The Geology of North America Vol. M, The Western North Atlantic Region The Geological Society of America, 1986 Chapter 3 A Jurassic to recent chronology Dennis V. Kent Lamont-Doherty Geological Observatory and Department ofGeological Sciences, Columbia University, Palisades, New York 10964 Felix M. Gradstein Geological Survey ofCanada, Bedford Institute ofOceanography, Dartmouth, Nova Scotia B2Y 4A2, Canada INTRODUCTION tively, allow the construction of a precise magnetochronologic framework. In contrast, the early Aptian to Santonian, and the We present an integrated geomagnetic polarity and geologic Cal10vian to late Oxfordian intervals are of predominantly con­ time scale for the Jurassic to Recent interval, encompassing the stant geomagnetic polarity. They correspond to the oceanic Cre­ age range of the modern ocean floor. The time scale is based on taceous and Jurassic Quiet Zones, respectively, and magneto­ the most recent bio-, magneto-, and radiochronologic data chronologic resolution is poor. A Sinemurian to Bathonian available. interval of frequent reversals has been documented in magneto­ The biostratigraphic bases for Jurassic, Cretaceous, and stratigraphic land sections, primarily from the Mediterranean re­ Cenozoic time-scales are discussed extensively elsewhere (e.g., gion. However, oceanic crust that might carry a magnetic Gradstein, this volume; Van Hinte, 1976b; Hardenbol and Berg­ anomaly signature of these early and middle Jurassic reversals is gren, 1978; Berggren and Van Couvering, 1974; Van Couver­ apparently not present. Consequently, the detailed sequence of ing and Berggren, 1977). Emphasis is placed here on magneto­ reversals is poorly known for this time interval. chronology and its integration with biochronology in the deriva­ tion of an internally consistent geologic time scale. The binary LATE CRETACEOUS AND CENOZOIC signal of normal and reversed geomagnetic polarity has little intrinsic absolute time value (ordinal scale), but it can be used to The chronology and chronostratigraphy of this time interval measure time according to its radiochronologic calibration (car­ is drawn directly from Berggren and others (1984a, b). In their dinal scale). The standard magnetic reversal sequence has a corre­ work, bio- and magnetostratigraphy in some European.Paleogene latable, characteristic pattern and is demonstrated to be and Neogene stratotype sections are integrated and an assessment continuous from numerous marine magnetic anomaly profiles of some 200 Cenozoic and Late Cretaceous calcareous plankton from the world ocean. The reversal sequence is recorded by datum events are directly correlated with magnetic polarity stra­ lateral accretion in sea-floor spreading and vertical accumulation tigraphy in deep-sea sediment cores and land sections. The data in sedimentary or lava sections, allowing independent checks on provide improved identification of the boundaries and durations the completeness and relative spacing of the reversal sequence as of chronostratigraphic units in terms of planktic biostratigraphy well as the opportunity to apply an assortment ofgeochronologic and geomagnetic polarity chrons. data for calibration. Although different phenomena and assump­ The geomagnetic polarity time scale is based on the radio­ tions are invoked in their derivation, both magneto- and bio­ metric dates and magnetic polarities on lavas for 0 to 4 Ma chronologic time estimates involve indirect assessment according (Mankinen and Dalrymple, 1979) and is extended in time by age to calculated rates ofsea-floor spreading, sedimentation, and biot­ calibration of the polarity sequence inferred from marine mag­ ic evolution. These extend the application of the relatively few netic anomalies. The polarity sequence compiled by laBrecque reliable radiometric dates available, so that a continuous geologic and others (1977) is taken as representative of the sea-floor time scale can be inferred. spreading record for the Late Cretaceous and Cenozoic. Six se­ The degree of magnetochronologic resolution possible de­ lected high-temperature radiometric ages are used for age calibra­ pends on the frequency of geomagnetic reversals and the availa­ tion in such a way as to minimize apparent