Impact Effects and Regional Tectonic Insights: Backstripping the Chesapeake Bay Impact Structure

Impact effects and regional tectonic insights: Backstripping the Chesapeake Bay impact structure

Travis Hayden* Department of Geosciences, Western Michigan University, 1187 Rood Hall, 1903 W. Michigan Avenue, Michelle Kominz Kalamazoo, Michigan 49008, USA David S. Powars United States Geological Survey, National Center, Reston, Virginia 20192, USA Lucy E. Edwards Kenneth G. Miller James V. Browning Department of Geosciences, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA Andrew A. Kulpecz

ABSTRACT day sediment thickness must be decompacted The Chesapeake Bay impact structure is a ca. 35.4 Ma crater located on the eastern sea- to estimate its thickness at the time of deposi- board of North America. Deposition returned to normal shortly after impact, resulting in a tion. This is done using porosity-depth curves unique record of both impact-related and subsequent passive margin sedimentation. We use generated from nearby cores. We used compac- backstripping to show that the impact strongly affected sedimentation for 7 m.y. through tion curves based on New Jersey Coastal Plain impact-derived crustal-scale tectonics, dominated by the effects of sediment compaction and cores (Van Sickel et al., 2004). Because these the introduction and subsequent removal of a negative thermal anomaly instead of the expected curves are based on sediment composition, positive thermal anomaly. After this, the area was dominated by passive margin thermal sub- detailed lithology is also required. Isostatic sidence overprinted by periods of regional-scale vertical tectonic events, on the order of tens of unloading of the sediment yields an estimate meters. Loading due to prograding sediment bodies may have generated these events. of how much of the hole was fi lled with sediment. In order to estimate the total subsidence, Keywords: impact processes, passive margin, Chesapeake Bay, tectonics, Eocene, backstripping. an estimate of paleowater depth must also be made. This water depth estimate is based on PREVIOUS WORK impact craters. The most recent studies include inferred depth preferences of selected benthic The Chesapeake Bay impact structure (Fig. 1) several continuously cored locations, both organisms. Organisms that live in shallower was identifi ed from core drilling and seismic inside and outside the crater (Fig. 1; e.g., water depths tend to have a more limited range refl ection studies in Chesapeake Bay region Poag 1997; Powars and Bruce, 1999; Powars, of suitable habitat, while organisms that live (e.g., Powars et al., 1992, 1993; Poag et al., 2000; Powars et al., 2005; Edwards et al., deeper tend to have wider habitable zones. This 1992, 1994). The Chesapeake Bay impact struc- 2005). These studies provided chronological, leads to increasing uncertainty in water depth ture correlates with the North American tektite paleobathymetric, and lithologic constraints on ranges for increasingly deeper water facies fi eld, which has been dated as 34.3–35.5 Ma the post-impact sediment packages. translating directly into increasing error ranges (Glass, 1989; Koeberl, 1989; Poag et al., 1994; Previous studies of impact processes at this for subsidence estimates. Horton and Izett, 2006). At 85 km in diam- and other impact locations have focused on The steps outlined above are codifi ed in eter, this crater is the largest known crater in immediate impact-related events, and were the following equation (modifi ed from Steckler the United States, the seventh largest known limited to hours and or years just after impact and Watts, 1978): on Earth, and one of the best preserved marine (e.g., Melosh and Ivanov, 1999; Melosh, 1989).

