to apply to the Gaskiers glaciation.

Natural and anthropogenic arsenic in soil in the Boston Basin

AbstractNaturally occurring and anthropogenic arsenic concentrations in soils, in conjunction with the current knowledge of its potential risk to human health, demonstrate a critical environmental and public health issue. These widespread arsenic concentrations potentially affect large human populations. There are locations worldwide where naturally occurring arsenic is considered to be and documented as a potential human health risk through the use of established risk assessment calculation approaches or as demonstrated by clearly measurable health or agricultural effects. This natural arsenic, exposed due to human activity and coupled with anthropogenic contributions, may be the number‐one soil contact environmental health issue in the world. Considering the relatively high natural arsenic concentrations in some New England locations, and with the advantage of access to a large soil arsenic database, this issue has been addressed as described herein for the Boston Basin and related areas. The article provides a summary of some large natural and anthropogenic arsenic data sets with comparison between them and a derivation of the natural and anthropogenic components in the Boston Basin. © 2010 Wiley Periodicals, Inc.

Plans d’eau glaciaires et isostasie dans les bassins de la haute Ware et de la rivière Otter, Massachusetts (2e partie)

The present study is one of the unpublished research projects which are known to have been conducted in New England and in Eastern Canada under the guidance of the late Richard J. LOUGEE, long-time professor of Geomorphology at Clark University. Over a number of years, this writer has worked in close relationship with Lougee and much evidence in the field was studied together. It is believed that here has been recorded a most detailed work of surveying, and this undoubtedly accounts for the somewhat exceptional results that will be presented. The area selected for this study (about 80% of it is woodland) is located in the highlands of Central Massachusetts in Worcester County, about twenty miles (32 kilo-meters) northwest of the city of Worcester. It consists of the valley of the Otter River draining north, and of a small portion of the East Branch of the Ware River draining south. Since completion of this study, parts of the low area which held the ancient glacial lakes have been flooded to become water reservoirs. That proglacial lakes, though temporary they may have been, once submerged much of the area under study, is shown by an abundance of deltas, kames, eskers and deltaic kames terraces. It is believed that all of these were built under water in such lakes. Other features, such as kettle-holes and glacial outlets, especially ice-marginal channels cut diagonally down the slope, have also been studied. By plotting on a profile of the most characteristic elevations (often carefully surveyed), it is possible to find the water planes of ancient proglacial lakes. To this must be added experiments conducted in a sedimentation tank as also measurements of both the imbrication of cobbles in eskers and the « smoothness indexes » of such stones and pebbles, using A. Cailleux' methods. Thus were obtained results which tend to show that : 1- the area under study probably was in a deep interlobate space created between the Connecticut Valley lobe to the west and the Boston Basin lobe to the east ; 2— ice-marginal channels are an indication of the existence of a thick, fast-retreating ice border ; 3- an isostatic balance restored itself by sometimes quick and strong adjustments of the crust of the earth ; 4— an early upwarping, made up of various zones of tilting articulated on hinge lines, has been referred to as Hubbard Uplift and is the earliest known in the post-Glacial history of New England.

Age of the Squantum "tillite," Boston Basin, Massachusetts; U-Pb zircon constraints on terminal Neoproterozoic glaciation

Age of the Squantum "tillite," Boston Basin, Massachusetts; U-Pb zircon constraints on terminal Neoproterozoic glaciation

Determining the source of felsitic lithic material in southeastern New England using neodymium isotope ratios

We introduce the use of Nd isotopes as a high-precision technique that can be used in conjunction with visual, petrographic, and trace element analyses to source indistinct felsitic debitage. This approach allows sourcing of debitage from southeastern New England, including samples that previously could not be constrained to one source. Combined with trace element data, isotopic data also provide the possibility of sourcing debitage to a particular quarry site within a volcanic complex, and even to a particular ash flow within a quarry site. Determining the origin of debitage so precisely is important for understanding acquisition, trade, and exchange networks in southeastern New England, where distances separating quarry sites within an individual volcanic complex (Lynn–Mattapan) are greater than distances between different volcanic complexes (Lynn–Mattapan, Blue Hill, and Wamsutta). The sourcing of debitage to Mattapan quarries suggests that Middle Archaic populations in the northwest part of the Boston Basin obtained nonlocal lithic material primarily from sources south of the basin. © 2000 John Wiley & Sons, Inc.

