Lorena Moscardelli of Statoil North America–Research, Development and Innovation in Austin is not the first to claim evidence to support the existence of a Martian ocean during the late Hesperian–early Amazonian. Viking Orbiter images did that throughout the 1980s and early 1990s.

Others have based their beliefs on alleged paleoshorelines, which has been heavily contested due to large variations in elevation (and some turned out to be of volcanic origin), but Moscardelli uses a new terrestrial, deep-water analogy. 

Moscardelli describes boulder-size rocks within the Vastitas Borealis Formation (VBF) on the northern plains of Mars that are similar to boulder- and kilometer-scale blocks transported to many terrestrial deep-water environments by subaqueous mass-transport events, such as teardrop-shaped islands, large-scale polygonal terrains, and high-albedo mounds. 

Knobby terrains in Arcadia Planitia (HiRISE ESP_019853_2410). DOI: 10.1130/GSATG197A.1

Moscardelli suggests that meter-scale rocks that make up part of the VBF might have been put there by catastrophic mass-transport events similar to those documented within continental margins on Earth.  Moscardelli says the comparison supports the existence of an ocean on Mars and a catastrophic mass-transport origin for the boulders not unlike events documented along continental margins on Earth.

"Discussion of the hypothetical existence of a late Hesperian–early Amazonian ocean on Mars should not revolve exclusively around an acceptance or rejection of a single line of evidence (e.g., the paleoshoreline argument). Search for additional evidence to support the idea of a hypothetically ancient ocean on the northern plains of Mars should also expand to areas presumably covered by water," the authors writes.

Citation: Lorena Moscardelli, 'Boulders of the Vastitas Borealis Formation: Potential origin and implications for an ancient martian ocean', GSA Today pp.4-10 DOI: 10.1130/GSATG197A.1