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Relative sea-level rise across the Eastern Border fault (Branford, Connecticut): evidence against seismotectonic movements
Marine Geology 184 (2002) 61^68 www.elsevier.com/locate/margeo
Relative sea-level rise across the Eastern Border fault (Branford, Connecticut): evidence against seismotectonic movements O. van de Plassche a; , K. van der Borg b , A.F.M. de Jong b a b
Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands R.J. Van de Graa¡ Laboratory, Universiteit Utrecht, P.O. Box 80000, 3508 TA Utrecht, The Netherlands Received 16 March 2001; accepted 18 September 2001
Abstract Basal peat age^depth data from four salt-marsh sites located 500^1500 m south of the Eastern Border fault (EBF) near Branford, Connecticut, document an error envelope for the lower limit of relative mean high water (MHW) rise during the past 3300 yr. The long-term rate of relative lower-limit-MHW rise during the past 3000 yr (1.1 mm yr31 ) corresponds with late Holocene rates of relative MHW rise found elsewhere in coastal Connecticut (0.9^1.2 mm yr31 ). This result invalidates repeated claims that multiple seismotectonic lowering of a crustal block towards the south has occurred along a segment of the EBF near Branford shortly after 815 cal yr BC and during the past 1200 yr and calls for a non-tectonic interpretation of biostratigraphic evidence of local short-term sea-level variations. 6 2002 Elsevier Science B.V. All rights reserved. Keywords: salt marsh; basal peat; sea-level variations
1. Introduction Re£ecting the di¡erence in tectonic setting of salt marshes on the Paci¢c and Atlantic coasts of North America, records of century-scale sealevel variations from west coast marshes have been analyzed primarily in terms of seismotectonic events (e.g. Atwater et al., 1995; Shennan et al., 1996), while those from the east coast have been
* Corresponding author. Tel.: +31-20-444-7380; Fax: +31-20-646-2457. E-mail address: [email protected] (O. van de Plassche).
correlated with climate-proxy records (e.g. Varekamp et al., 1992; van de Plassche et al., 1998a; Gehrels, 1999). Whereas the database for west coast marshes (mainly in the states of Oregon and Washington) is large enough to detect coherent regional space^time patterns of relative sealevel (RSL) change, that for the east coast (mainly Connecticut) is still very small and published records of century-scale sea-level variations di¡er more than they agree (van de Plassche, 2000). Theoretically, these di¡erences may be attributed to methodological errors or inaccuracies, variations in local tidal range, compaction e¡ects, and/or di¡erences in crustal movement. Here, we focus on the latter possibility following recent
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claims of seismotectonic events in coastal Connecticut during the late Holocene. Sea-level studies in the Connecticut River estuary and in the Hammock River and East River marshes, Connecticut, have found average rates of RSL rise close to 1 mm yr31 during the past 1800 yr or more (Bloom and Stuiver, 1963; Patton and Horne, 1991; van de Plassche, 1991; Varekamp et al., 1992; Nydick et al., 1995; van de Plassche et al., 1998a,b) (Fig. 1). On the basis of a single basal peat date (3670 H 140 yr BP, 4 m below marsh surface), Orson et al. (1987) also found a rate of ca. 1.1 mm yr31 for RSL rise in Pattagansett River marsh, located ca. 14 km east of the mouth of the Connecticut River. Thompson et al. (1997, 1999; unpublished data) and Varekamp et al. (1999; unpublished data) report a comparable mean rate of RSL rise of ca. 1.2 mm yr31 since ca. 800 cal yr AD for a site in the eastern lobe of the Farm River marsh, but, remarkably, double that rate (ca. 2.5 mm yr31 ) for a site in Kelsey Island marsh, located 1.9 km to the south of their Farm River marsh study site (Fig. 1). They suggest that the di¡erence between the two RSL records can be attributed to repeated neotectonic activity along a segment of the (reversely reactivated) Eastern Border fault (EBF) (Rodgers, 1985), which runs just south of their Farm River marsh study site (Fig. 1). This interpretation is based on stratigraphic and radiocarbon data, which suggest a seismic event with an o¡set of ca. 1 m to the south shortly after 815 cal yr BC and a step-wise lowering, totalling 1.45 m, of the Kelsey Island marsh study site during the past 1200 yr. Thompson et al. (2000; unpublished data) claim that new evidence from a site in Farm River marsh, immediately to the south of the EBF, con¢rms that intermittent downthrow has occurred here during the past 1200 yr. With regard to the implications of their seismotectonic interpretation of the di¡erences in RSL data from Farm River marsh and Kelsey Island marsh, Thompson et al. (1999) point out that ‘A similar mechanism may be responsible for smaller relative sea level rise variations seen in other core locations’ (in Connecticut marshes nearby). The objective of this study is to verify the repeated claim that multiple seismotectonic down-
throw has occurred south of the EBF near Branford. It is motivated primarily by the fact that such fault activity, if unequivocally demonstrated, can have signi¢cant consequences for the interpretation of century-scale sea-level £uctuations, that have been and will be reconstructed for these and other Connecticut marshes, in terms of climate^ocean variability (Varekamp et al., 1992; Nydick et al., 1995; van de Plassche et al., 1998a; van de Plassche, 2000). The study is also relevant with regard to risk assessment of future earthquakes in the study area and surroundings, where seismicity is not uncommon and has occurred as recently as February 2001.