In the Chesapeake Bay impact structure, the (ρm −ρs) (ρm ) preservation of the impact-related sequences, as TS = S* −ΔSL + Wd. (1) (ρm −ρw) (ρm −ρw ) 76°N well as the immediate post-impact sequences, Washington D.C. Del provides a record of the effects of the impact aware on time scales of millions of years. In addi- This equation allows for the calculation of tion, the impact structure provides a relatively tectonic subsidence (TS) from decompacted ρ Maryland complete record of the post-impact, normal, sediment thicknesses (S*), densities ( s), and shallow-marine sedimentation. water depths (Wd), given the change in sea Chesapeake Bay Δ ρ ρ level ( SL). The values of m and w represent ρ 3 METHODS the density of the mantle ( m = 3.33 g/cm ) and Fentress Central In this work we use backstripping to esti- seawater (ρ = 1.03 g/cm3). S*, ρ , and Wd are Virginia w s MW4-1 Crater Bethany Beach mate the subsidence of the basement under the unknowns that can be estimated from a W Exmore ′ water (Bond and Kominz, 1984). Backstrip- sedimentary section. Kiptopeke 37°16 Langley Annular ping removes the variable sedimentation by Our analysis of backstripping results is Trough Dismal Swamp compensating for the subsidence caused by the time-dependent, requiring numerical age esti- Eyreville sediment load. This yields both tectonic and mates of the sediment packages. In the case Virginia eustatic basement subsidence. of this study, low age resolution makes the North Carolina Backstripping quantitatively estimates the study of only slower (a million years or more) Figure 1. Location map showing Chesapeake depth of the hole or accommodation space that processes possible. Bay impact structure and coreholes used in is ﬁ lled with a combination of sediments and The goal of our backstripping is to deter- this study. water. The thickness of the sediments deposited mine the subsidence of the basement under provides a limit on the minimum amount of sub- water. This is the ﬁ rst reduction, or R1, of Bond *E-mail: [email protected]. sidence that took place. However, the present- et al. (1989):