Determining the source of felsitic lithic material in southeastern New England using neodymium isotope ratios

We introduce the use of Nd isotopes as a high-precision technique that can be used in conjunction with visual, petrographic, and trace element analyses to source indistinct felsitic debitage. This approach allows sourcing of debitage from southeastern New England, including samples that previously could not be constrained to one source. Combined with trace element data, isotopic data also provide the possibility of sourcing debitage to a particular quarry site within a volcanic complex, and even to a particular ash flow within a quarry site. Determining the origin of debitage so precisely is important for understanding acquisition, trade, and exchange networks in southeastern New England, where distances separating quarry sites within an individual volcanic complex (Lynn–Mattapan) are greater than distances between different volcanic complexes (Lynn–Mattapan, Blue Hill, and Wamsutta). The sourcing of debitage to Mattapan quarries suggests that Middle Archaic populations in the northwest part of the Boston Basin obtained nonlocal lithic material primarily from sources south of the basin. © 2000 John Wiley & Sons, Inc.

Application of petrographic and geochemical methods to sourcing felsitic archaeological materials in Southeastern New England

At least four volcanic complexes of different age and petrologic character occur in southeastern New England. Each complex contains a variety of fine-grained felsitic rocks, and three of these are known to have been quarried by prehistoric people for material used in the production of stone tools. These volcanics include a Late Proterozoic calcalkaline suite (Lynn-Mattapan) and several alkaline suites of Ordovician to Carboniferous age (Blue Hill, Spencer Hill, and Wamsutta suites). Each suite exhibits unique petrographic and geochemical features that help to constrain sources of felsitic archaeological materials. Distinctive petrographic features are: (1) Lynn/Mattapan: mostly pyroclastic rocks that typically contain broken crystals, volcanic clasts, and relict pumice and glass shards; phenocrysts of plagioclase, quartz, and perthite, commonly in glomeroporphyritic clusters; accessory sphene and prominent late-stage epidote; some varieties exhibit distinct flow-banding. (2) Blue Hill and Spencer Hill rocks: mainly lava flows; abundant perthite and quartz phenocrysts, with minor or no plagioclase; accessory minerals may include fluorite, aegerine, riebeckite, zircon, and allanite. (3) Wamsutta rhyolite: phenocrysts solely of anorthoclase; quartz restricted to late-stage filling of vesicles, and to planar, subparallel fractures in-filled as lithophysae; lava flows with devitrified glass matrix. Major element geochemistry is of limited use in distinguishing the volcanic groups, but trace element signatures are distinct and provide excellent criteria to discriminate rocks from each suite. Compared to the Lynn/Mattapan suite, the Blue Hill and Spencer Hill rocks exhibit higher concentrations of Rb, Y, Zr, Nb, La, Ce and Zn, and lower concentrations of Sr and Ba. Wamsutta rhyolite is intermediate in composition, but distinct. Examination of material from six prehistoric quarries, and debitage collected at seven archaeological sites, demonstrates that most samples can be assigned to one of the above volcanic suites based on combined petrographic and geochemical attributes. These geologic attributes add a significant element of quantification to archaeological sourcing problems that lead to improved identification of materials compared to hand sample characterization alone. The volcanics from source areas proximal to the Boston basin were important to Early and Middle Archaic period populations across most of southeastern Massachusetts. During the Late/Terminal Archaic period, these materials were being transported extensively throughout eastern and southeastern Massachusetts, the Narragansett Bay area, and parts of Rhode Island. Thus, routinely applied geological methods can provide useful approaches to constrain the sources of felsites from southeastern New England found in archaeological contexts. © 1997 John Wiley & Sons, Inc.

Submerged and eroded drumlins off northeastern Massachusetts

Streamlined, oval-shaped, oriented topographic highs in Massachusetts Bay are identified as the erosional remnants of drumlins. The topographic highs correlate with outlines of lag gravel deposits on the sea floor and both the highs and lag gravel seafloor footprint have a distinct east-southeast long axis trend. This trend is similar to the preferred orientation of the long axes of drumlins in the Boston Basin and indicates the flow direction of the late Wisconsinan Laurentide Ice Sheet. Modification of the drumlins occurred during two passages of the shoreline, the first during the late Wisconsinan regression when the drumlins were only slightly eroded. The second passage of the shoreline occurred during the marine transgression, when erosion, in the form of cliff-face retreat, removed the upper part of the drumlins.

