The origin of third-order eustatic sequences is reviewed by comparing recent sequence stratigraphic data to the latest, best-constrained astronomical model. Middle Eocene to Holocene icehouse sequences correspond to ~ 1.2 myr obliquity cycles. Constraints from oxygen isotope records highlight the link between "icehouse" sea-level lowerings, sequence boundaries, and ~ 1.2 myr obliquity nodes. Mesozoic greenhouse sequences show some relation with the ~ 2.4 myr eccentricity cycles, suggesting that orbital forcing contribute to sea-level change. We suggest that during the icehouse, large ice sheets associated with significant glacioeustatic changes (>>25. m up to 120. m changes) were mainly governed by obliquity forcing. During icehouse worlds, obliquity forcing was the stongest control on global sea-level and depositional sequences. Additionally, during the Middle Eocene, third-order sequences were glacioeustatically driven in tune with ~ 1.2 myr obliquity cycle, suggesting that the presence of significant ice sheets is earlier than previously supposed (i.e., Early Eocene). In contrast, during greenhouse worlds (e.g., ephemeral, small to medium sized or no ice sheets; 0- ~ 25. m glacioeustatic changes), the expression of obliquity in the sedimentary record is weak and intermittent. Instead, the eccentricity signature, which is the modulator of climatic precession, is documented. Moreover, we presume that greenhouse sequences on the myr scale are global and hence cannot be caused by regional tectonism (e.g., intraplate stress or mantle "hot blobs"). Instead, the eccentricity link implies a weaker glacioeustatic control because thermoeustasy is too small to explain the sea-level changes. Stratigraphically well-documented fourth-order sequences may be linked to the astronomically stable (strongest amplitude) 405-kyr eccentricity cycle and possibly to ~ 160-200-kyr obliquity modulation cycles, fifth-order sequences to the short (~ 100-kyr) eccentricity cycles, and finally sixth-order sequences to the fundamental obliquity (~ 40 kyr) and climatic precession (~ 20 kyr) cycles. These astronomical cycles could be preserved in the sedimentary record, and have been demonstrated to control sea-level changes. Accordingly, by placing depositional sequence orders into a high-resolution temporal framework (i.e., orbital periodicities), standardization of eustatic sequence hierarchy may be possible.
All Science Journal Classification (ASJC) codes
- Earth and Planetary Sciences(all)
- Eustatic sequence hierarchy
- Third-order eustatic sequences
- ~1.2- and ~2.4-myr astronomical cycles