President’s Column – March 2011

Title: Rock Creek Anticline

Author: Ira Pasternack

Publication: The Outcrop, March 2011, p.8, 10

Article Type: President’s Column

Over a century ago, G.K. Gilbert (1895) proposed a relationship between cyclical sediment deposition and periodic variations in the Earth’s orbit, now known as Milankovitch cycles. Gilbert described outcrops of the Benton, Niobrara and Pierre groups along the Arkansas River valley at Rock Creek anticline, near Pueblo, Colorado. Four intervals scattered within the Benton and Niobrara were noted to contain rhythmically alternating layers of shale, calcareous shale and limestone. The lower two rhythmic intervals contain shale-limestone couplets that average 18 inches thick. The upper two intervals consist of alternating beds of calcareous and less calcareous shale. There are as many as fifteen repeated cycles in each of the four rhythmic intervals and the intervals are separated by 90- to 475-foot sections that are dominantly shale.

Gilbert deduced there were no terrestrial causes that could produce the rhythmic interbedding he observed. He proposed that the interbedding could be the result of periodic variations in the Earth’s orbit. Gilbert dismissed the annual cycle as being too short a period to allow for the accumulation of one 18-inch thick depositional cycle. He considered the eccentric orbit cycle, thought to have a period of about 91,000 years at the time of Gilbert’s writing, (46 years prior to Milankovitch’s work), to be too long. Gilbert settled on the 21,000-year precessional cycle as the likely time frame.

The Greek astronomer Hipparchus of Nicea was the first to recognize variations in Earth’s orbit in approximately 130 BC. Hipparchus compared earlier observations to his own, and concluded that during the preceding 169 years, the position of the Sun at equinox (the two days of the year when the length of day is exactly equal to the length of night) had shifted by two degrees. Hipparchus termed this motion “precession of the equinoxes.” “Precession” has been retained by subsequent workers to describe this motion of the Earth’s rotational axis, similar to that exhibited by the axis of a spinning toy top that is about to fall over.

Adhé mar (1842) often is cited as one of the first to suggest that climatic changes might be controlled by variations in Earth’s orbital patterns. Croll (1875) elaborated on Adhé mar’s work by describing periodic shifts in Earth’s orbital pattern that affect the distribution of solar radiation, or insolation, and interpreted the shifts as responsible for instigating multiple ice ages. It was Serbian mathematician Milutin Milankovitch, however, who is credited with making the earliest scientifically accurate calculations. Milankovitch determined that variations in the Earth’s orbital path could cause up to a 30 percent variation in solar insolation. Orbital variations include: (1) precession of the Earth’s axis over a 21,000-year cycle; (2) tilt of the Earth’s axis relative to the plane of its orbit (obliquity), which changes over about 41,000 years from 21.5 degrees to 24.4 degrees; and (3) degree of the orbit eccentricity which was found to vary over 100,000 year periods from being nearly circular to noticeably elliptical.

Milankovitch developed insolation curves for the last 650,000 years during a 30-year study published in 1941, Kanon der Erbestrahlung und seine Anwendung auf das Eiszeitenproblem (“Canon of Insolation and the Ice Age Problem”). Four groups of temperature minima (calculated for latitude 65 degrees north and for the northern-hemisphere summer season only) were postulated to be the driving mechanism for the four glacial advances.

Gilbert selected the 21,000-year precessional cycle period as a basis to estimate the time required for deposition of the entire 3,900-foot Benton-Niobrara-Pierre section at approximately 20 million years. Demonstrating his penchant for multiple, working, hypotheses, Gilbert also provided a possible time range for deposition between 10 and 40 million years. The accuracy of Gilbert’s depositional time estimate can be evaluated by considering regional upper Cretaceous cross sections of Weimer (1960). Weimer’s Figure 2 is a cross section that terminates in the southern part of the Denver Basin, reasonably close to the outcrops described by Gilbert. Weimer places the age of the Benton-Niobrara-Pierre interval (which encompasses his first transgression “T1” through his final regression “R4,” Figure 2) at mid-Turonian to mid-Maestrichtian (Weimer, Table 1). Obradovich (1993) places the midpoint ages of these two stages at 91 and 68.4 million years, respectively, reflecting a time span of 22.6 million years. Therefore, Gilbert’s estimate of 20 million years was surprisingly accurate—certainly within the range he speculated was possible—and an excellent effort considering this was the pioneering attempt at estimating depositional rates through consideration of variations in the Earth’s orbit.

The outcrops at Rock Creek anticline also are significant because they are the site of the Global Boundary Stratotype Section and Point (GSSP) for the Turonian-Cenomanian boundary (Kennedy, et al., 2005). GSSPs are subdivisions of the global geologic record formally defined by The International Commission on Stratigraphy (ICS). The intention of GSSP definition is to address issues that develop when subdividing the geologic record on the basis of provincial type sections, floras, or faunas that cause gaps or overlaps in the record when extended to a global scale. The primary basis for GSSP definition is their global correlatability and includes considerations of biostratigraphy, geochronometry, magnetostratigraphy, chemostratigraphy, cyclostratigraphy and astrochronology. Details of each GSSP are posted on the ICS website (www. stratigraphy.org). The Turonian-Cenomanian boundary GSSP at Rock Creek anticline was ratified by the ICS in 2003 because it is well-exposed and accessible, contains several widespread biostratigraphic markers, has a well-defined carbon-13 isotope signature profile, and contains bentonites correlatable to other sections that have been radiometrically age dated at 93.0 to 93.5 Ma.

In Colorado, we are fortunate to have outcrops that are unique and geologically noteworthy on a global scale. One of the most significant is located less than two hours from Denver at Rock Creek anticline along the Arkansas River near Pueblo. Anybody interested in going on a field trip? I’m game!

References:

Adhé mar, J.A., 1842, Revolucion des Mares, Deluges periodique, Paris

Croll, J., 1875, Climate and Time in their Geological Relations, A Theory of Secular Changes of the Earth’s Climate, Dalby, Ibister & Company, London.

Gilbert, G.K., 1895, Sedimentary measurement of geologic time: Geology, v. 3, p. 121-127.

Kennedy, W.J., I. Walaszczyk, and W.A. Cobban, 2005, The Global Boundary Stratotype Section and Point for the base of the Turonian Stage of the Cretaceous: Pueblo, Colorado, U.S.A: Episodes, Vol. 28, no.2, p. 93-104.

Obradovich, J. D., 1993, A Cretaceous time scale, in W. G. E. Caldwell and E. G. Kauffman eds., Evolution of the Western Interior Basin: Geological Association of Canada, Special Paper 39, p. 379-396.

Weimer, R. J., 1960, Upper Cretaceous stratigraphy, Rocky Mountain area: AAPG Bull., v. 44, p. 1-20.

Advertisements