© 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, April April 2008; 2008 v. 36; no. 4; p. 327–330; doi: 10.1130/G24408A.1; 4 fi gures. 327 (ρ −ρ ) One of the major limiting factors for core removal of part of the upper crust. The scale R1 = S* m S + Wd (ρ −ρ ) selection was paleoenvironmental data. For the of the diagram does not allow for a detailed m w (ρ ) (2) purposes of this study, previous work provided view of this event, which occurred in hours or = TS +ΔSL m . paleoenvironmental indicators. When possible, days (Poag et al., 1994). All three of the crater- (ρ −ρ ) m w the authors of those works were contacted to interior cores record higher than expected sub- better understand and interpret these into a sidence rates after impact, that decrease into a It is important to note that this R1 curve framework suitable for backstripping. hiatus ~3 m.y. after impact (Fig. 3, event B). includes both eustatic and tectonic signals. R1 The other major limiting factor for selec- The rapid subsidence event was followed by a data can be compared to a theoretical model of tion of cores was availability of age estimates. dramatic uplift event ranging between 50 m and passive margin subsidence based on the work of Relative dates based on strontium isotope tech- 125 m in all three crater-interior cores (Fig. 3). McKenzie (1978) or Royden and Keen (1980). niques were used to backstrip the Oligocene– The uplift is documented by an unconformity These are theoretical models of the expected Miocene sections of the Bethany Beach core (Fig. 3, event C), and deposition (Fig. 3, event amount and timing of subsidence due to thermal (Browning et al., 2006) and the Miocene in the D) above the hiatus was shallower than below cooling of stretched continental lithosphere. As Kiptopeke core (Powars and Bruce, 1999). In the hiatus. The uplift magnitudes range from the Chesapeake Bay impact structure is under- most cases, however, age estimates were based 100 ± 5 m recorded at Kiptopeke to between 50 lain by the rifted passive margin of North Amer- on biostratigraphy. These age estimates were and 125 m at Langley. These uplifts occurred ica, these theoretical models are applicable. The made using the Gradstein et al. (2004) bio- between 0.8 and 2.9 m.y. in Kiptopeke and general forms of the R1 backstripping results stratigraphic time scale. Langley, respectively. Subsequent to the uplift are similar to the McKenzie (1978) models event, Exmore and Kiptopeke both record (Fig. 2A). The difference between expected sub- RESULTS unconformities between 28.6 and 18.8 Ma sidence from thermal cooling and the actual R1 Of the four R1 curves from outside the impact and between 31.7 and 16.5 Ma, respectively. subsidence recorded by the sediments is a com- crater, three show remarkable internal consis- The Langley core documents slow deposition bination of eustatic and non-thermal tectonic tency in the magnitude and timing of subsidence throughout this time. Improved age control for effects. The interpretation of these results is the (Fig. 2B). At the time of the impact none of these this core may reveal hiatuses during this inter- basis of this paper and we refer to more or less cores recorded deposition (the Bethany Beach val (Fig. 3, event D). Note that the Langley R1 subsidence in comparison to the predicted core, however, did not penetrate sediments core shows evidence of water depth changes McKenzie (1978) model of thermal subsidence older than ca. 28 Ma.) An excess sub sidence at