 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; January 2006; v. 34; no. 1; p. 29–32; doi: 10.1130/G21918.1; 3 figures. 29 Magnetic record of Milankovitch rhythms in lithologically noncyclic marine carbonates Diana K. Latta* ² David J. Anastasio* Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA Linda A. Hinnov* Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, USA Maya Elrick* Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA Kenneth P. Kodama* Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA ABSTRACT Rock magnetic variations record cyclicity within lithologically homogeneous basinal lime mudstones of the Lower Cretaceous San Angel Limestone, northeastern Mexico. Var- iations in ferromagnetic mineral concentrations, as measured by anhysteretic remanent magnetization (ARM), occur at frequencies consistent with Milankovitch orbital rhythms. Magnetic mineral compositions, grain-size distributions, and grain shapes from digested samples are congruent with far-traveled atmospheric dust. Prevailing winds and the prox- imity of the Cretaceous basin to an African eolian source support the encoding of orbitally modulated changes in wind intensity or source-area aridity. ARM measurements offer great potential to calibrate the pace of depositional processes in carbonates and to inves- tigate high-frequency orbitally driven climate change in basinal strata throughout geologic time. Keywords: rock magnetics, Milankovitch theory, carbonates, anhysteretic remanent magneti- zation, paleoclimate. Figure 1. Paleogeographic reconstruction of Atlantic during middle Cretaceous. Star de- notes field site in northeastern Mexico, shal- low marine (light gray) and land (dark gray). Arrows denote trade winds from African continent at 115 Ma. Figure modified from Scotese et al. (1989). INTRODUCTION Outer shelf to deep-ocean sedimentary rocks are excellent recorders of paleoenviron- mental change because of their ubiquity, tem- poral stability, and continuity, but the absence of detailed chronologies leaves many of these data underutilized. Some marine successions display climatically controlled lithologic or bi- ologic variations characterized by visually ob- vious changes in rock type, color, bioturba- tion, sedimentary structures, and/or organic matter, that are unraveled by time-series anal- yses of the varying characteristics (see discus- sions in Shackleton et al., 1999; Hinnov, 2000; D’Argenio et al., 2004). Many deeper marine carbonate successions, however, lack obvious lithologic or biogenic variations that can be measured from outcrop or core, but still may contain important paleoenvironmental records beneath a monotonous veneer. Rock magnetics provide a high-resolution tool for detecting environmental change from *E-mails: diana.k.latta@exxonmobile.com; dja2@lehigh.edu; hinnov@jhu.edu; Elrick—dolo- mite@unm.edu; kpk0@lehigh.edu. ²Current address: ExxonMobile Exploration Company. decadal to orbital time scales (Maher and Thompson, 1999). Within marine carbonates, changes in magnetic mineral concentration can be externally driven and reflect changes in detrital influx, variations in carbonate con- tent (dilution), or diagenesis (von Dobeneck and Schmieder, 1999). Magnetic susceptibility (MS), a commonly measured bulk-rock prop- erty, integrates the concentration of diamag- netic, paramagnetic, and ferromagnetic grains. MS is often used as a proxy for varying car- bonate content and has been interpreted to re- flect changes in climate (e.g., Mayer and Ap- pel, 1999; Kashiyama et al., 2003). In carbonates, MS is dominated by calcite, mut- ing variation in paramagnetic and ferromag- netic concentrations, which are known climate proxies. Anhysteretic remanent magnetization (ARM), a measurement of the concentration of fine-grained (20 m) ferromagnetic min- erals, is insensitive to carbonate concentration and thus can elucidate climate change in car- bonate rocks. We used ARM to identify and track orbital-scale climatic change in visually noncyclic carbonates. Identification of Milan- kovitch cycles (Milankovitch, 1941) allows precise stratigraphic correlation and accumu- lation rate determination. GEOLOGIC SETTING In northeastern Mexico, platform and shelf deposits accumulated from the Late Jurassic to middle Cretaceous with the opening of the Gulf of Mexico (Longoria, 1998). The units were deformed during the Late Cretaceous– Tertiary Sevier-Laramide orogeny and crop out within large-scale de ´collement folds with- in the Sierra Madre Oriental fold belt (Hum- phrey, 1956). At Cerro de la Silla anticline, a 2-km-thick Mesozoic succession is exposed in La Boca Canyon (Fig. 1). Here, the San Angel