Paleozoic paleogeography of North America, Gondwana, and intervening displaced terranes: Comparisons of paleomagnetism with paleoclimatology and biogeographical patterns

New paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the Silurian and Devonian. In this paper, the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data. The data from the three fields of paleomagnetism, paleoclimatology, and biogeography are generally in excellent agreement, and an internally consistent paleogeographic evolutionary picture of the interactions between North America, Gondwana, and intervening displaced terranes is emerging.During the interval of the Ordovician, Silurian, and Devonian, North America stayed in equatorial paleoposition, while rotating counter-clockwise. The northwest African part of Gondwana was in high southerly latitudes during the Late Ordovician and was fringed by peri-Gondwanide terranes, such as southern Europe (Armorica) and Avalonian basement blocks now found in eastern Newfoundland, Nova Scotia, the Boston Basin, the Appalachian Piedmont, and northern Florida. Subsequently, Gondwana and the peri-Gondwanide terranes displayed rapid drift with respect to the pole. This drift translates into the following pattern of movement for northwest Africa. During the latest Ordovician-Early Silurian, this area moved rapidly northward from polar to subtropical latitudes, followed by equally rapid southward motion from subtropical to intermediate (about 50°S) paleolatitudes during the Late Silurian-Middle Devonian. It is likely that significant east-to-west motion accompanied the latter shift in paleolatitudes, with the Caledonian-Acadian orogeny the result of Silurian to Early Devonian convergence and collision between Gondwana and North America. This collision sandwiched several of the intervening displaced terranes between Gondwana and North America. Subsequent to this collision, Gondwana was separated in the Late Devonian by a medium-width ocean from North America and the Avalonian and southern European blocks which were left behind adjacent to North America. This new ocean closed during the Carboniferous, and the resulting convergence and collision were the cause of the Hercynian-Alleghanian orogenic belt. Problems remaining for future research, besides the further gathering of reliable paleopoles, involve the uncertain pre-Devonian position of the southern British Isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid Late Ordovician-Silurian apparent polar wander for Gondwana.

Eocambrian paleomagnetism of the Boston Basin: Evidence for displaced terrane

A paleomagnetic study of the Late Precambrian Roxbury formation, consisting of extrusives and clastic sediments, has yielded a steeply downward directed and pre‐folding characteristic magnetization (D/I = 219°/71°, α95 = 5°, k = 311, paleopole at 13°N, 267°E, N = 4 sites), which indicates a significantly higher paleolatitude (55°) than would be expected if the Boston basin were part of the equatorial North American craton in the latest Precambrian and earliest Paleozoic. This characteristic magnetization reveals dual polarities and is further supported by a positive conglomerate test. A ubiquitous post‐folding late Paleozoic overprint is present in nine sites (D/I = 183°/14°, α95 = 8°, k = 42), with a paleopole at 41°S, 285°E. The pre‐folding magnetization resides in hematite, which is inferred to have formed during early oxidation of the rocks; the high stability of this hematite may have prevented its magnetization from being reset during the late Paleozoic chemical event responsible for the magnetic overprint. The Boston basin has a marked geological similarity to the Avalon basement terranes in Nova Scotia and Newfoundland, as well as the Armorican Massif in France, and the high paleolatitudes observed for all these terranes suggest a common paleogeographical affinity; a likely paleolocation is near the northwestern margin of Gondwana which was located at the southpole in the latest Precambrian and Early Cambrian.

Paleomagnetism and magnetic mineralogy of the Nahant gabbro and tonalite, eastern Massachusetts

Paleomagnetic analyses of the Nahant gabbro and an associated tonalite of eastern Massachusetts reveal five separate populations of paleomagnetic directions. The gabbro and tonalite, exposed in the northernmost portion of the Boston Basin, were selected for this study because of the lack of significant macroscopic evidence of penetrative metamorphism or deformation. The tonalite has not been radiometrically dated; however, an Ordovician date has been reported for the gabbro. Detailed petrographic data indicate that the five populations of paleomagnetic directions obtained for the gabbro and tonalite correspond to five styles of magnetic mineral alteration.A mean alteration index was calculated for each site in the Nahant gabbro and tonalite. There is an apparent relationship between magnetite alteration and normalized intensity at a given demagnetization level, with normalized intensity decreasing as magnetite alteration increases. This relationship illustrates the interdependence between paleomagnetic properties and magnetic minerals. Alteration of these minerals is probably associated with periods of magnetic overprinting.Autometasomatism is believed to have occurred in the gabbro. Fluid flushing initiated plagioclase decomposition, liberating Ca + Al + Si to form Al-rich (4–10 wt.% Al2O3) titanite (sphene, CaTiSiO5). Whether the metasomatism was deuteric or the result of postdeuteric hydrothermal activity has not been determined.The Nahant gabbro poles are interpreted as representing either a deuteric or a metasomatic chemical remanent magnetization (CRM). The tonalite pole is interpreted as a deuteric CRM. The gabbro and tonalite poles are similar to a number of poles from Ordovician to Devonian intrusives in the Acadia terrane, including the St. Stephen and St. George plutons.Petrographic examination of polished sections from the Devonian St. Stephen and St. George plutons reveals evidence for alteration of primary magnetic mineral assemblages. Similarities exist between these alterations and alterations observed in the Nahant suite, most notably, the formation of titanite from Fe–Ti oxides. The repeated determination of similar poles on intrusive rocks in the Acadia terrane suggests there may be some tectonic or geomagnetic significance to poles from intrusives; however, without more data pertaining to the timing of metasomatic events observed in the Nahant suite and the St. Stephen and St. George plutons, the age and significance of these poles cannot be interpreted with certainty.

