Ancient Patagonian Lake Reveals How Jurassic Climate Swings Controlled Organic Matter

Deep in the extra-Andean badlands of Patagonia, Argentina, a 323-metre-thick stack of Jurassic lake sediments has yielded a remarkably detailed record of how climate swings between wet and dry periods shaped one of the most fundamental questions in sedimentary geology: what determines whether organic matter is buried, preserved, or destroyed in ancient lake basins. A new study of the Cañadón Asfalto Formation, published in the Journal of Micropalaeontology, combines palynofacies analysis with lacustrine sequence stratigraphy to reconstruct the full life cycle of a lake system that existed roughly 170 million years ago, during the late Early to Middle Jurassic. The research, led by Daniela Elizabeth Olivera of Shandong University of Science and Technology together with colleagues from Argentina, demonstrates that the balance between precipitation and evaporation exerted a decisive control on the nature, quantity, and preservation quality of the organic material that accumulated in the basin.

The Cañadón Asfalto Basin formed as part of the tectonic upheaval that accompanied the break-up of Gondwana, beginning in the Early Jurassic and continuing into the Late Cretaceous. North-westerly oriented half-grabens created isolated depocentres, and the Cerro Cóndor depocentre, in the middle Chubut River valley, preserves one of the most complete lacustrine successions of the period. The Cañadón Asfalto Formation itself is constrained between the Late Toarcian and Late Bajocian stages, an age anchored by biostratigraphically useful palynomorphs and a high-precision uranium-lead zircon date. At the Cañadón Lahuincó section, the team measured and sampled the succession bed by bed, collecting 51 outcrop samples of very fine-grained sandstones and siltstones for palynological processing using non-oxidative hydrochloric and hydrofluoric acids, a protocol chosen specifically to protect the natural colour and fluorescence of hydrogen-rich organic particles.

Palynofacies analysis, the technique at the heart of the study, examines the entire acid-resistant organic fraction of a rock rather than only the diagnostic spores and pollen grains. Each sample was quantified by counting a minimum of 500 particles under transmitted light microscopy, classifying them into structured organic matter such as palynomorphs, translucent and opaque phytoclasts, and unstructured amorphous organic matter. The researchers also assessed particle size, shape, colour, and fluorescence, and applied the thermal alteration index to gauge maturity. Cluster analysis using the Euclidean distance and unweighted pair group method grouped the samples into four distinct palynofacies types, designated PT-A through PT-D, plus one outlier sample whose organic matter had been baked by a thick andesite sill that intrudes the section. Total organic carbon measurements on 25 selected samples ranged from a mere 0.13 per cent to an impressive 11.96 per cent.

The sedimentological analysis revealed two complete depositional sequences, each telling a different chapter of the lake’s evolution. Sequence I, extending to about 197 metres above the base of the section, records a full transgressive-regressive cycle. Its lower part consists of fining- and thinning-upward cycles up to 10 metres thick, deposited when the lake was underfilled, meaning that water and sediment inflow could not keep pace with the accommodation space created by subsidence. In this state, lake levels fluctuated dramatically below the basin’s spill point, and marginal areas were periodically exposed, producing arid palaeosols and restricted water conditions that favoured phytoplanktonic productivity. The upper part of Sequence I flips the story: stacked coarsening-upward successions of littoral delta deposits indicate that the lake had transitioned to balanced-fill and ultimately overfilled conditions, in which the water level stabilised at the spill point and coastal progradation became possible.

Sequence II begins with an abrupt increase in accommodation, likely driven by renewed tectonic subsidence, and initially returns to underfilled conditions marked by shales interbedded with sandstones and thin limestones. The limestones, the authors interpret as punctuated episodes of lake restriction and alkaline waters during high-frequency regressive pulses. Then, at roughly 222 metres above the base, a sharp boundary marks what the researchers identify as a forced-regression surface, produced when the lake breached its spill point and entered fully overfilled conditions. Above this surface, thick coarsening-upward sandy cycles up to 30 metres record persistent progradation of littoral deltas into a freshwater lake, an overall progradational parasequence set that continues to the top of the measured section.