2. Salt-marsh vegetation New England salt marshes generally have three main vegetation zones, called low, high, and upper marsh (e.g. Red¢eld, 1972). The low marsh zone develops when mud- or sand£ats in the intertidal zone reach a level high enough for colonization by the cord grass Spartina alterni£ora (tall). When the low marsh zone reaches (to within 10 cm of) local mean high water (MHW), high marsh grasses such as Spartina patens and Distichlis spicata begin to replace the tall cord grass (e.g. Gordon, 1980; Lefor et al., 1987). The high marsh zone can reach as high as the highest spring tide level, but where su⁄cient fresh water enters the marsh, upper marsh plants (e.g. Phragmites australis, Scirpus robustus) can replace the high marsh vegetation down to about MHW spring level, or lower with increasing fresh water input. Relevant for this study is our observation in several Connecticut marshes that D. spicata can occur, exceptionally, as high as ca. 40 cm above the low marsh^high marsh boundary (i.e. ca. 30^40 cm above local MHW). On Kelsey Island, where MHW is estimated at ca. +1 m NGVD29 (National Geodetic Vertical Datum of 1929), the highest observed occurrence of D. spicata is at +1.35 m NGVD.
3. Approach and methods According to Thompson et al. (1999, 2000), the
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Fig. 1. Location of study area southwest of Branford (B) and rates of long-term RSL rise for previously studied sites in central coastal Connecticut.
EBF trace across eastern Farm River marsh marks the boundary between tidal sediments and low marsh peat to the south and high marsh peat deposits to the north. They interpret this apparent spatial correlation between lithofacies distribution
and fault line position as a possible expression of tectonic activity. The distribution of tidal sediments, and low and high marsh peat is, however, also dependent on the (former) position and activity of tidal creeks. We mapped the marsh area
Fig. 2. Sub-surface distribution of marine clay in the marsh stratigraphy in eastern Farm River marsh north of the EBF. The position of this fault is taken from the Bedrock Geological Map of Connecticut (Rodgers, 1985).
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Fig. 3. Location of basal peat sampling sites in the study area. FR: Farm River marsh, DB: Double Beach marsh, M: Momauguin marsh, KI: Kelsey Island marsh (key as in Fig. 2).
north of the approximate fault-trace position and found extensive sub-surface distribution of tidal sediments and low marsh peat there too (Fig. 2). This mapping result does not, however, exclude the possibility of multiple tectonic downthrow to the south along a segment of the EBF. If a crustal block on the south side of this fault in the Branford area has been repeatedly lowered seismotectonically during the past 1200 yr or more, one expects to ¢nd the following structural, morphological, stratigraphic, and age^depth evidence: (1) slippage phenomena in outcrops of hard rock; (2) a step(-like) feature in the soft rock substrate, traceable in the stratigraphy of the overlying salt-marsh deposits present prior to the onset of neotectonic activity ; (3) a repetition of (sudden) transgressive^re-
gressive facies changes in marsh deposits south of the fault trace, and (4) provided compaction is excluded, older MHW data from sites south of the EBF plot increasingly deeper below contemporaneous MHW data from sites north or well east of this fault. Given that slippage in hard rock cannot be dated, that our (reconnaissance) core data indicate complete erosion of basal salt-marsh peats (immediately) south and north of the EBF trace, and that transgressive^regressive sequences may result also from non-tectonic causes, we decided to focus on the last of these four possibilities to test the hypothesis of repeated neotectonic activity. We dated rhizomes of Distichlis spicata collected from the top of the substrate or the very base (of remnants) of salt-marsh peat overlying the substrate at sites in Farm River marsh, Double Beach marsh, Momauguin marsh, and Kelsey
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65
elevation error for each sample is estimated at H 3 cm. We inspected the samples for 14 C dating by AMS technique for penetration by younger rootlets using a binocular microscope. The 14 C ages were calibrated according to Method A of Stuiver and Reimer (1993).
