Hiatuses and condensation: an estimation of time lost on a shallow carbonate platform

Abstract

Abstract On shallow carbonate platforms, the sedimentary record is highly fragmentary because low accommodation commonly leads to non‐deposition, erosion, reworking and condensation. Consequently, it is difficult to quantify the time that is actually recorded and to estimate sedimentation rates. The Berriasian platform in the Swiss and French Jura Mountains offers an opportunity to address this problem. Facies evolution through time displays deepening‐shallowing trends of different orders, resulting in a hierarchical stacking of sequences. The sequence‐ and cyclostratigraphic analysis relates these sequences to orbital cycles with durations of 20, 100, and 400 kyr. Many surfaces in the record of shallow‐water carbonates can be interpreted as hiatuses due to non‐deposition in the supratidal realm, reactivation of shoals or scouring by currents. Strongly bioturbated intervals and hardgrounds commonly result from sediment starvation. These same processes and products can be observed on modern carbonate platforms, and the duration of the hiatuses or condensed intervals can be inferred (a few hours to a few hundreds of years). Amalgamations and narrow stacking of such surfaces create sequence‐boundary zones that can be followed over the entire platform. Time lost or condensed in these zones may correspond to hundreds of thousands of years. Time distribution in the studied sections thus is highly irregular: short periods of sedimentation alternated with long periods of non‐deposition, erosion, reworking, and condensation. Furthermore, due to substrate morphology and laterally variable depositional environments, the depositional record of a given time interval varied significantly across the platform. In one example, it can be estimated that – applying modern sedimentation rates – a sedimentary record spanning 800 kyr could have been deposited within 44 kyr if there were no hiatuses or condensations. It becomes clear that, when estimating carbonate production and sediment accumulation rates in ancient records, the shortest possible time spans must be used to minimize the effect of time lost in hiatal surfaces and condensed intervals.