higher frequency than we can analyze, due (MK means) (e.g., Figs. 2A, 2B). event is recorded in the Bethany Beach core to limited age control during this time frame ( Browning et al., 2006) beginning at 22 Ma. (Powars et al., 2005). DATA This event began at 14 Ma in the MW4–1 core, Sedimentation resumed ca. 18.8 Ma in the The input data sets were collected and inter- and at 13 Ma in the Fentress core. Finally, start- Exmore core, and began progressively later preted from several cores drilled inside and ing ca. 2.5 Ma subsidence decreased in both the to the south (Fig. 3, event E). The subsidence outside the impact crater (Fig. 1). This duality Fentress and MW4–1 cores, suggesting a small- recorded in event E is ~20–30 m between 19 allows comparison between crater-exterior scale uplift of the region. and 5 Ma and is seen both inside and outside and crater-interior subsidence signals. Of the From 20 Ma to the present, the subsidence of the crater. However, both the Exmore and numerous cores located throughout the area, history of the cores located within the crater is Kiptopeke cores record a period of increased only seven cores provide suffi cient age, litho- similar to that of the cores located outside the subsidence rate before an unconformity. Thus, logic, and paleodepth data to perform back- crater. These cores also record several events these cores suggest a larger and less continuous stripping analysis. These cores included four immediately post-impact (Fig. 3). A vertical subsidence event in comparison to tailing off cores located outside the crater, and three inside line just before 35 Ma on the R1 plot (Fig. 3, into the more general regional, crater-exterior the crater (Fig. 1) that were drilled between event A) records the dramatic, instantaneous trend. The Exmore core shows the largest 1986 and 2006 (Powars et al., 1992, 2005; removal of the pre-impact sediment pile, fol- subsidence event E, with a magnitude of Edwards et al., 2005; Powars, 2000; Powars lowed by virtually instantaneous redeposition ~75–100 m between 18.8 and 14.3 Ma. The and Bruce, 1999; Mixon, 1985). We reconfi g- for each of the crater-interior cores. None of the Kiptopeke core records a similar amount of ured the existing data into a framework suitable cores analyzed were located within the center- excess subsidence but it starts later, at 16.5 Ma, for backstripping. most portion of the crater that also underwent and occurs in two events, one from 16.5 to

B Figure 2. A: R1 plots for A Time (Ma) Time (Ma) 40 35 30 25 20 15 10 5 0 two representative crater- 160 140 120 100 80 60 40 20 0 0 exterior cores. Heavier 0 100 lines represent range of R1 200 results due to water depth ) 200 uncertainty. MK means 400 represent the McKenzie th (m 300 600 (1978) best-fit thermal Dep 1

models to the R1 curves R 800 400 assuming best-estimate R1 Depth (m) water depths. Subsidence 1000 MW4-1 MK mean Bethany Beach MK mean - 900 m 500 pattern for both cores is MW4-1R1mean Bethany Beach R1 mean - 900 m 1200 600 dominantly passive mar- MW4-1 MK mean Bethany Beach MK mean Fentress MK mean Dismal Swamp MK mean gin type. B: Expansion of MW4-1 R1 range Bethany Beach R1 range Fentress R1 mean Dismal Swamp R1 mean the part of A from 40 Ma to present. Water depth ranges have been removed to aid readability, and all four crater-exterior cores are represented. Subsidence patterns of Bethany Beach, MW4–1, and Fentress are quite similar, while the trends at Dismal Swamp do not follow those of these wells.