4. Results
Fig. 4. Stratigraphic position of basal high marsh peat samples (Distichlis spicata rhizomes) from Farm River marsh (FR-1), Momauguin marsh (M-1), Double Beach marsh (DB-1), and Kelsey Island marsh (KI-1, KI-2). See Fig. 3 for location of sampling sites.
Island marsh (Figs. 3 and 4). The depth and geographical distributions of the samples re£ect our intention to obtain a record going back several thousand years and to detect hidden active faults in the study area. The samples were collected by means of a 1-mlong gouge auger with a diameter of 6 cm. The surface elevation of coring sites was determined relative to NGVD29 by means of a total station and a regular surveying instrument. The total
The age^depth data for Distichlis spicata rhizomes sampled from the base of salt-marsh peat at three mainland sites located well south of the EBF (FR-1, M-1, and DB-1) and at two sites on Kelsey Island (KI-1, -2) (Fig. 3) are summarized in Table 1. The position of these ¢ve samples within, at or just above the surface of the substrate implies that no or very little lowering due to compaction has occurred (Fig. 4). Rhizomes of D. spicata generally grow 4^12 cm below the marsh surface (van de Plassche et al., 1998a), but we observed that when rooted in a sandy substrate they regularly spread their rhizomes as deep as 20 cm below the marsh surface. We accept here that a D. spicata rhizome at or just above the substrate surface indicates that the former marsh surface occurred 4^16 cm higher, and at most 20 cm higher for a rhizome collected from below the substrate surface. Each of the ¢ve paleomarsh-surface estimates yields a minimum MHW estimate by assuming that the associated marsh surface occurred 35 cm above local MHW. An age^depth plot of these ¢ve minimum MHW estimates from south of the EBF shows a consistent long-term rate of relative lower-limit-MHW rise of ca. 1.1 mm yr31 since ca. 3000 cal yr BP (Fig. 5).
5. Discussion and conclusions A rate of late Holocene relative lower-limitMHW rise of ca. 1.1 mm yr31 for our study area south of the EBF is consistent with longterm rates of relative MHW rise as established for Farm River marsh north of the EBF and for the East River, Hammock River, and Connecticut River marshes (Fig. 1), proving conclusively that
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that of the eastern Farm River marsh suggesting a smaller tidal range in the latter marsh. Varekamp et al. (1999) noted that ‘Records from the Farm River marsh (CT) show strong evidence for repeated small fault movements during the last 1500 yr, which result in an unusually steep RSLR[ise] curve at locations in the downfaulted block. Some steps in these curves (presumably indicating activity along the faults) correlate with similar steps in RSLR curves from other marshes, suggesting the possibility of repeated local tectonic movements over a wide area.’ With the hypothesis of seismotectonic movements conclusively invalidated, this statement invites, pending the publication of relevant data from the Farm River and Kelsey Island marshes, three points of future discussion. Firstly, di¡erent Connecticut marshes, including Kelsey Island marsh, recorded synchronous MHW variations during the past 1200 yr related to one or more non-tectonic forcings. If the claim for synchronous MHW variations is supported by the data (which remain to be published), then regional changes in the rate of sea-level rise (van de Plassche et al., 1998a) would be a likely explanation. Secondly, some ‘steps’ in the Kelsey Island marsh MHW curve are not present, or not synchronous with ‘steps’ in the MHW curve from Farm River marsh just north of the EBF and in records from other Connecticut marshes (or the hypothesis of seismotectonic activity along this fault would not have emerged). Given the previous point, the question arises why these RSL records di¡er at the scale of MHW ‘steps’.
Fig. 5. Age^depth diagram comparing the error envelope for the lower limit of relative MHW rise in the area south of the EBF near Branford, including Kelsey Island, with the average rate of relative MHW rise (ca. 1 mm yr31 ) based on previous studies in Connecticut (see Fig. 1) and a reported average rate of MHW rise (ca. 2.5 mm yr31 ) for a site in Kelsey Island marsh.
no seismotectonic lowering has occurred along the EBF near Branford during the past 3000 yr. The marked in£ection in the lower-limit-MHW envelope, around 2900 cal yr BP need not re£ect a sudden decrease in the rate of regional MHW rise. More likely, the relatively low position of index point FR-1 re£ects a smaller tidal range in Farm River marsh caused by damping of the tidal wave as it moved up the relatively long and narrow Farm River estuary set in bedrock (Fig. 3). The elevation of the modern high marsh surface in the Double Beach, Kelsey Island, and Momauguin marshes is, on average, 15^20 cm higher than
Table 1 Sample information for age^depth data used in this study Index point
Dated materiala Sample depth (m NGVD29)b
FR-1
DS rhiz.
32.74^2.77
M-1 DB-1 KI-1 KI-2
DS DS DS DS
31.90^192 31.07^1.08 30.39^0.41 30.02^0.04
a b c
rhiz. rhiz. rhiz. rhiz.