accelerations in bility of a well-defined record of the polarity sequence best sea-floor spreading history. These key ages are for Anomalies 2A developed in marine magnetic lineations (Vogt, this volume, Ch. (3.40 Ma), 5 (8.87 Ma), 12 (32.4 Ma), 13 (34.6 Ma), 21 (49.5 15). Thus the Campanian to Recent and the latest Oxfordian to Ma) and 34 (84.0 Ma) (see Berggren and others, 1984a). Calcu­ early Aptian intervals of frequent reversals, corresponding to the lated ages for magnetic polarity intervals are shown in Table 1. mid-ocean ridge and the M-sequence magnetic lineations, respec- Relative precision ofboundaries in the reversal sequence depends Kent, D. V., and Gradstein, F. M., 1986, A Jurassic to recent chronology; in Vogt, P. R., and Tucholke, B. E., eds., The Geology of North America, Volume M, The Western North Atlantic Region: Geological Society of America. 45 46 D. V. Kent and F. M Gradstein TABLE 1. REVISED GEOMAGNETIC POLARITY TIME-SCALE FOR OXFORDIAN TO RECENT TIME Normal Polarity Anomaly Normal Polarity Anomaly Anomaly Normal Polarity Anomaly Interval (Mal Interval (Mal (Reversed) Interval (Ma) (Normal) - 118.00 Cretaceous Quiet Zone 0.00 - 0.73 1 24.04 - 24. 21 6C MO 118.70 - 121.81 0.91 - 0.98 25.50 - 25.60 7 M1 122.25 - 1n.01 M2 1. 66 - 1. 88 2 25.67 - 25.97 7 M3 125.36 - 126.46 M4 2.47 - 2.92 2A 26.38 - 26.56 7A M5 127.05 - 127.21 2.99 - 3.08 2A 26.86 - 26.93 8 M6 127.14 - 127.52 3.18 - 3.40 2A 27.01 - 27.74 8 M7 127.97 - 128.11 3.88 - 3.97 3 28.15 - 28.74 9 M8 128.60 - 128.91 4.10 - 4.24 1 28.80 - 29.21 9 M9 129.41 - 129.82 4.40 - 4. 47 3 29.73 - 30.03 10 M10 130.19 - 130.57 4.57 - 4.77 3 30.09 - 30.13 10 110.63 - 131. 00 5.35 - 5.53 3A 31. 23 - 31. 58 11 131.02 - 131. 36 5.68 - 5.89 3A 31. 64 - 32.06 11 M10N 131.65 - 112.51 6.37 - 6.50 32.46 - 32.90 12 M11 131.03 - 133.08 6.70 - 6.78 4 35.29 - 35.47 13 Mll 131.50 - 1 3 4.11 6.85 - 7.28 4 35.54 - 35.87 13 134.42 - 134.75 7.35 - 7.41 4 37.24 - 37.46 15 M12 115.56 - 115.Fi6 7.90 - 8.21 4A 37.48 - 37.68 15 135.88 - 136.24 8.41 - 8.50 4A 38.10 - 38.34 16 116.37 - 136.64 8.71 - 8.80 38.50 - 38.79 16 M13 137.10 - 137.39 8.92 - 10.42 5 38.83 - 39.24 16 M14 138.10 - 119.01 10.54 - 10.59 39.53 - 40.43 17 M15 139.58 - 141. 20 11. 03 - 11. 09 40.50 - 40.70 17 M16 141. 85 - 142.27 11. 55 - 11. 73 5A 40.77 - 41.11 17 M17 143.76 - 144.13 11. 86 - 12.12 5A 41. 29 - 41.73 18 M18 144.75 - 144.88 12.46 - 12.49 41. 80 - 42.23 18 144.96 - 145.98 12.58 - 12.62 42.30 - 42.73 18 M19 146.44 - 146.75 12.83 - 13.01 5AA 43.60 - 44.06 19 146.81 - 147.47 13.20 - 13.46 5AB 44 .66 - 46.17 20 M20 148.33 - 149.42 13 .69 - 14.08 5AC 48.75 - 50.34 21 M21 149.89 - 151. 46 14.20 - 14.66 5AD 51. 95 - 52.62 22 151.51 - 151.56 14.87 - 14.96 5B 53.88 - 54. 03 21 151.61 - 151. 69 15.13 - 15.27 5B 54.09 - 54.70 23 M22 152.53 - 152.66 16.22 - 16.52 5C 55.14 - 55.37 24 152.84 - 153.21 16.56 - 16.73 5C 55.66 - 56.14 24 153.49 - 151.52 16.80 - 16.",8 5C 58.64 - 59.24 25 M23 154.15 - 154.48 17.57 - 17.90 50 60.21 - 60.75 26 154.85 - 154.88 18.12 - 18.14 50 63.03 - 63.54 27 M24 155.08 - 155.21 18.56 - 19.09 5E 64.29 - 65.12 28 155.48 - 155.84 19.35 - 20.45 6 65.50 - 66.17 29 156.00 - 156.29 20.88 - 21.16 6A 66.74 - 68.42 30 M25 15Fi.55 - 156.70 21. 38 - 21. 71 6A 68.52 - 69.40 31 156.78 - 156.88 21. 90 - 22.06 6AA 71.17 - 71.65 32 156.96 - 157.10 22.25 - 22.35 6AA 71. 91 - 73.55 32 157.20 - 157.30 22.57 22.97 6B 73.96 - 74. 01 15 7 .38 - 157.46 23.27 23.44 6C 74.30 - 80.17 33 157.53 - 157.61 23.55 23.79 6C 84.00 -118.00 34 157.66 - 157.85 PM26 158.01 - 158.21 PM27 158.37 - 158.66 PM28 158.87 - 159.80 PM29 160.33 - (169.00) Jurassic Quiet Zone on the spatial resolution of the magnetic anomaly data and on the thickness) to obtain precise age estimates for various boundaries assumption that the compiled reversal sequence represents a lin­ in accordance with magnetobiostratigraphic correlations. Numer­ ear and continuous record over time intervals of at least tens of ical ages on the Late Cretaceous and Cenozoic geologic time­ million years (but see Vogt, this volume, Ch. 24). The accuracy of scale (Fig. 1; Plate 1, in pocket inside back cover) are therefore the reversal chronology ultimately depends on the radiometric age based on the revised magnetochronology summarized above.