328 GEOLOGY, April 2008 Pliocene Eocene Oligocene Miocene Pleistocene Temperature (°C) AB C D E F Age (Ma) 40 35 30 25 20 15 10 5 0 0

100

200

300 Depth (km) Normal Geothermal 400 Initial Post-Impact R1 Depth (m) Maximum Subsidence 500 Final

600 Figure 4. Theoretical temperature versus

Events of Interest depth plots at key stages. The negative 700 AB C D E F thermal anomaly is generated by rapid sedimentation and occurs immediately after Kiptopeke R1 range Exmore R1 range Langley R1 range Kiptopeke R1 sediments Exmore R1 sediments Langley R1 sediments impact. Initially, heating of the sediments Background signals from crater-exterior cores results in cooling of the uppermost crust. This occurred ~3 m.y. after impact and rep- Figure 3. Results for the three crater-interior cores. Background crater-exterior results are resents maximum thermal subsidence. The shown in gray for comparison. R1 (fi rst reduction) sediment lines illustrate only sub sidence fi nal post-impact line has higher tempera- represented by the sediments without any water depth information. R1 ranges indicate the tures than the initial profi le due to thermal subsidence when water depth changes are taken into account. Event A represents the impact blanketing of sediments that are still more event. Events B and C are various stages of the post-impact response inside crater. Events loosely compacted than those originally E and F are regional. present and is ~7 m.y. after impact.

14 Ma, accounting for 55 m of subsidence, and thermal subsidence. What actually occurred thicknesses, resulting in differing amounts of the other from 7.7 to 5.5 Ma, accounting for was initial subsidence, followed by uplift compaction and thermal blanketing. 50 m of additional subsidence. (Fig. 3). In detail, the immediate post-impact The thermal anomaly due to the impact was Between 5 and 2.5 Ma, all three of the event (event B in Fig. 3) is characterized by removed by ca. 30 ± 2 Ma. From 30 to 20 Ma crater-interior cores record an unconformity rapid subsidence that tails off to nondeposition a hiatus dominated in the study region. Low leading to a thin veneer of recent sediments and/or erosion by ~3 m.y. after impact. The average sedimentation rates are observed in the (Fig. 3, event F). This suggests a slight regional subsequent uplift event is indicated by deposi- Langley core during this time interval, which uplift, which is also seen in two crater-exterior tion at a shallower level (Fig. 3, event C). records a slight uplift (Fig. 3, event D). How- cores, MW4–1 and Fentress. We attribute the immediate post-impact ever, the requisite age and paleowater depth data subsidence (event B) to the rapid compaction lack the precision required to quantify this inter- INTERPRETATIONS of the impact breccia. Having been deposited pretation through backstripping. The subsidence histories of the crater-exterior in a few tens of minutes to hours (Collins The fi rst regional post-impact event occurred cores are remarkably similar, with one excep- and Wuennemann, 2005), these sediments progressively from north to south. Excess sub- tion. The Dismal Swamp core is located farthest must have been poorly consolidated so that sidence started at Bethany Beach at 22.0 Ma south (Fig. 1), and subsided differently from the subsequent compaction was time-dependent (Browning et al., 2006), and proceeded south- other coreholes (Fig. 2B). This core is located (Schmoker and Gautier, 1989). ward to Exmore at 20.5 Ma, then to Kiptopeke near the Norfolk Arch and may belong to a dif- The subsequent uplift event (Fig. 3, event at 16.5 Ma, to Langley at 14.5 Ma, to MW4–1 ferent tectonic regime than the rest of the cores C) suggests crustal heating. We postulate that at 14.0 Ma, and to Fentress at 13.0 Ma (Fig. 2B (Powars et al., 1992). a crustal-scale negative thermal anomaly was and event E in Fig. 3). Between 22 and 5 Ma, The similarity in subsidence among the caused by the deposition of cold sediments, this event generated ~20–30 m of subsidence, remaining crater-exterior cores suggests that resetting the geothermal gradient (Fig. 4). in excess of that expected in this passive mar- these cores record the general subsidence trend This fi rst cooled the upper crust, adding to the gin setting. Event E is interpreted as the fl exural of the Chesapeake Bay region (Fig. 2B). They accommodation space generated immediately response of the basement to the deposition of a provide a framework for comparison with the after the impact (event B). Subsequently the large sedimentary load (Browning et al., 2006) crater-interior results that allows us to sepa- upper crust and overlying sediment package that prograded from north to south along the rate impact-related and non-impact-related reheated (Fig. 4), generating uplift. The exact coast. Marine seismic refl ection surveys con- processes (Fig. 3). interaction between subsidence due to sediment ducted in the Chesapeake Bay and offshore are Inside the crater, the subsidence was more compaction and the effects of the crustal-scale consistent with this interpretation. However, the complex than outside. A large impact would thermal event is a topic for future modeling. profi les are not high resolution and do not pro- be expected to generate crustal-scale heating Each of the three crater-interior records shows vide defi nitive details of the southward progra- of the basement. This would result in initial slightly different timing and magnitudes of dation (Powars and Bruce, 1999). uplift, followed by cooling, producing accom- uplift (Fig. 3, event C). We interpret this to A small-scale uplift is recorded in all of modation space in excess of the background, be due to differential sediment properties and the crater-interior cores and two of the crater-