14
C age (yr BP)
N13 C (x)
Age (cal yr BP)c
Min. MHW (m NGVD29)
9 139
3 092 H 31
318.1
32.90^3.10
9 140 9 262 10 439 10 440
2 814 H 34 2 124 H 37 1 560 H 40 1 133 H 37
314.2 315.8 315.1 315.5
3 357^3 319/3 307^3 299/ 3 291^3 266 2 951^2 865 2 148^2 041 1 520^1 408/1 397^1 394 1 063^972
Lab. no. UtC
DS rhiz.: rhizome of Distichlis spicata. Measurement error: H 0.03 m. H 1c; CALIB 4.1.2. decadal curve (Stuiver and Reimer, 1993).
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32.06^2.25 31.23^1.41 30.51^0.71 30.14^0.26
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And thirdly, the high average rate of RSL rise during the past 1200 yr (ca. 2.5 mm yr31 ), reported by Thompson et al. (1999, 2000) and Varekamp et al. (1999) for their study site in Kelsey Island marsh, and the relatively high rate (1.6 mm yr31 ) of RSL rise obtained for the core GA site in West River marsh (Nydick et al., 1995) (Fig. 1) require a non-tectonic explanation. These high(er) rates of relative MHW rise are likely to result from one or more of the following causes: (a) stronger compaction, (b) underestimation of former marsh-surface elevations, and (c) erroneous MHW reconstruction due to, for instance, postmortem changes of the foraminiferal assemblages (see Varekamp et al., 1992; van de Plassche et al., 1998a for methodology). Finally, we agree with Kafka (2000) that announcements of locally heightened risks of seismotectonic events must be based on a tested hypothesis and not just on interpretation. Media attention can motivate insurance companies to raise their rates for coverage of damage by earthquakes in that area. Our data demonstrate beyond question that there are no grounds for justifying such a rate increase in the study area.
Acknowledgements With pleasure we thank Marit Brommer, Stefan Buurman, Enith de Boer, Raymond Hazebroek, Marion Rensink, Michel van de Riet, Lisette van der Burgh, and Wiebe Wijnja for dedicated fieldwork. We are grateful to the Kelseys for permission to study Kelsey Island marsh and for indispensable nautical assistance to and from the island. Financial support was provided by the Vrije Universiteit Amsterdam and the Dutch National Research Program on Global Air Pollution and Climate Change (Project 951268).
References Atwater, B.F., Nelson, A.R., Clague, J.J., Carver, G.A., Yamaguchi, D.K., Bobrowski, P.T., Borgeois, J., Darienzo, W.C., Grant, M.E., Hemphil-Haley, E., Kelsey, H.M., Jacoby, G., Nishenko, S.P., Palmer, S.P., Peterson, C.D.,
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Reinhart, M.A., 1995. Summary of coastal geologic evidence for past great earthquakes at the Cascadia subduction zone. Earthq. Spectra 11, 1^18. Bloom, A.L., Stuiver, M., 1963. Submergence of the Connecticut coast. Science 139, 332^334. Gehrels, W.R., 1999. Middle and late Holocene sea-level change in eastern Maine reconstructed from foraminiferal saltmarsh stratigraphy and AMS 14 C dates on basal peat. Quat. Res. 52, 350^359. Gordon, R.B., 1980. The sedimentary system of Long Island Sound. In: Salzman, B. (Ed.), Estuarine Physics and Chemistry: Studies in Long Island Sound. Adv. Geophys. 22, 1^39. Kafka, A.L., 2000. Public misconceptions about faults and earthquakes in the eastern United States: is it our own fault? Seismol. Res. Lett. 71, 311. Lefor, M.W., Kennard, W.C., Civco, D.L., 1987. Relationships of salt-marsh plant distributions to tidal levels in Connecticut, USA. Environ. Manage. 11, 61^68. Nydick, K.R., Bidwell, A.B., Thomas, E., Varekamp, J.C., 1995. A sea-level rise curve from Guilford, Connecticut, USA. Mar. Geol. 124, 137^159. Orson, R.A., Warren, R.S., Niering, W.A., 1987. Development of a tidal marsh in a New England river valley. Estuaries 10, 20^27. Patton, P.C., Horne, G.S., 1991. A submergence curve for the Connecticut River estuary. J. Coast. Res. SI 11, 181^196. Red¢eld, A.C., 1972. Development of a New England salt marsh. Ecol. Monogr. 