Linda Mary Olivine
Those are the trilobite trails near Idanha. But a lot older than your burrows near Cadiz.
The Bornos Embalse is surrounded by sand, silt etc deposited about 5 – 7 million years ago during the Messinian, for a time the Mediterranean dried out completely but when the sea flooded back in through the Straits of G8braltar a lot of the low lying basins in southern Spain were flooded. Ideal for shrimps etc to burrow in the soft sediment