Taphofacies analysis of late early Givetian fossil assemblages of the Parnaíba Basin (State of Piauí, northeast Brazil)

Taphonomic signatures of brachiopod-dominated fossil assemblages from the Passagem Member (Middle Devonian: late early Givetian) are described in the present study. This rock unit (originally assigned to the Cabeças Formation, but actually related to the underlying Pimenteira Formation) is restricted to the eastern border of the Parnaíba Basin. It consists mostly of fine-grained sandstones with sigmoidal clinoform structures (bearing asymptotic cross-stratification and climbing ripples) and interbedded hummocky cross-stratified (HCS) parallel-sided, graded sandstone lobes. Three taphofacies — identified and characterized in four outcrops in the municipalities of Pimenteiras (Oiti and Rio Banguê), Picos (BR-316/km 305), and Sussuapara (Barreiro Branco), State of Piauí — are based on the qualitative and quantitative analysis of the taphonomic signatures of marine macrofossils and land plant debris. Their origin is associated with shallow-water turbidity currents (hyperpycnal flows) generated by a flood-dominated fluvio-deltaic system entering shallow marine settings. The taphonomic signatures of Taphofacies 1 indicate a proximal environment, consistent with distal mouth-bar deposits. This taphofacies, dominated by the brachiopod Pleurochonetes comstocki (Rathbun, 1874), comprises sandstones with asymptotic cross-stratification and climbing ripples. These deposits probably originated from the deceleration of medium-density, essentially unidirectional turbidity currents. Basinward of Taphofacies 1, Taphofacies 2 is considered to have accumulated via the deceleration of low-density turbidity currents in which the oscillatory component of the flow prevailed. This taphofacies is characterized by delta-front sandstone lobes with HCS, and is rich in terebratulid brachiopods. Lastly, Taphofacies 3 was generated by settlement from the upper part of the hyperpycnal flow, charged with fine-grained sediments, plant debris, and mica, and deposited as laterally extensive silt/sand couplets in more distal and marginal areas of the flow.

► A flood-dominated fluvio-deltaic model conforms with the Passagem fossil content.
► The Passagem fossil assemblages were deposited from hyperpycnal flows.
► High-density shallow-water turbidity currents transported macrofossils into the basin.
► Middle Devonian fossils are preserved in mouth-bar deposits and delta-front lobes.
► The Passagem Member is a proximal facies of the upper Pimenteira Formation in Piauí.