42, 201^237. Rodgers, J., 1985. Bedrock Geological Map of Connecticut (in cooperation with US Geological Survey). Shennan, I., Long, A.J., Rutherford, M.M., Green, F.M., Innes, J.R., Lloyd, J.M., Zong, Y., Walker, K.J., 1996. Tidal marsh stratigraphy, sea-level change and large earthquakes, I: a 5000 year record in Washington, USA. Quat. Sci. Rev. 15, 1023^1059. Stuiver, M., Reimer, P.J., 1993. Extended 14 C database and revised CALIB radiocarbon calibration program. Radiocarbon 35, 215^230. Thompson, W.G., Kreulen, B., Tobey, E., Thomas, E., Varekamp, J.C., 1997. The Farm River Marsh, CT: Evolution and pollution. EOS, Transactions, Am. Geophys. Union 1997 Spring Meeting 78, 17, April 29, Supplement, S136. Thompson, W.G., Varekamp, J.C., Thomas, E., 2000. Fault motions along the eastern border fault, Hartford Basin, CT, over the past 2800 years. EOS, Transactions, Am. Geophys. Union 2000 Spring Meeting, 81, 19, Supplement, S311. Thompson, W.G., Varekamp, J.C, Thomas, E., de Boer, J.Z., 1999. Neotectonics on coastal Connecticut: evidence from the Farm River marsh, Branford. EOS, Transactions, Am. Geophys. Union 1999 Spring Meeting, 80, 18, Supplement, S86. van de Plassche, O., 1991. Late Holocene sea-level £uctuations on the shore of Connecticut inferred from transgressive and regressive overlap boundaries in salt-marsh deposits. J. Coast. Res. Spec. Issue 111, 159^180. van de Plassche, O., 2000. North Atlantic climate^ocean var-
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iations and sea level in Long Island Sound, Connecticut, since 500 cal yr A.D.. Quat. Res. 53, 89^97. van de Plassche, O., van der Borg, K., de Jong, A.F.M., 1998a. Sea level^climate correlation during the past 1400 yr. Geology 26, 319^322. van de Plassche, O., van der Borg, K., de Jong, A.F.M., 1998b. A correction. Geology 26, 672.
Varekamp, J.C., Thomas, E., Thompson, W.G., 1999. Relative sea level rise along the NE USA seaboard. EOS, Transactions, Am. Geophys. Union 1999 Spring Meeting, 80, 18, Supplement, S85. Varekamp, J.C., Thomas, E., van de Plassche, O., 1992. Relative sea-level rise and climate change over the last 1500 years. Terra Nova 4, 293^304.
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Relative sea-level rise across the Eastern Border fault (Branford, Connecticut): evidence against seismotectonic movements O. van de Plassche a; , K. van der Borg b , A.F.M. de Jong b a b
Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands R.J. Van de Graa¡ Laboratory, Universiteit Utrecht, P.O. Box 80000, 3508 TA Utrecht, The Netherlands Received 16 March 2001; accepted 18 September 2001
Abstract Basal peat age^depth data from four salt-marsh sites located 500^1500 m south of the Eastern Border fault (EBF) near Branford, Connecticut, document an error envelope for the lower limit of relative mean high water (MHW) rise during the past 3300 yr. The long-term rate of relative lower-limit-MHW rise during the past 3000 yr (1.1 mm yr31 ) corresponds with late Holocene rates of relative MHW rise found elsewhere in coastal Connecticut (0.9^1.2 mm yr31 ). This result invalidates repeated claims that multiple seismotectonic lowering of a crustal block towards the south has occurred along a segment of the EBF near Branford shortly after 815 cal yr BC and during the past 1200 yr and calls for a non-tectonic interpretation of biostratigraphic evidence of local short-term sea-level variations. 6 2002 Elsevier Science B.V. All rights reserved. Keywords: salt marsh; basal peat; sea-level variations
1. Introduction Re£ecting the di¡erence in tectonic setting of salt marshes on the Paci¢c and Atlantic coasts of North America, records of century-scale sealevel variations from west coast marshes have been analyzed primarily in terms of seismotectonic events (e.g. Atwater et al., 1995; Shennan et al., 1996), while those from the east coast have been
* Corresponding author. Tel.: +31-20-444-7380; Fax: +31-20-646-2457. E-mail address: [email protected] (O. van de Plassche).