The palynological evidence maps beautifully onto this stratigraphic framework. Throughout the succession, land-derived material dominates, with phytoclasts making up between 31 and 100 per cent of the total organic assemblage, testifying to a strong terrestrial input into the basin. Yet the subtle shifts between this allochthonous plant debris and autochthonous algal material encode the climatic rhythm. Palynofacies types PT-A and PT-D signal wetter periods, when intensified rainfall drove extraordinary fluvial discharges, possibly including hyperpycnal flows, that swept high volumes of reworked terrestrial phytoclasts into the lake. PT-B, by contrast, marks drier intervals, and PT-C captures the transition between these opposing states. The ratio of equidimensional to blade-shaped opaque particles, a proxy for flow velocity, further supports this interpretation, with low values indicating distal accumulation from waning flows.

One of the most striking findings concerns the green colonial alga Botryococcus, whose well-preserved colonies with strong yellow fluorescence bloom in the assemblages during drier periods. Botryococcus is a tenacious coloniser of shallow, alkaline, oligotrophic to mesotrophic waters, equipped with desiccation-resistant walls and the ability to store large reserves of food, adaptations that allow it to survive conditions lethal to other algae. When evaporation outpaced runoff and the water body contracted, these algal blooms flourished, and their decay depleted oxygen in bottom waters, creating the dysoxic to anoxic conditions that allowed organic matter to be preserved with exceptional fidelity. The researchers note parallels with modern studies from southern Brazil, where algal dominance in lagoon sediments has been directly linked to twentieth-century drought episodes and falling water levels.

The taphonomic contrast between lake stages proved equally revealing. During the underfilled stage, terrestrial palynomorphs are scarce and poorly preserved, a pattern the authors attribute less to the absence of vegetation nearby than to the hostile chemistry of the lake water, whose pH conditions likely destroyed sporopollenin, the resilient biopolymer forming spore and pollen walls. The robust walls of Botryococcus and plant cuticles survived where sporomorphs did not. In the balanced-fill stage, by contrast, sporomorph-rich assemblages with well-preserved grains appear, dominated by Classopollis pollen from the Hirmerellaceae, which reaches up to 85 per cent of the sporomorph group, alongside araucariacean and podocarpacean pollen whose distribution patterns help distinguish proximal delta-plain settings from more distal depositional environments.

The study also carries practical weight for petroleum geoscience. Kerogen in the analysed samples ranges from gas-prone Type III, dominated by terrestrial material, to mixed Type I/III in the algae-rich intervals, and thermal alteration indices between 2+ and 3− indicate an early to middle-mature, oil- and gas-prone stage. Given the predominance of Type III kerogen, the authors conclude that the Cañadón Asfalto Formation deposits at this locality represent a potential unconventional shale gas reservoir. More broadly, the work underscores that lacustrine basins cannot be treated as miniature oceans: their water levels, chemistry, and organic archives respond to climate with a sensitivity that marine systems rarely match. By reading the microscopic organic particles locked in Patagonian mudstones, geologists can now reconstruct, cycle by cycle, how a Jurassic lake breathed with the rhythm of ancient monsoons, and how those same rhythms determined which carbon was destined to become tomorrow’s energy resource.

Subject of Research: Palynofacies analysis and lacustrine sequence stratigraphy of the Jurassic Cañadón Asfalto Formation in Patagonia, Argentina

Article Title: Palynofacies and lacustrine sequence stratigraphy: a case example from the Jurassic Cañadón Asfalto Formation, Cañadón Asfalto Basin, Extra-Andean Patagonia, Argentina

Article References: Olivera, D. E., Zavala, C., Quattrocchio, M. E., Soreda, M. E., Scasso, R., & Yang, R. (2026). Palynofacies and lacustrine sequence stratigraphy: a case example from the Jurassic Cañadón Asfalto Formation, Cañadón Asfalto Basin, Extra-Andean Patagonia, Argentina. Journal of Micropalaeontology, 45(1), 335-357. https://doi.org/10.5194/jm-45-335-2026

Image Credits: AI Generated

DOI: 10.5194/jm-45-335-2026

Keywords: palynofacies, lacustrine sequence stratigraphy, Cañadón Asfalto Formation, Jurassic, Patagonia, Botryococcus, organic matter preservation, kerogen, total organic carbon, paleoclimate, underfilled lake, source rock