GEOLOGY, April 2008 329 exterior cores (Fig. 3, event F). This uplift is of Society of Economic Paleontologists and Min- tional origin of upper Eocene bolide-generated unknown origin, and appears to be ~2–20 m. It eralogists Special Publication 44, p. 39–61. sediments along the U.S. East Coast: Geo- Browning, J.V., Miller, K.G., McLaughlin, P.P., logical Society of America Abstracts with Pro- shows no clearly identifi able trends with respect Kominz, M.A., Sugarman, P.J., Monteverde, grams, v. 24, no. 7, p. 172–173. to location or magnitude. D., Feigenson, M.D., and Hernandez, J.C., Poag, C.W., Powars, D.S., Poppe, L.J., and 2006, Quantifi cation of the effects of eustasy, Mixon, R.B., 1994, Meteoroid mayhem in CONCLUSIONS AND FUTURE WORK subsidence, and sediment supply on Miocene Ole Virginny: Source of the North Ameri- Backstripping of the Chesapeake Bay sequences, mid-Atlantic margin of the United can Tektite strewn fi eld: Geology, v. 22, States: Geological Society of America Bulletin, p. 691–694, doi: 10.1130/0091–7613(1994)022 impact structure has increased our understand- v. 118, p. 567–588, doi: 10.1130/B25551.1. <0691:MMIOVS>2.3.CO;2. ing of both long-term impact-related processes Collins, G.S., and Wuennemann, K., 2005, How Powars, D.S., 2000, The effects of the Chesapeake and normal tectonic and depositional processes big was the Chesapeake Bay impact? Insight Bay impact crater on the geologic frame- that prevail along a passive margin. Our results from numerical modeling: Geology, v. 33, work and correlation of hydrogeologic units p. 925–928. of southeastern Virginia, south of the James show that impact-related tectonism is not a Edwards, L.E., Barron, J.A., Bukry, D., Bybell, River: U.S. Geological Survey Professional major contributor to depositional patterns after L.M., Cronin, T.M., Poag, C.W., Weems, R.E., Paper 1622, 53 p. ~7 m.y. We postulate that accommodation and Wingard, G.L., 2005, Paleontology of the Powars, D.S., and Bruce, T.S., 1999, The effects of space was generated by sediment compaction upper Eocene to Quaternary postimpact section the Chesapeake Bay impact crater on the geo- in and the introduction and subsequent removal in the USGS-NASA Langley core, Horton, logical framework and correlation of hydrogeo- J.W., et al., eds., Studies of the Chesapeake Bay logic units of the lower York-James Penisula, of a negative thermal anomaly. After the impact impact structure—The USGS-NASA Langley Virginia: U.S. Geological Survey Professional effects ceased, the crater interior operated as a core, Hampton, Virginia, and related coreholes Paper 1612, 82 p. part of the surrounding passive margin, which, and geophysical surveys: U.S. Geological Sur- Powars, D.S., Mixon, R.B., and Bruce, T.S., 1992, in this case, was quite complex. These results vey Professional Paper 1688, p. H1–H47. Uppermost Mesozoic and Cenozoic geological Glass, B.P., 1989, North American tektite debris and cross section, outer coastal plain of Virginia, in show that a bolide impact can add unusual impact ejecta from DSDP site 612: Meteoritics, Gohn, G.S., ed., Proceedings of the 1988 U.S. complexities to an already complex late-stage v. 24, p. 209–218. Geological Survey workshop on the geology and passive margin. Gohn, G.S., Koeberl, C., Miller, K.G., Reimold, W.U., geohydrology of the Atlantic Coastal Plain: U.S. Future work in this area is in progress and Cockell, C.S., Horton, J.W., Jr., Sanford, W.E., Geological Survey Circular 1059, p. 85–101. includes numerical forward modeling of the and Voytek, M.A., 2006, Chesapeake Bay impact Powars, D.S., Poag, C.W., and Mixon, R.B., 1993, structure drilled: Eos (Transactions, American The Chesapeake Bay “impact crater”—Stratig- negative thermal anomaly. This modeling Geophysical Union), v. 87, p. 349–355. raphy and seismic evidence: Geological Soci- work focuses on the 10 m.y. immediately fol- Gradstein, F.M., Ogg, J.G., and Smith, A.G., 2004, ety of America Abstracts with Programs, v. 25, lowing the impact, and focuses on the inter- A geologic time scale 2004: Cambridge, UK, no. 6, p. 378. action between the negative thermal anomaly, Cambridge University Press, 384 p. Powars, D.S., Bruce, T.S., Edwards, L.E., Gohn, G.S., Horton, J.W., Jr., and Izett, G.A., 2006, Crystalline-rock Self-Trail, J.M., Weems, R.E., Johnson, G.H., the sediment compaction, and the observed ejecta and shocked minerals of the Chesapeake Smith, M.J., and McCartan, C.T., 2005, Physi- sedimentation patterns (Fig. 4). In addition, Bay impact structure, USGS-NASA Langley cal stratigraphy of the upper Eocene to Qua- detailed high-resolution analysis and the core, Hampton, Virginia, with supplemental ternary postimpact section in the USGS-NASA backstripping of the Eyreville A/B and C cores constraints on the age of impact, in Horton, Langley Core, Hampton, Virginia, in Horton, acquired in 2005–2006 will yield insight about J.W., et al., eds., Studies of the Chesapeake Bay J.W., et al., eds., Studies of the Chesapeake Bay impact structure—The USGS-NASA Langley impact structure—The USGS-NASA Langley the subsidence history within the central crater core, Hampton, Virginia, and related coreholes core, Hampton, Virginia, and related coreholes (Gohn et al., 2006). and geophysical surveys: U.S. Geological Sur- and geophysical surveys: U.S. Geological Sur- vey Professional Paper 1688, p. E1–E34. vey Professional Paper 1688, p. G1–G44. 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