correlated with climate-proxy records (e.g. Varekamp et al., 1992; van de Plassche et al., 1998a; Gehrels, 1999). Whereas the database for west coast marshes (mainly in the states of Oregon and Washington) is large enough to detect coherent regional space^time patterns of relative sealevel (RSL) change, that for the east coast (mainly Connecticut) is still very small and published records of century-scale sea-level variations di¡er more than they agree (van de Plassche, 2000). Theoretically, these di¡erences may be attributed to methodological errors or inaccuracies, variations in local tidal range, compaction e¡ects, and/or di¡erences in crustal movement. Here, we focus on the latter possibility following recent
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claims of seismotectonic events in coastal Connecticut during the late Holocene. Sea-level studies in the Connecticut River estuary and in the Hammock River and East River marshes, Connecticut, have found average rates of RSL rise close to 1 mm yr31 during the past 1800 yr or more (Bloom and Stuiver, 1963; Patton and Horne, 1991; van de Plassche, 1991; Varekamp et al., 1992; Nydick et al., 1995; van de Plassche et al., 1998a,b) (Fig. 1). On the basis of a single basal peat date (3670 H 140 yr BP, 4 m below marsh surface), Orson et al. (1987) also found a rate of ca. 1.1 mm yr31 for RSL rise in Pattagansett River marsh, located ca. 14 km east of the mouth of the Connecticut River. Thompson et al. (1997, 1999; unpublished data) and Varekamp et al. (1999; unpublished data) report a comparable mean rate of RSL rise of ca. 1.2 mm yr31 since ca. 800 cal yr AD for a site in the eastern lobe of the Farm River marsh, but, remarkably, double that rate (ca. 2.5 mm yr31 ) for a site in Kelsey Island marsh, located 1.9 km to the south of their Farm River marsh study site (Fig. 1). They suggest that the di¡erence between the two RSL records can be attributed to repeated neotectonic activity along a segment of the (reversely reactivated) Eastern Border fault (EBF) (Rodgers, 1985), which runs just south of their Farm River marsh study site (Fig. 1). This interpretation is based on stratigraphic and radiocarbon data, which suggest a seismic event with an o¡set of ca. 1 m to the south shortly after 815 cal yr BC and a step-wise lowering, totalling 1.45 m, of the Kelsey Island marsh study site during the past 1200 yr. Thompson et al. (2000; unpublished data) claim that new evidence from a site in Farm River marsh, immediately to the south of the EBF, con¢rms that intermittent downthrow has occurred here during the past 1200 yr. With regard to the implications of their seismotectonic interpretation of the di¡erences in RSL data from Farm River marsh and Kelsey Island marsh, Thompson et al. (1999) point out that ‘A similar mechanism may be responsible for smaller relative sea level rise variations seen in other core locations’ (in Connecticut marshes nearby). The objective of this study is to verify the repeated claim that multiple seismotectonic down-
throw has occurred south of the EBF near Branford. It is motivated primarily by the fact that such fault activity, if unequivocally demonstrated, can have signi¢cant consequences for the interpretation of century-scale sea-level £uctuations, that have been and will be reconstructed for these and other Connecticut marshes, in terms of climate^ocean variability (Varekamp et al., 1992; Nydick et al., 1995; van de Plassche et al., 1998a; van de Plassche, 2000). The study is also relevant with regard to risk assessment of future earthquakes in the study area and surroundings, where seismicity is not uncommon and has occurred as recently as February 2001.
2. Salt-marsh vegetation New England salt marshes generally have three main vegetation zones, called low, high, and upper marsh (e.g. Red¢eld, 1972). The low marsh zone develops when mud- or sand£ats in the intertidal zone reach a level high enough for colonization by the cord grass Spartina alterni£ora (tall). When the low marsh zone reaches (to within 10 cm of) local mean high water (MHW), high marsh grasses such as Spartina patens and Distichlis spicata begin to replace the tall cord grass (e.g. Gordon, 1980; Lefor et al., 1987). The high marsh zone can reach as high as the highest spring tide level, but where su⁄cient fresh water enters the marsh, upper marsh plants (e.g. Phragmites australis, Scirpus robustus) can replace the high marsh vegetation down to about MHW spring level, or lower with increasing fresh water input. Relevant for this study is our observation in several Connecticut marshes that D. spicata can occur, exceptionally, as high as ca. 40 cm above the low marsh^high marsh boundary (i.e. ca. 30^40 cm above local MHW). On Kelsey Island, where MHW is estimated at ca. +1 m NGVD29 (National Geodetic Vertical Datum of 1929), the highest observed occurrence of D. spicata is at +1.35 m NGVD.
3. Approach and methods According to Thompson et al. (1999, 2000), the
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63
Fig. 1. Location of study area southwest of Branford (B) and rates of long-term RSL rise for previously studied sites in central coastal Connecticut.
EBF trace across eastern Farm River marsh marks the boundary between tidal sediments and low marsh peat to the south and high marsh peat deposits to the north. They interpret this apparent spatial correlation between lithofacies distribution
and fault line position as a possible expression of tectonic activity. The distribution of tidal sediments, and low and high marsh peat is, however, also dependent on the (former) position and activity of tidal creeks. We mapped the marsh area
Fig. 2. Sub-surface distribution of marine clay in the marsh stratigraphy in eastern Farm River marsh north of the EBF. The position of this fault is taken from the Bedrock Geological Map of Connecticut (Rodgers, 1985).
