Geology Department, Temple University, Philadelphia, PA 19122
andy@astro.temple.edu

The Lower Cretaceous Sierra del Pozo Formation is totally divisible into meter-scale cycles that can be bundled in a three-tiered hierarchy, Fig. 1 - Column: 4th Order Sequence I (130-143M). The smallest scale cycles (6th order), thought to be the product of precessionally forced sea-level change average less than a meter in thickness, shallow upward and are bounded by surfaces where deeper facies abruptly overlie shallower facies. These 6^th order cycles may be grouped into sets (5^th order sequences) of 2 to 6 cycles by periodic change (100 ka) in the eccentricity of the Earth’s orbit. These larger-scale, 5th order sequences, are recognized by an asymmetric pattern in the degree of facies change internal to successive, constituent meter-scale cycles. Near the base of a 5^th order set (i.e. in the first or second 6^th order cycle) the magnitude of facies change at the basal cycle boundary is greater than that in subsequent cycles in the set. The next larger tier in the hierarchy (4th order) comprises sets of four 5th order sequences. This tier is the product of a longer term variation (400 ka) in eccentricity. Typically a maximum degree of eccentricity (and a resulting maximum degree of facies change) is observed near the base of the second 5^th order sequence of the 4th order bundle. That is high eccentricity causes the strongest precessional signals in the second 5th order sequence in a 4th order set.

Jimenez de Cisneros and Vera (1993) described 141 cycles in a 280 m interval in the Sierra del Pozo section. Their cycle boundaries (shown to the left side of the columns in Figures 1. and 2.) are defined at surfaces where subtidal-facies abruptly overly tidal-flat facies, where there is evidence of exposure (i.e. karst or soil) or at marked discontinuities. Cycle thicknesses on their log (varying between 1-4 m) are precise and and reproducible. They conclude that the average duration of the cycles they describe is approximately 40 ka and that this time interval suggests that obliquity-forced sea-level change is the most probable explanation of the origin of the cycles, Fig. 2 - Column: 4th Order Sequence II (143-156M).

Re-evaluation of the cyclicity in this section applying an  orbitally forced, hierarchic genetic model (Anderson and Goodwin, 1990; Goodwin and Anderson, 1997) indicates that the thicker cycles described by Jimenez de Cisneros and Vera are composite cycles and that meter-scale cycles (some of which are recognized by Jimenez de Cisneros and Vera) are bundled in a 3 tiered hierarchy. This hierarchy conforms well with the tiered cyclic structure described by Strasser and Hillgärtner (1998) in rocks of the same age (and similar facies) in the French Jura. The 3-tiered stacking patterns and time limitations observed in both areas support a ‘Milankovitch’ orbitally forcing mechanism. Work supported by NSF Grant No. 9902680.

    Figures 1 and 2 above show stratigraphic logs of two successive 4th order sequences located between 132 and 156 meters in the Sierra del Pozo section.  Black tick marks indicate the location of 6th order cycles.  The orange bar to the left of the column show the location of cycles 53 to 64 of Jimenez de Cisneros and Vera.  The cross-hatched interval between 151 and 154 meters is missing in their column.

    Photographs (scale one meter) documenting 4th order sequence I  shown in Figure 1.  Photo 1. shows the karst at the 3rd order boundary located at 132.5 meters. Photo 1. 3rd Order Boundary.jpg    Photo 2. shows the A - 5th order sequence at the base of the I - 4th order sequence shown (Fig. 1).  This approximates cycle 55 of  of Jimenez de Cisneros and Vera (excluding the shale on top).  Photo 2. 5th Order Sequence A.jpg  Photo 3. shows 5th order sequence B in the I - 4th order sequence.  The facies changes at the first and third 6th order boundaries are extremely marked while facies changes at the second and fourth order boundaries are muted.  This might be explained by the obliquity signal exactly being in phase with one and three.  Photo 3. 5th Order Sequence B.jpg  Photo 4. shows the boundary between the A and B  5th order sequences above.  Photo 4. A-B sequence boundary.jpg   Photo 5 (top). shows the C and D 5ths of 4th order sequence I  and Photo 5 (bottom). the 4th order boundary and the overlying A 5th order sequence of  4th order sequence II.  Photo 5. 4th order I-II sequence boundary.jpg

    Photographs 6. and 7. documenting the B and C  5th  order sequences of  4th order sequence II shown in Figure 2.  Photo 6. 5th Order Sequence B of II 4th and Photo 7. 5th Order Sequence B/C of II 4th

Reference List 

Anderson, E.J. and Goodwin, P.W. (1990) The significance of meter-scale allocycles in the quest for a fundamental stratigraphic unit.-Jour. of the Geol. Soc., London, 147, 507-518.

Goodwin, P.W. and Anderson, E.J. (1997) Stratigraphic incompleteness: Milankovitch in the Manlius at the margin.-Field Trip Guide, New York State Geol. Assoc., 69^th Annual Meeting, 237-249.

Jimenez de Cisneros, C. and Vera, J.A. (1993) Milankovitch cyclicity in Purbeck peritidal limestones of the Prebetic (Berriasian, southern Spain).- Sedimentology, 40, 513-537.

Strasser, A. and Hillgärtner, H. (1998) High-frequency sea-level fluctuations recorded on a shallow carbonate platform (Berriasian and Lower Valanginian of Mount Salève, French Jura). Eclogae geol. Helvetica, 92, 375-390.