Paleoenvironmental and sequence-stratigraphic interpretation of the Middle Eocene Río Turbio (upper member) and Man Aike Formations of southern Patagonia, Argentina

The Middle Eocene of southern Patagonia, Argentina, consists of the Río Turbio and Man Aike formations. These units comprise a succession of siliciclastic sediments deposited on the southwestern margin of Austral Basin. Lateral and vertical variations in lithofacies, trace- and body-fossil content, and stratal-stacking patterns were documented so that their distribution could be used to interpret paleoenvironmental and paleoecological conditions, and construct a sequence-stratigraphic framework. The outcrop succession for the Upper Member of the Río Turbio Formation includes six facies associations. Outer-estuarine successions contain facies that were deposited in channel-thalweg (FAA), subtidal-sandbar complex (FAB, FAD–FAE), and oyster-reef and reef-margin (FAC) environments. The coastal-plain association (FAF) comprises facies that accumulated in distributary-channel, floodplain, salt-marsh, swamp, and swamp-margin environments. Five facies associations were documented in the outcrop succession for the Man Aike Formation. The fluvial association (FAA) accumulated in a braided river, and tidal-embayment successions (FAB–FAE) accrued in subtidal channel-thalweg and compound-dune environments. The Middle Eocene succession consists of deposits housed within tide-dominated incised-valley systems. The Río Turbio incised valley was characterized by transgressive outer-estuarine, and highstand normal regressive coastal-plain conditions. Transgressive deposits are composed of at least seven-stacked, upward-fining, outer-estuarine units deposited during the retrogradation of the paleoshoreline. The transgressive character of the estuarine infill is clearly indicated by the vertical superposition of marine facies—i.e., coarse-grained, glauconite-bearing, higher ichnodiversity, high bivalve diversities, large bivalves—over brackish-water—i.e., fine-grained, low ichnodiversity, low bivalve diversity, smaller bivalves—facies. Highstand deposits comprise approximately three-stacked, upward-coarsening units deposited during the progradation of the paleoshoreline. The vertical superposition of coastal-plain deposits over outer-estuarine facies reflects the progradational nature of this phase of incised-valley infill. The Man Aike incised valley was exemplified by lowstand normal regressive fluvial, and transgressive coastal-embayment conditions. Lowstand deposits accumulated prior to the headward migration of the shoreline. Early fluvial deposition took place when the rate of accommodation creation was low—i.e., initial base-level rise—hence channel amalgamation was frequent, leading to the formation of a coarse-grained succession in which muddy overbank sediment was absent. Transgressive deposits comprise at least five-stacked, upward-fining, coastal embayment units deposited during the retrogradation of the paleoshoreline. The transgressive character of the tidal embayment infill is clearly indicated by the vertical superposition of marine facies over brackish-water facies. Trace-fossil analysis of the Upper Member of the Río Turbio Formation evaluates the applicability of the brackish-water trace-fossil model to Middle Eocene outer-estuarine environments. Previously, the rarity of preserved examples hindered extensive work categorizing the ichnological signature of the estuary-mouth. The ichnofauna from the Río Turbio and Man Aike formations display many characteristics that indicate deposition occurred in salinity-stressed environments. However, these assemblages demonstrate departures from the typical model—e.g., a gradual increase in ichnodiversity, burrows lacking significant size reduction, and the presence of more complex biogenic structures—suggesting a transition in the paleovalley from brackish to more normal marine salinities over time. This research supports that brackish-water outer-estuarine ichnofauna from the Middle to Late Eocene of southern Patagonia display a continuity of behavioral strategies through the Cretaceous–Paleogene transition. Autogenic and allogenic expressions of the Glossifungites ichnofacies demarcate stratigraphic surfaces within the studied sections of the Upper Member of the Río Turbio Formation. Results show that the scale and significance of the erosional discontinuities can be ascertained by integrating data derived from substrate-controlled ichnofacies—such as driving process, period of exposure, length of depositional hiatus, relation within sediment, compaction, and associated trace fossils—with vertical and lateral distribution of the paleoecological data. Trace fossils documented within the autogenic-stiffground assemblages are Thalassinoides, Psilonichnus, and Bergaueria; and Gastrochaenolites is found in the autogenic- and allogenic-firmground assemblages. Allogenically produced surfaces occurred on a regional scale marking major stratal discontinuities, while minor autogenically produced surfaces occurred over a few meters laterally. High-resolution ichnological data, presented herein, combined with the distributions of the paleoecological data provide new insights on the importance and scale of erosional discontinuities observed. Building on the trace-fossil analysis of the Man Aike Formation, a detailed description of Macaronichnus ichnofabrics present in the tidal-embayment facies, as well as the relationship between inferred ecological parameters and Macaronichnus ichnofabric distribution is supplied. This investigation revealed how colonization trends in a high-energy, tide-dominated coastal embayment are controlled by ecological factors specific to different sub-environments within the subtidal compound-dune field. Differences in the observed Macaronichnus ichnofabrics were linked to changes in local hydraulic conditions in the upper and lower trough areas of the compound dunes. A comparison of the common ecological controls on Macaronichnus ichnofabrics in shallow- and marginal-marine environments is provided. This study demonstrates that in the Man Aike Formation, as in many other environments reviewed, the Macaronichnus tracemaker was the first to colonize the shifting sandy substrates. This tracemaker appears to inhabit an ecological niche that is intolerable for other organisms. Overprinting by other ichnoguilds occurred only after physiochemical conditions become favorable and colonization window length increased. Fluctuations in ecological conditions—i.e., hydraulic energy, sedimentation rates, physical reworking—within each depositional environment triggered variations in colonization window lengths, which subsequently produced different colonization and tiering styles—i.e., Macaronichnus ichnofabrics. The opheliid polychaetes that produce Macaronichnus-like traces have adapted to life within specific sand-rich environments—i.e., felsic-rich, fine- to medium-grained sand deposited under moderate- to high-energy conditions producing a moist substrate that is well oxygenated, with an abundant and regularly replenished food source—and as such demonstrate an extremely high degree of substrate specificity. This investigation highlighted that numerous shallow- and marginal-marine environments possess the exact physiochemical properties that this tracemaker requires in order to colonize. This physiochemical specificity illuminates the true importance of Macaronichnus