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Fig. 3. Location of basal peat sampling sites in the study area. FR: Farm River marsh, DB: Double Beach marsh, M: Momauguin marsh, KI: Kelsey Island marsh (key as in Fig. 2).
north of the approximate fault-trace position and found extensive sub-surface distribution of tidal sediments and low marsh peat there too (Fig. 2). This mapping result does not, however, exclude the possibility of multiple tectonic downthrow to the south along a segment of the EBF. If a crustal block on the south side of this fault in the Branford area has been repeatedly lowered seismotectonically during the past 1200 yr or more, one expects to ¢nd the following structural, morphological, stratigraphic, and age^depth evidence: (1) slippage phenomena in outcrops of hard rock; (2) a step(-like) feature in the soft rock substrate, traceable in the stratigraphy of the overlying salt-marsh deposits present prior to the onset of neotectonic activity ; (3) a repetition of (sudden) transgressive^re-
gressive facies changes in marsh deposits south of the fault trace, and (4) provided compaction is excluded, older MHW data from sites south of the EBF plot increasingly deeper below contemporaneous MHW data from sites north or well east of this fault. Given that slippage in hard rock cannot be dated, that our (reconnaissance) core data indicate complete erosion of basal salt-marsh peats (immediately) south and north of the EBF trace, and that transgressive^regressive sequences may result also from non-tectonic causes, we decided to focus on the last of these four possibilities to test the hypothesis of repeated neotectonic activity. We dated rhizomes of Distichlis spicata collected from the top of the substrate or the very base (of remnants) of salt-marsh peat overlying the substrate at sites in Farm River marsh, Double Beach marsh, Momauguin marsh, and Kelsey
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65
elevation error for each sample is estimated at H 3 cm. We inspected the samples for 14 C dating by AMS technique for penetration by younger rootlets using a binocular microscope. The 14 C ages were calibrated according to Method A of Stuiver and Reimer (1993).
4. Results
Fig. 4. Stratigraphic position of basal high marsh peat samples (Distichlis spicata rhizomes) from Farm River marsh (FR-1), Momauguin marsh (M-1), Double Beach marsh (DB-1), and Kelsey Island marsh (KI-1, KI-2). See Fig. 3 for location of sampling sites.
Island marsh (Figs. 3 and 4). The depth and geographical distributions of the samples re£ect our intention to obtain a record going back several thousand years and to detect hidden active faults in the study area. The samples were collected by means of a 1-mlong gouge auger with a diameter of 6 cm. The surface elevation of coring sites was determined relative to NGVD29 by means of a total station and a regular surveying instrument. The total
The age^depth data for Distichlis spicata rhizomes sampled from the base of salt-marsh peat at three mainland sites located well south of the EBF (FR-1, M-1, and DB-1) and at two sites on Kelsey Island (KI-1, -2) (Fig. 3) are summarized in Table 1. The position of these ¢ve samples within, at or just above the surface of the substrate implies that no or very little lowering due to compaction has occurred (Fig. 4). Rhizomes of D. spicata generally grow 4^12 cm below the marsh surface (van de Plassche et al., 1998a), but we observed that when rooted in a sandy substrate they regularly spread their rhizomes as deep as 20 cm below the marsh surface. We accept here that a D. spicata rhizome at or just above the substrate surface indicates that the former marsh surface occurred 4^16 cm higher, and at most 20 cm higher for a rhizome collected from below the substrate surface. Each of the ¢ve paleomarsh-surface estimates yields a minimum MHW estimate by assuming that the associated marsh surface occurred 35 cm above local MHW. An age^depth plot of these ¢ve minimum MHW estimates from south of the EBF shows a consistent long-term rate of relative lower-limit-MHW rise of ca. 1.1 mm yr31 since ca. 3000 cal yr BP (Fig. 5).
