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 sedi- ment. 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. Sub sequent 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.