Sequence stratigraphic controls on synsedimentary cementation and preservation of dinosaur tracks: Example from the lower Cretaceous, (Upper Albian) Dakota Formation, Southeastern Nebraska, U.S.A.

Authors

Preston Lee Phillips Jr., University of North Carolina at Pembroke
Greg A. Ludvigson, Kansas Geological Survey, The University of Kansas, Lawrence
R. Matthew Joeckel, University of Nebraska-LincolnFollow
Luis A. González, The University of Kansas, Lawrence
Robert L. Brenner, University of Iowa
Brian J. Witzke, University of Iowa

Date of this Version

4-2007

Comments

Published in Palaeogeography, Palaeoclimatology, Palaeoecology 246:2-4 (April 6, 2007), pp. 367–389; doi: 10.1016/j.palaeo.2006.10.013 Copyright © 2006 Elsevier B.V. Used by permission.

Abstract

A thin cemented sandstone bed in the Upper Albian Dakota Formation of southeastern Nebraska contains the first dinosaur tracks to be described from the state. Of equal importance to the tracks are stable-isotope (C, O) analyses of cements in the track bed, especially in the context of data derived from generally correlative strata (sandstones and sphaerosiderite-bearing paleosols) in the region. These data provide the framework for interpretations of paleoenvironmental conditions, as well as a novel approach to understanding mechanisms of terrestrial vertebrate track preservation.

High minus-cement-porosity (> 47%) and low grain-to-grain contacts (~2.5) in the track bed indicate early (pre-compaction) lithification. Although phreatic cements dominate, the history of cementation within this stratigraphic interval is complex. Cathodoluminescence petrography reveals two distinct calcite zones in the track-bearing horizon and four cement zones in stratigraphically equivalent strata from a nearby section. The earliest calcite cements from both localities are likely coeval because they exhibit identical positive covariant trends (δ¹⁸O values of − 9.89 to − 6.32‰ and δ¹³C values of − 28.01 to − 19.33‰ VPDB) and record mixing of brackish and meteoric groundwaters. All other calcite cements define meteoric calcite lines with δ¹⁸O values clustering around − 9.42‰ and − 8.21‰ VPDB from the track-bearing horizon, and − 7.74‰, − 5.81‰, and − 3.95‰ VPDB from the neighboring section. Distinct meteoric sphaerosiderite lines from roughly correlative paleosols serve as a proxy for locally recharged groundwaters. Back-calculated paleogroundwater δ¹⁸O estimates from paleosol sphaerosiderites range from − 7.4 to − 4.2‰ SMOW; whereas, meteoric calcite lines from the track horizon are generally more depleted.

Differences in cement δ¹⁸O values record changes in paleogroundwater recharge areas over time. Early calcite cements indicate mixing of fresh and brackish groundwaters during the syndepositional lithification of the track horizon. Later calcite cements, however, indicate recharge from a larger catchment basin that extended far inland. Therefore, the cements likely record a rise and subsequent fall in relative sea level. We conclude that scrutiny of the cement isotope geochemistry of genetically significant surfaces, especially track beds, can provide new data for interpreting sea level change.