Precambrian Age of the Boston Basin: New Evidence from Microfossils

A Vendian (Late Proterozoic Z) age has been determined for the Boston Basin by comparison of a microflora from the Cambridge Argillite with other late Precambrian assemblages. The microfossils, which include Bavlinella cf. faveolata, are preserved as petrifactions in pyrite. This age designation for the sedimentary rocks of the Boston Basin should allow for the reinterpretation of the structure of the basin and its regional correlations.

Ordovician cauldron subsidence of the Blue Hills Complex, eastern Massachusetts

The Blue Hills Complex, which lies south of the Boston Basin in eastern Massachusetts, is composed of Cambrian sedimentary rocks and Late Ordovician alkalic plutonic and volcanic rocks. On the north, the Complex is separated by a fault from the Boston Bay Group, which very recently has been shown to be late Precambrian. On the south, the Complex is overlain unconformably by the Pennsyvanian rocks of the Norfolk Basin. The bedding and volcanic flow structures of the Cambrian, Ordovician, and Pennsylvanian rocks are essentially parallel and dip steeply south. I believe that the Blue Hills Complex is a Late Ordovician cauldron subsidence that was tilted during the Alleghenian Orogeny and was thrust northward over the Boston Bay Group.

Geologic profile across Southeastern New England

This progress report describes major geological features of three segments of a profile across southern New England: (1) the Clinton—Boston, (2) the Boston—Narragansett Basins, and (3) the Narragansett Bay cross sections. Segment (1) is a traverse across four tectonic blocks from the Merrimack synclinorium across the Clinton—Newbury and Bloody Bluff fault zones and across the southwestern tip of the Boston Basin. The Clinton—Newbury and Bloody Bluff fault zones together may define the suture between the North American plate and the Avalonian terrain of the Eur-African plate. The Avalonian plate may have been overridden by the North American or vice versa, these two faults in any case mark the northwestern and southeastern boundaries of crustal underplating. These tectonic features probably formed during the Acadian, although their present geometry was defined during the Alleghanian orogeny. The Boston and Narragansett basins, segments 2 and 3 are within the Avalonian platform. The age of the cover rocks of the former is uncertain but arguments for a Late Precambrian—Cambrian age are presented; those of the Narragansett Basin are fossiliferous Pennsylvanian. The northwestern part of the Avalonian platform is characterized by SE-directed, NW-dipping, thrusts and highangle reverse faults; the southeastern part of the platform, on the other hand, is characterized by NW-directed, SE-dipping faults. In the Narragansett Basin the Alleghanian (Variscan) orogeny is a major orogenic event consisting of the following elements: (1) isoclinal folding and refolding associated with thrusting; (2) upper amphibolite facies Barrovian regional metamorphism; and (3) intrusion of probably anatectic granites.

Areal Parameters of the Sea Breeze and Its Vertical Structure in the Boston Basin

Abstract The sea breeze is a mesoscale wind whose frequency of occurrence is 40–50 times annually in the Boston Basin. Boston's sea breeze is among the best developed of all midlatitude sites studied. The complexity of site conditions and an urban concentration alter the characteristics of the sea breeze with inland penetration. Vertical temperature and dew point data have provided the first detailed look at the changes effected in Boston's atmosphere by this local wind. The data have also revealed that Boston's sea breeze is not always a moist flow of marine air but that the moisture content of the air is directly related to the regional wind speed and direction prior to onset of the sea breeze. Transformations of the vertical characteristics of the atmosphere suggest subtle but serious geographic and environmental variations in the spatial distribution of atmospheric contaminants.