5. Discussion and conclusions A rate of late Holocene relative lower-limitMHW rise of ca. 1.1 mm yr31 for our study area south of the EBF is consistent with longterm rates of relative MHW rise as established for Farm River marsh north of the EBF and for the East River, Hammock River, and Connecticut River marshes (Fig. 1), proving conclusively that
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that of the eastern Farm River marsh suggesting a smaller tidal range in the latter marsh. Varekamp et al. (1999) noted that ‘Records from the Farm River marsh (CT) show strong evidence for repeated small fault movements during the last 1500 yr, which result in an unusually steep RSLR[ise] curve at locations in the downfaulted block. Some steps in these curves (presumably indicating activity along the faults) correlate with similar steps in RSLR curves from other marshes, suggesting the possibility of repeated local tectonic movements over a wide area.’ With the hypothesis of seismotectonic movements conclusively invalidated, this statement invites, pending the publication of relevant data from the Farm River and Kelsey Island marshes, three points of future discussion. Firstly, di¡erent Connecticut marshes, including Kelsey Island marsh, recorded synchronous MHW variations during the past 1200 yr related to one or more non-tectonic forcings. If the claim for synchronous MHW variations is supported by the data (which remain to be published), then regional changes in the rate of sea-level rise (van de Plassche et al., 1998a) would be a likely explanation. Secondly, some ‘steps’ in the Kelsey Island marsh MHW curve are not present, or not synchronous with ‘steps’ in the MHW curve from Farm River marsh just north of the EBF and in records from other Connecticut marshes (or the hypothesis of seismotectonic activity along this fault would not have emerged). Given the previous point, the question arises why these RSL records di¡er at the scale of MHW ‘steps’.
Fig. 5. Age^depth diagram comparing the error envelope for the lower limit of relative MHW rise in the area south of the EBF near Branford, including Kelsey Island, with the average rate of relative MHW rise (ca. 1 mm yr31 ) based on previous studies in Connecticut (see Fig. 1) and a reported average rate of MHW rise (ca. 2.5 mm yr31 ) for a site in Kelsey Island marsh.
no seismotectonic lowering has occurred along the EBF near Branford during the past 3000 yr. The marked in£ection in the lower-limit-MHW envelope, around 2900 cal yr BP need not re£ect a sudden decrease in the rate of regional MHW rise. More likely, the relatively low position of index point FR-1 re£ects a smaller tidal range in Farm River marsh caused by damping of the tidal wave as it moved up the relatively long and narrow Farm River estuary set in bedrock (Fig. 3). The elevation of the modern high marsh surface in the Double Beach, Kelsey Island, and Momauguin marshes is, on average, 15^20 cm higher than
Table 1 Sample information for age^depth data used in this study Index point
Dated materiala Sample depth (m NGVD29)b
FR-1
DS rhiz.
32.74^2.77
M-1 DB-1 KI-1 KI-2
DS DS DS DS
31.90^192 31.07^1.08 30.39^0.41 30.02^0.04
a b c
rhiz. rhiz. rhiz. rhiz.
14
C age (yr BP)
N13 C (x)
Age (cal yr BP)c
Min. MHW (m NGVD29)
9 139
3 092 H 31
318.1
32.90^3.10
9 140 9 262 10 439 10 440
2 814 H 34 2 124 H 37 1 560 H 40 1 133 H 37
314.2 315.8 315.1 315.5
3 357^3 319/3 307^3 299/ 3 291^3 266 2 951^2 865 2 148^2 041 1 520^1 408/1 397^1 394 1 063^972
Lab. no. UtC
DS rhiz.: rhizome of Distichlis spicata. Measurement error: H 0.03 m. H 1c; CALIB 4.1.2. decadal curve (Stuiver and Reimer, 1993).
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32.06^2.25 31.23^1.41 30.51^0.71 30.14^0.26
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And thirdly, the high average rate of RSL rise during the past 1200 yr (ca. 2.5 mm yr31 ), reported by Thompson et al. (1999, 2000) and Varekamp et al. (1999) for their study site in Kelsey Island marsh, and the relatively high rate (1.6 mm yr31 ) of RSL rise obtained for the core GA site in West River marsh (Nydick et al., 1995) (Fig. 1) require a non-tectonic explanation. These high(er) rates of relative MHW rise are likely to result from one or more of the following causes: (a) stronger compaction, (b) underestimation of former marsh-surface elevations, and (c) erroneous MHW reconstruction due to, for instance, postmortem changes of the foraminiferal assemblages (see Varekamp et al., 1992; van de Plassche et al., 1998a for methodology). Finally, we agree with Kafka (2000) that announcements of locally heightened risks of seismotectonic events must be based on a tested hypothesis and not just on interpretation. Media attention can motivate insurance companies to raise their rates for coverage of damage by earthquakes in that area. Our data demonstrate beyond question that there are no grounds for justifying such a rate increase in the study area.
Acknowledgements With pleasure we thank Marit Brommer, Stefan Buurman, Enith de Boer, Raymond Hazebroek, Marion Rensink, Michel van de Riet, Lisette van der Burgh, and Wiebe Wijnja for dedicated fieldwork. We are grateful to the Kelseys for permission to study Kelsey Island marsh and for indispensable nautical assistance to and from the island. Financial support was provided by the Vrije Universiteit Amsterdam and the Dutch National Research Program on Global Air Pollution and Climate Change (Project 951268).
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